Urban, pluvial flooding

2018-05-08

Ph. D. thesis Johanna Sörensen

Appended papers

I. Sörensen, J., Persson, A., Sternudd, C., Aspegren, H., Nilsson, J., Nordström, J., Jönsson, K., Mottaghi, M., Becker, P., Pilesjö, P., Larsson, R., Berndtsson, R., and Mobini, S. (2016). “Re-thinking urban flood management - Time for a regime shift.” Water, 8(332), 1–15.

II. Sörensen, J., and Mobini, S. (2017). “Pluvial, urban flood mechanisms and characteristics – Assessment based on insurance claims.” Journal of Hydrology, 555, 51–67.

III. Johanna Sörensen and Tobias Emilsson (n.d.) Flood Risk Reduction by Urban Stormwater LID – an Evaluation using Insurance Data [under review for Journal of Water Resources Planning and Management]

IV. M. Wihlborg, J. Sörensen and J. Alkan Olsson. (n.d.) Drivers and barriers for implementation of blue-green infrastructure in Swedish municipalities – what could be done to overcome them [manuscript]

V. Johanna Sörensen, Johanna Alkan Olsson and Anna Persson. (n.d.) A framework for city planning with nature-based solutions (NBS) [manuscript for Land Degradation and Development, special issue on Nature-based solutions]

Author’s contribution to appended papers

I. The author coordinated the writing process. All co-authors contributed with texts and ideas. The author synthesised the material into an article.

II. The author collected data, developed the method, analysed the data and discussed the results independently. The author wrote the article, with supervision on the structure and with proof-reading help.

III. The author collected data, developed the method, analysed the data and discussed the results independently together with the co-author.

IV. The study was conducted by a master’s student (first author of the article). The author supervised the thesis of which the article is based on together with the other co-author. All three authors developed the theory for the study further during the article writing process.

V. The author took the initiative for collaboration on blue-green infrastructure and nature-based solutions. A couple of brainstorming sessions were conducted together with the co-authors. The author, together with one of the co-authors, made three interviews with in total six practitioners.

Urban, pluvial flooding

Blue-green infrastructure as a strategy for resilience

Johanna Maria Lykke Sörensen

DOCTORAL DISSERTATION

by due permission of the Faculty of Engineering, Lund University, Sweden.

To be defended at John Ericssons väg 1, V:B, on 14 September 2018 at 10:00.

Faculty opponent

Dr Ir Marie-claire ten Veldhuis

Associate professor, Civil Engineering and Geosciences

Delft University of Technology

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LUND UNIVERSITY

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I, the undersigned, being the copyright owner of the abstract of the above-mentioned dissertation, hereby grant to all reference sources permission to publish and disseminate the abstract of the above-mentioned dissertation.

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Urban, pluvial flooding

Blue-green infrastructure as a strategy for resilience

Johanna Maria Lykke Sörensen

Coverphoto by Johanna Sörensen

Copyright 2018 Johanna Sörensen

Paper I © 2016 by the authors (open access)

Paper II © 2017 by Elsevier B.V.

Paper III ©

Paper IV ©

Paper V ©

Faculty of Engineering

Department of Water Resources Engineering

ISBN print version: 978-91-7753-734-2

ISBN digital version: 978-91-7753-735-9

ISSN, report number

Printed in Sweden by Media-Tryck, Lund University

Lund 2018

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ICSWAN ECO

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1234 5678

Det går ett träd omkring i regnet,

skyndar förbi oss i det skvalande grå.

Det har ett ärende. Det hämtar liv ur regnet

som en koltrast i en fruktträdgård.

Då regnet upphör stannar trädet.

Det skymtar rakt, stilla i klara nätter

i väntan liksom vi på ögonblicket

då snöflingorna slår ut i rymden.

Tomas Tranströmer

Trädet och skyn

ur Den halvfärdiga himlen, 1962

ii

Contents

Introduction ..............................................................................................................1

Adaptation of systems for urban drainage ......................................................3

Blue-green infrastructure as a sustainable urban drainage solution ...............4

Objectives .......................................................................................................4

Structure of this thesis ....................................................................................5

Theoretical background ............................................................................................6

Extreme precipitation .....................................................................................6

Pluvial flooding ..............................................................................................8

Historical development of urban drainage systems ........................................9 Problems related to current stormwater management .........................11

Blue-green infrastructure ..............................................................................11

Transition theory ..........................................................................................13

Flood risk and resilience ..............................................................................16 Future uncertainties and strategies for adaptation ...............................16

Flood risk management ................................................................................17

Study area ...............................................................................................................19

City of Malmö ..............................................................................................20

Augustenborg Eco-City ................................................................................20

Municipalities and water utilities in Scania .................................................20

Methodology...........................................................................................................22

iii

Insurance claims as a measure of flooding ...................................................22

Flow path analysis ........................................................................................23

Selection of nearby areas for comparison ....................................................23

Interviews .....................................................................................................23

Data ..............................................................................................................24

Results ....................................................................................................................25

Mechanisms and characteristics of urban, pluvial flooding (Paper2) ..........25

Flood risk reduction with blue-green infrastructure (Paper3) ......................27

Barriers and drivers for implementation of blue-green infrastructure in

Swedish municipalities (Paper4) ..................................................................28

A framework for city planning with blue-green infrastructure and nature-

based solutions (Paper5) ...............................................................................29

Discussion...............................................................................................................31

Measures to decrease flood risk and increase resilience ..............................31

Future stormwater control ............................................................................32

Transition towards increased resilience .......................................................33

Ambiguity and conceptual confusion ...........................................................33

Further research ............................................................................................34 Relation between rainfall and flooding ...............................................34 Responsibility, policy changes, and updated guidelines .....................34 Mainstreaming of blue-green infrastructure in urban planning ...........34 More… ................................................................................................34

Conclusions ............................................................................................................35

References ..............................................................................................................36

Online ...........................................................................................................36

Not in Mendeley ...........................................................................................36

In Mendeley .................................................................................................37

(Last page) ....................................................................................................41

iv

Acknowledgement

Introduction

Floods cause enormous damage around the world and were the second largest nat-

ural cause of economic loss (after storms) and account for 47% of all weather-re-

lated disasters between 1995 and 2015 (CRED & UNISDR 2015). During 1980–

2010, floods affected almost 90 million people worldwide. 15% of the world’s pop-

ulation will live in flood-prone areas by 2050, not taking climate change into ac-

count, meaning they are threatened by flooding (Ligtvoet et al. 2014). Floods have

a large impact on human well-being and economy (CRED & UNISDR 2015). Be-

sides loss of human life and human health effects (Hajat et al. 2005), flooding leads

to ecosystem degradation and damage to economic, historical and cultural values,

as well as decrease of socio-economic welfare (REF Jongman, Appleton från Jonk-

man). Most of the death tolls from natural disasters are reported from low-income

countries, while the economic losses are higher in high-income countries, reflecting

the accumulation of economic wealth there (CRED & UNISDR 2015).

The worldwide frequency of floods has been demonstrated to already have increased

during the twentieth century (Milly et al. 2002) and several reasons for the increase

have been suggested, such as climate change (Milly et al. 2002), increased popula-

tion and economic growth (Ligtvoet et al. 2014), and land use changes (REF). In

Europe, a small increase in flood frequency between 1970 and 2005 is reported for

major floods (direct damage larger than 0.005% of the EU GDP and/or more than

70 casualties), while number of total floods (including smaller events) shows a

greater increase (Barredo 2007). The total losses in Europe are estimated to approx-

imately €4.9 billion annually (2000–2012) and expected to increase to approxi-

mately €23.5 billion by 2050 (Jongman et al. 2014). The projection is how-ever

highly uncertain (ibid.).

2

Severe flooding has hit highly developed cities like Prague, Dresden, and several

other cities (2002), Bern and several other cities (2005), New Orleans (2005), Co-

penhagen (2010, 2011, and 2014), and New York (2012), as well as areas like

Queensland (2010), south-western England (2013–2014), and the French Riviera

(2015). The societal consequences are severe and flooding in urban areas is costly,

especially in central areas. In Nordic countries recent floods have caused severe

losses. In 2002, several villages on the island Orust outside Gothenburg, Sweden

were isolated, and the damages covered by insurance companies were estimated to

123 million SEK (~15 million €) (MSB 2013). The insurance costs after the pluvial

flood in Copenhagen, Denmark in 2011 was estimated to more than 800 million

USD (~580 million €) (Swiss Re 2011). The direct economic losses of an extreme

rainfall event in Malmö, Sweden in 2014 was estimated to 600 million SEK (~60

million €) (Malmö Stad 2016). There are few flood events reported to the EM-DAT

from Nordic countries between 1970 and 2005, compared to countries like Italy,

Spain, France and Germany (Barredo 2007). One major event was reported from

Sweden, a spring flood in Bergslagen 1977. River floods, flash floods, and storm

surge cause most damage and casualties in Europe (Barredo 2007).

Urban flooding problems are increasing due to numerous reasons. Extensive urban

and suburban growth (UN 2015, Ligtvoet et al. 2014) in combination with insuffi-

cient sewer systems (Swan 2010) as well as climate change (Semadeni-Davies et al.

2008a,b) aggravates the problem. The effects associated with global warming, such

as sea level rise, more intensive precipitation and higher river discharges, may in-

crease the frequency and the extent of flooding on a worldwide scale. Global aver-

age precipitation is projected to increase, but both increases and decreases are ex-

pected at the regional and continental scales (REF IPCC). In northern Europe, both

annual precipitation and extreme rainfall events during summer is projected to in-

crease (REF IPCC, REF Olsson).

In this work, focus is on pluvial flooding because of its close connection to storm-

water management and the urban landscape. The situation in small cities, like

Malmö, where some of the studies in this work have been conducted, is different

from the most vulnerable megacities, like Dhaka, Kolkata, Shanghai, Mumbai, Ja-

karta, Bangkok and Hoh Chi Minh City (Ligtvoet et al. 2014). While the megacities

mentioned are threatened by both riverine and coastal flooding, Malmö has no major

river and is, in comparison, well protected from storm surge. It is therefore natural

to focus on pluvial flooding, which has led to large damages several places in

Malmö and other cities in Nordic countries and elsewhere (REF MSB, Houston et

al. 2011). Despite the difference in urban density compared to the megacities, there

have been several casualties reported during urban floods in Europe (REF MSB,

REF), adding on to the importance to manage urban flooding also in developed

countries. Pluvial flooding has in common with fluvial flooding the climatological

(Glaser et al. 2010) and hydrological (Berghuijs et al. 2016) drivers as well as the

3

effect of human activities (Zhang et al. 2014). However, pluvial flooding acts on a

different scale than fluvial flooding, where mechanisms and characteristics are func-

tioning differently. Consequently, research is needed to investigate mechanisms and

characteristics also for pluvial flooding, despite extensive research on causality of

fluvial flooding. Also, the measures to control pluvial flood differs from those to

control fluvial and coastal flooding.

Adaptation of systems for urban drainage

The traditional engineering approach to manage urban drainage is by combined or

separate sewers. In urban catchments, drainage systems may include different types

of storage and detention facilities to cope with floods. However, during recent dec-

ades alternative ways to manage floods have evolved since traditional methods often

harm the riverine ecosystems in urban as well as in rural areas and increase the flood

risk (Liao 2012, REF Smits). The current, centralised sewers put a pressure on urban

areas and their surroundings, both by increased flood risk and by high pollution

loads related to combined sewer overflow (CSO) as well as stormwater runoff from

polluted surfaces.

Densification has become a dominating urban planning strategy, as many cities

strive to reduce their negative, environmental impact (REF Ståhle, REF Talen).

With a high amount of impermeable surfaces, urban land is more vulnerable to

flooding than the surrounding environment (REF). To mitigate negative effects of

densification, such as loss of ecosystem functions and services, alternative storm-

water management solutions have been used since the 1970s. During later years,

green infrastructure planning with the social perspective in mind has been called

for. Schifman (2017) claims that the perspective must be shifted from hydrologi-

cally driven to an integrated, socio-hydrological approach, where values such as

increased property value, greenspace aesthetics, heat island amelioration, carbon se-

questration, and habitat for biodiversity are included. In this thesis, the term blue-

green infrastructure is used to clarify the need of integrated solutions with holistic

water cycle as well as ecosystem perspective.

Another problem with current stormwater management is related to aging sewers

and/or lack of proper maintenance. Many pipes have begun to show signs of deteri-

oration and it expensive to meet the need for new investments (REF SVU-rapport)

with new pipes and enlarged capacity. To avoid replacement of existing pipe net-

work, some areas can be disconnected from the sewers to reduce load from them,

and mains and distribution pipes can be rehabilitated with no-dig methods like pipe

lining. These methods are often less expensive than traditional open cut replacement

methods.

4

The combination of climate change adaptation, densification, the call for more green

spaces, and a need to restore aging sewers, leads to strong interest in retrofitting of

urban areas with blue-green infrastructure.

Blue-green infrastructure as a sustainable urban drainage

solution

Blue-green infrastructure can contribute to the urban environment with multiple

benefits: water supply, flood mitigation, terrestrial biodiversity, urban cooling, re-

silience to climate change, urban agriculture, and human well-being (Walsh et al.

2016). In this work, the main focus is on how blue-green infrastructure can contrib-

ute to improve the urban water cycle by detention (REF), infiltration (REF), and

evapotranspiration (REF) of urban stormwater to reduce pluvial flood risk (REF).

These hydrological processes act differently for different time scales, i.e. precipita-

tion durations. As this work concerns urban, pluvial flooding, the most extreme rain-

fall events are considered, typically with short duration.

Incorporation of blue-green infrastructure and resilience into decision-making and

ways to handle integrative and multi-criteria aspects in the legal and organisational

system are still to a great extent undeveloped. The current regime for stormwater

management, through piped drainage, is dominating (Ashley et al. 2011, REF Cett-

ner). An urban planning approach integrating technical, social, environmental, legal,

and institutional aspects of stormwater management is crucial (Zhou 2014). Intro-

ducing such an approach is faced with barriers that are largely socio-institutional

rather than technical (Brown & Farrelly 2009).

Objectives

This study has its base in a hydrological perspective on urban, pluvial flooding.

From this perspective, it reaches further to study socio-technological transition in

the context of changed urban environment and climate change. Strategies for retrofit

of urban areas and urban drainage systems in a decentralised manner with the use

of blue-green infrastructure are examined. The overall purpose is to better under-

stand if urban, pluvial flooding could be mitigated with blue-green infrastructure

and how wide-spread implementation of such measures could be done. While the

studies have been conducted in Sweden, most of the conclusions are applicable in

other regions with similar socio-technical circumstances.

5

Through elaborated analysis of flood insurance data, in combination with study of

drainage system, elevation, and land use, characteristics and mechanisms of pluvial

flooding are examined. While the study is limited to temperate climate, many of the

conclusions are applicable also in other parts of the world. Drivers and barriers of

the socio-technological transition from pipe-bound to blue-green drainage is inves-

tigated through interviews with municipal officials. A similar method is used to pro-

pose a framework for data collection and management for planning of blue-green

infrastructure.

The detailed objectives are:

• to analyse the spatial distribution of flooding and its spatial relation to drain-

age system, flow paths, rainfall patterns, and sea level (Paper2),

• to understand the characteristics and mechanisms that governs the effect of

flooding (Paper2),

• to understand the role of blue-green infrastructure for risk reduction (Pa-

per3),

• to investigate the barriers and drivers for a socio-technological transition

from pipe-bound drainage to blue-green structures for stormwater manage-

ment (Paper4), and

• to develop a framework for data collection and management for planning of

blue-green infrastructure in urban areas (Paper5).

Structure of this thesis

After the introduction in chapter 1, a theoretical background is given in chapter 2

that serves as a foundation for the studies (Paper2, Paper3, Paper4, and Paper5) pre-

sented and discussed in the following chapters, including some of the concepts pre-

sented in Paper1. The work is based on five scientific papers (appended). Paper1

presents a framework for flood risk management. Effects of extreme precipitation

in urban areas are then studied in full city scale (Paper2) and in a neighbourhood

retrofitted with blue-green infrastructure (Paper3). In Paper4, barriers and drivers

for implementation of blue-green infrastructure are assessed and in paper5 a frame-

work for GIS data management in the planning of blue-green infrastructure is pre-

sented. The studies in Paper2, Paper3, Paper4, and Paper5 are described in chapter

3. Methodology, 4. Study area, 5. Data, and 6. Results, while Paper1 serves as a

theoretical framework presented in chapter 2.8. Flood risk management. In chapter

7, the results from the studies are analysed in relation to findings from other studies

and in chapter 8, some implications of the work and my deliberate suggestions for

the future are discussed. In chapter 9 the work is finally concluded.

Theoretical background

Extreme precipitation

An extreme event is only extreme in relation to the normal. Extreme precipitation

cannot be defined by its impact on the society, i.e. how severe hazard it leads to, or

what the systems normally are designed for, i.e. the design standard. Both these

perspectives would lead to a definition where an event would be considered less

extreme as the society gets more well-prepared, or as the standards are changed. It

is more correct to define the extremeness in relation to how common the precipita-

tion in itself is; the lower frequency the more extreme. In urban hydrology, a 100

years event is often considered extreme (Hoang and Fenner 2016, Madsen et al.

2014). However, even this definition is problematic. As the climate changes and

heavy precipitation gets more common, the pillars also of this definition shake. This,

common, definition needs to be re-evaluated as climate changes.

For extreme precipitation to fall, either sufficient moisture must be available for a

convective thunderstorm to be developed by solar heating, or moisture must be ad-

vected into the region and released by an uplift mechanism (Gustafsson et al. 2010).

Extreme daily summer rainfall in Sweden from advection is governed by atmos-

pheric circulation characterised by air-masses that collect moisture over the Euro-

pean continent and the Baltic Sea (Gustafsson et al. 2010). While convectional rain-

falls are typical for the tropics, they are also common at higher latitudes, particularly

in the summer. As the ground gets heated, air bubbles raise 10–12 km and form

cumulonimbus clouds from which intense rainfall is released when it cools to con-

densation temperature (Shaw 1988). The rain cells are often too small to be captured

7

by point measurements (Wern 2012). The most severe downpours in Sweden typi-

cally hits Scania, eastern Götaland, Svealand and the southern coast of Norrland

(Wern 2012). There is no correlation between high annual precipitation and extreme

daily rainfall in Sweden (Wern 2012, Bengtsson & Rana 2014). Most events of ex-

treme precipitation in Sweden are reported in July and August and a weak correla-

tion between high summer temperatures and number of days with extreme daily

rainfall was found in a study by Wern (2012).

In Sweden, about 70% (range 52–81%, but most often within 70–80%) of the ex-

treme events occurred during cyclonic weather type, compared to only 45% for the

non-extreme events (Hellström 2005). However, the southernmost region of Swe-

den, including Scania, differed from the other regions. Here the difference in

weather type is less distinct (ibid.). Extreme events were in the southernmost region

to 52% related to cyclonic weather (non-extreme 27%), to 39% related to anticy-

clonic weather (non-extreme 23%) and to 10% related to directional weather types

(non-extreme 50%) (ibid.).

The Swedish Meteorological and Hydrological Institute (SMHI) mainly measures

daily precipitation. Therefore, most Swedish studies are based on daily precipita-

tion. Only 120 of 750 stations uses automatic registration, where typically registra-

tion is done every 15 minutes since the 1990s. For the rest, manual registration is

done once a day (SMHI 2017). As the measurements are registered as 24 hours totals

recorded at 0700 local time, it is difficult to compare studies done with these data

with statistics from other places (Hernebring & Salomonsson 2009). High resolution

data from e.g. tipping buckets are only registered by municipalities, utility compa-

nies and on private initiative. In Sweden, extreme precipitation is often defined as

40 mm of daily precipitation (Hellström 2005, Gustafsson et al. 2010). The return

period for such an event is between 1 and 5 years (Wern 2012). The 10-years event

is in the range 45–55 mm/day and the 100-years event in the range 60–100 mm/day

(Bengtsson & Rana 2014). SMHI defines downpour (in Swedish: skyfall) as mini-

mum 50 mm in an hour or minimum 1 mm in a minute (Wern 2012), which in

Malmö corresponds to a return period between 50 and 100 years (Hernebring et al.

2015).

Many studies from Northern Europe have found an increased number of extreme

rainfall events (Madsen et al. 2014). However, in Sweden no significant trend has

been found (Bengtsson & Rana 2014). The 1970s were a very dry period in Sweden,

especially in southern Sweden (Lindström and Bergström 2004). During 1807–

2002, the years of 1951 and 1924 stand out as most notable (riverine) flood years,

but also during the 1990s several big floods where noted (Lindström and Bergström

2004). Future precipitation patterns cannot be predicted from analysis of historic

data. Instead, climate models must be used (Wern 2012), projecting an increase in

short-duration extreme intensities of 5-10% in Stockholm, Sweden by 2011-2040

8

and 10-20% in 2071-2100 (REF Olsson in press, from Madsen) and 20-60% in-

crease in 30-min intensities in Kalmar, Sweden (REF Olsson 2009, from Madsen).

In Denmark extreme precipitation for durations between 1 and 24 hours are pro-

jected to increase by 10-50% within the next 100 years (REF Arnbjerg-Nielsen

2012, from Madsen).

Pluvial flooding

To write: Vad är pluvial översvämning, vad orsakar den, hur kan den motverkas

Floods are usually categorised by the governing mechanism. The most common

flood types are coastal (from storm surge, sometimes in combination with high tide),

fluvial (or riverine) and pluvial (rain induced) flooding. Flooding can also appear as

groundwater flooding or flooding caused by dam breaks, damage to water supply or

drainage system. Riverine floods in mountainous landscape is often called flash

floods, because of their sudden appearance. (REF) In this work, pluvial flooding

includes surface water flooding and flooding by exceedance water from the drainage

system.

To write: Pluvial flooding is a hydrological mechanism and is defined as unintended

inundation of land that causes damage. When rainfall volumes exceed drainage ca-

pacity of natural and constructed systems, low-lying areas are inundated with water.

To write: Causes for pluvial flooding. Rainfall main driver. The mechanisms and

characteristics of pluvial flooding is further investigated in this work. Extreme pre-

cipitation and its predicted increase with a changing climate has already been dis-

cussed above. The main drivers for increased flood hazard are presented below.

Runoff from both pervious and impervious surfaces (Boyd et al. 1993) is the gov-

erning hydrological process for urban flooding (Berggren et al. 2013). The effect of

impervious surfaces is constant for all storms, while runoff from pervious surfaces

are related to the depth of rain. For storms larger than 50 mm, soil saturation before

the storm was also important (Boyd et al. 1993). During extreme rainfall, the

green/pervious areas contribute to runoff (Berggren et al. 2013). Changes in the in-

filtration capacity impact the urban runoff (ibid.).

To write: Design standards for sewers

In urban areas, flooding may be associated with inadequate sewer systems. With

high property values of buildings and other structures, potential damage from flood-

ing can easily extend into large amounts. However, drainage systems designed to

cope with the most extreme storms would be too expensive to build and operate

(REF 3PA). In establishing tolerable flood frequencies, the safety of the residents

9

and the protection of their valuables must be in balance with the technical and eco-

nomic restrictions. The response of the drainage system to rain events in the urban

environment is characterized by two main components (Bengtsson et al. 1993). The

first is the surface runoff on natural slopes, i.e. the major system. The second com-

ponent consists of the artificial drainage system, i.e. the minor system. In most of

the cities, the artificial drainage system is controlled by a combined sewer network,

which collects both stormwater and wastewater and leads it to the treatment plant.

While the major system often is neglected in the urban planning, leading to con-

struction in low-laying, high risk areas, the minor system is typically designed ac-

cording to a certain, locally or nationally set design standard (REF butler). However,

the way to set design levels has been criticised (Knighton & Walter 2016).

To write: Precipitation events prior to extreme rainfall events have a significant im-

pact on insurance claims (Torgersen 2015).

Historical development of urban drainage systems

Figure XXhist. Stages of development of urban drainage systems in Sweden. A)

sewerage deposited to street, B) drainage of streets through piped sewers, C) pipe

system connected with interceptor sewers, D) wastewater treatment plants to treat

sewerage, E) separate foul and surface water sewers, F) detention and treatment of

stormwater in vegetated swales, canals and ponds.

Urban drainage in Sweden has gone through a long development, from the first

piped solutions, to introduction of wastewater treatment and later separation of san-

itary sewers and stormwater pipes. Similar development is seen worldwide (Brown

10

et al. 2009, Cettner et al. 2012, de Feo et al. 2014). Before pipes were constructed

for drainage in urban areas, sewerage was deposited through a hole in the wall, the

door, or a window and flushed away with rain. Faeces were stored in the backyard,

collected, and used as fertiliser on agricultural land (Figure XXhist, A). The cities

were smelly and hygiene was bad. After several outbreaks of diseases like cholera,

action was called for and pipes were constructed to drain the streets. During 1930s

outhouses were replaced by water closets in many homes (REF SKBL Agnes

Hedvig Charlotta Lagerstedt [2018-03-10]). At this time, sewers were also seen as

a way to promote civic pride (Cettner et al. 2012). Through construction of pipelines

for drainage, cities could be promoted as well developed and with future perspec-

tives (REF Cettner). In the beginning, the outlets were typically located in a nearby

canal, stream or lake (Figure XXhist, B). Later, about 19XX, outlets close to the city

were shut down and interceptor sewers were constructed, often incorporating

stretches of polluted, urban watercourses, which lead the sewage further away to

bigger rivers, lakes, or to the coast (Figure XXhist, C). Still, sewage was discharged

without treatment. In 1950s and 60s, pollution of streams and waterborne diseases

lead to public health interventions for treatment of sewage in waste water treatment

plants (Figure XXhist, D). As sewerage and … started to construct separated sewers,

with a foul sewer to carry contaminated wastewater and a surface water sewer to

carry rainwater to receiving waters (Figure XXhist, E). In the 1970s, ideas of envi-

ronmentally friendly drainage of stormwater started to develop (Cettner et al. 2012,

Fletcher et al. 2014). The main idea was to retain and treat stormwater in retention

and detention basins, urban wetlands, etc. (Stahre 2006) (Figure XXhist, F). The

piped drainage has a central role for clean and modern cities (Cettner et al. 2012),

but the fast and efficient conduit of stormwater from urban spaces also leads to

problematic pollution of downstream recipients like rivers, lakes and coastal water

(Weibel et al. 1964), increased risk of flooding in urban areas (REF, not mine) and

erosion and other disturbances to aquatic wildlife (Walsh et al. 2016).

To write: Piped network works fine under most circumstances.

At present about 54% of the global population live in cities and by 2050, two thirds

of the world’s population will live in urban environments (UN 2015). In Europe, the

current, urban population is 73% (ibid.). The hydrology in urban areas differs from

the surrounding landscape. While infiltration and evapotranspiration are signifi-

cantly reduced with urbanisation, both overall discharge and peak flow increase.

The rapid flow, caused by shorter lag time to peak flow, leads to increased flood

risk. For receiving waters, erosive flow frequencies increase, at the same time as

base flow in urban streams are reduced because of the low infiltration rates. Urban-

isation leads to changes in the aquatic ecosystems by i.e. reduction of fish popula-

tion, and decreased diversity of algae and macrophytes. Pollution from heavy met-

als, PAHs, PCBs, pesticides, and pharmaceuticals as well as macronutrients (N/P/K)

11

increases. (McGrane 2016) Urban areas pollute with more heavy metals, but with

less nutrients, compared to farmland (Berndtsson & Bengtsson 2006).

Problems related to current stormwater management

To write: Pollution from CSO and stormwater

To write: Floods – little capacity and centralised sewers

To write: Runoff from impervious land

To write: Loss of ecosystem functions and services

To write: Degraded pipes. Aging system. Need for new investments.

To write: Need to develop urban green

Misconnections, where wastewater is discharged into surface water sewers, are

common (Ellis & Butler 2015). In some parts of London, as many as one in three

households lead their wastewater untreated to rivers and streams because of such

domestic misconnections (REF Thames Water) The misconnections are difficult to

find and correct, as the sewers are hidden underground. The opposite kind of mis-

connections, where stormwater runoff is discharged into foul sewers, leads to unin-

tentionally high pressure on wastewater treatment plants (REF Ashley, NORIS pro-

ject) and increased risk of flooding.

The development of urban drainage over time has led to inclusion of more and more

different perspectives (Brown et al. 2009). Blue-green infrastructure is highly com-

plex (Hoang & Fenner 2016) and it is a challenge how we best organise the man-

agement among a diverse group of professionals and stakeholders. There is today a

need for stakeholders to understand more than one discipline or perspective, such

as hydrology, ecology, landscape architecture, urban planning, etc. (Brown et al.

2009).

Blue-green infrastructure

Blue-green infrastructure is a concept for landscape planning where urban storm-

water is controlled in a decentralised manner with vegetated structures (Liao et al.

2017, O’Donnell et al. 2017). There are a vast number of concepts to describe de-

centralised and sustainable drainage of stormwater through vegetated measures, e.g.

Low Impact Development (LID), Sustainable (Urban) Drainage Systems

(SuDS/SUDS), Green Infrastructure (GI), and Water Sensitive Urban Design

(WSUD) (Fletcher et al. 2014). The concept blue-green infrastructure is chosen in

12

this work, as it emphasises the importance of both blue (water) and green (vegeta-

tion) and the interaction between them. The word infrastructure underline that dif-

ferent elements need to be interlinked to work as a connected web of measures (Len-

non 2015). Water obviously follows flow paths, natural or constructed, but urban

ecological networks and connectivity is also important in perspective of ecology

(Ahern 2013), where it benefits spread of flora and fauna between the different green

elements (REF se referenser från Anna). The connected parts are more than the sum

of the single elements. Sandström (2002) similarly choose the concept green infra-

structure, instead of the traditionally more commonly used green space, to signal

the multiple purpose of green space. The word infrastructure gives the concept the

same dignity as other kinds of (technological) infrastructure (Sandström 2002, Len-

non 2015). In countries with a neoliberal agenda, the concept shows that such

measures complement rather than prevent economic development (Matthews et al.

2015). Every element of blue-green infrastructure is in itself a nature-based solution

and mimics natural ways to handle water (European Commission 2015). The eco-

nomic benefits of such solutions have been put forward by the European Commis-

sion as well as through research (ibid., Ossa-Moreno et al. 2017).

Stahre (2006) categorises the different elements of blue-green infrastructure de-

pending on their role in the stormwater control system and on the stakeholder that

can allocate space for them. Most upstream, source control is used to describe small

scale facilities, like green roofs, permeable paving and local ponds, typically on pri-

vate land. On public land, onsite control measures for detention, infiltration and

evaporation, such as soakaways, special surfaces for temporary flooding, and

smaller ponds, are found. Water from these structures are retained in canals, swales,

and other structures for slow transport. Larger retention ponds, lakes and wetlands

serve as downstream control.

Green spaces are important in urban areas for six reasons, according to Boverket

(1992) (REF From Sandström 2002): recreation, maintenance of biodiversity, city

structure, cultural identity, environmental quality of the urban area, and as biologi-

cal solutions to technical problems in urban areas. Swedish cities are recommended

to develop a green plan where urban green spaces are identified together with their

value to the public (Sandström 2002). This plan should be a part of the mandatory

structure plan. However, Sandström (2002) showed that these plans do not take into

consideration the multiple purposes of green spaces in the urban environment. The

relation between urban green and water is only to a limited degree mentioned and

discussed in the evaluated plans. The importance to work integrated with water and

green spaces have been mentioned also by others (REF, REF, REF). The European

Commission defines Green Infrastructure as “a strategically planned network of

high quality natural and semi-natural areas with other environmental features, which

is designed and managed to deliver a wide range of ecosystem services and protect

biodiversity in both rural and urban settings.” (European Commission 2013). This

13

definition does not explicitly include a focus on water. On the other hand, the US

Environmental Protection Agency use Green Infrastructure as the narrow, water fo-

cused concept to describe vegetated, stormwater technologies. They claim that

“green infrastructure is a costeffective, resilient approach to managing wet weather

impacts that provides many community benefits.” (US EPA, webpage) and exem-

plify with technologies like rain gardens, bioswales, downspout disconnection, ur-

ban tree canopy, green streets, and rainwater harvesting.

Stormwater control measures are known for their ability to treat stormwater from

polluting particles and solved compounds. Nutrition uptake in urban wetlands re-

duces pollution of downstream recipients (REF) and XXXX are degraded by XXXX

(REF). However, caution must be taken as stormwater carriers heavy metals from

the urban fabric (REF Lars). These metals are not treated or degraded but detained

by the stormwater control measures. They will therefore stay in the water or the

vegetation of the control measure, or infiltrate and potentially pollute the ground-

water.

To write: Challenges with the multi-disciplinary basis of the concept.

To write: Hur hydrologin påverkas generellt

To write: Vad sägs om översvämning och blå-grönt?

Stormwater control measures of different kinds have been shown to reduce flood

risk, like swales (Qin et al. 2013), green roofs (Qin et al. 2013), permeable pavement

(Qin et al. 2013, Zahmatkesh et al. 2014), ponds (REF), urban wetlands (REF Mi-

klas?). For instance, swales can lower the flood risk as they can store large volumes

of water (Qin et al. 2013). Initial wet conditions reduce the ability of blue-green

infrastructure to handle a large storm (Villarreal et al. 2004). A combination of

stormwater control measures are often recommended in the literature (Qin et al.

2013, Liu et al. 2014).

Transition theory

Blue-green infrastructure is a fairly new technology that goes outside the currently

dominating socio-technological structure for stormwater management. Research has

shown that barriers for transition is institutional rather than technical (Brown & Far-

relly 2009). In this work, transition theory with a multi-level perspective is used as

an analytical tool to explore barriers and drivers that may exist in relation to an

altered management of stormwater in Sweden. Transition theory sees socio-techno-

logical systems, in this case the stormwater management system, as composed of

three constellations: regime, niche and niche-regime. Surrounding these is the land-

scape that forms the prerequisites for the system (de Haan & Rotmans 2011). The

14

three constellations and their surrounding landscape is shown in Figure XXtrans.

While Rotmans et al. (2001) relate the levels to bureaucratic levels (macro, meso,

and micro level) or nested hierarchies, we perceive the levels as including organisa-

tional, technical as well as social systems as suggested by Geels (2011). In the fol-

lowing section the features of the multi-level perspective are briefly explained and

thereafter the process of change according to transition theory is described.

Figure XXtrans. Transition theory according to Ashley et al (2011) and de Haan &

Rotmans (2011).

The regime dominates the system and is the way that societal needs are met (de

Haan & Rotmans 2011). A regime includes institutions, technologies, practical ap-

plications and social relationships (Geels 2002, Smith et al. 2005). The regime is

the most powerful constellation of the system (de Haan & Rotmans 2011). In Swe-

den, the current regime of urban drainage management consists of a pipe-bound

infrastructure with a centralised management. Several actors are involved in devel-

oping and maintaining it (Cettner et al. 2012). A shift to a new regime is more than

just an evolutionary transformation of a previous regime. There is a significant dif-

ference before and after a regime shift. For instance, to change from combined to

separate sewers is an evolutionary transformation, while the change from pre-sew-

ered cities to sewered cities can be considered as a transition to a new regime (Ash-

ley et al. 2011).

A niche is a system that only meets certain, specific societal needs (de Haan & Rot-

mans 2011). The niche level is mainly referred to where emerging innovations are

developed that are fundamentally different from solutions at the regime level (Geels

2011).

A niche-regimes is a regime that is not currently dominating, while it has significant

power to compete with the regime for the functioning of the system (de Haan &

15

Rotmans 2011). Thus, a niche-regime is somewhere between niches and the regime

in strength.

The landscape represents the wider context (legal systems, demography, economy

and the natural environment) that in a long term perspective is able to influence and

affect the development of the practices at regime, niche-regime and niche levels

(Geels 2002; Geels 2011; Koppenjan et al. 2012).

A central idea of the transition theory is that change emerges from niche through

alternative solutions to niche-regime to displaced or replaced regime, but a regime

can also be influenced by actions and ideas emerging from landscape level (Kop-

penjan et al. 2012, Geels 2011, Smith et al. 2005). The challenges of the regime

emerge from the dispersal of new social norms concerning how to solve a problem,

know-how and motivation that alternative solutions bring with them when devel-

oped and implemented at the niche level (Koppenjan et al. 2004; Van de Bruges et

al. 2005; de Haan & Rotmans 2011; Ashley et al. 2011 Farrelly & Brown 2011).

Drivers for this type of socio-technical transition has been divided into three cate-

gories: pressure, stress, and tensions (de Haan & Rotmans 2011). Pressure comes

from alternative technologies that become viable competitors to the regime. Stress

emerges when the regime is inadequate or internally inconsistent in meeting the

needs of the system. Tension is when the system compromises in its relation to its

natural or social environment. The tensions can either be structural, related to phys-

ical aspects of the regime, legal or cultural, related to cognitive or discursive aspects

of the regime (ibid.). Structural tensions are more frequently related to pressures

from the landscape level where as the cultural tensions can emerge from both niche

and landscape level (Ashley et al. 2011).

Due to entrenched technological path dependency and cognitive lock-in, actors may

be unable or hindered to implement new solutions (Brown & Farrelly 2009a). Such

implementation barriers, have different origins and can be technological, legal, or-

ganisational, financial, social, educational or related to political will (Holtz et al.

2008; Brown & Farrelly 2009a; Brown & Farrelly 2009b; van de Meene et al. 2011;

Cettner et al. 2013; Cettner et al. 2014; Mguni et al. 2015). As a consequence, a

transformation of a regime must consist of both physical, administrative and struc-

tural changes in the regime (Bettini et al. 2015) and change usually has to occur at

multiple levels for a regime to alter (Geels 2002).

16

Flood risk and resilience

To write: In this work, risk is seen as a potentioally negative deviation from a pre-

ferred, expected development over time (Paper1). Increased resilience through con-

struction of measures, increased awareness, ...

Since planners must cope with uncertainty (Godschalk 2003) and a floodevent al-

ways can be bigger than what the system is designed for (Liao 2012), cities must be

built resilient. An urban resilience to floods could be conceptualized as the capacity

to remain in a desirable regime while experiencing a flood (Liao 2012). While a

flood resistant city can resist floods to a certain degree, but not bigger floods, a flood

resilient city is flexible and adaptive, and learning from historic events. It is in many

cases not possible for a city to be both resistant and resilient. A city that accepts

smaller floods, will be better prepared when a bigger flood occurs. To be able to

handle floods, floodable areas are needed in the city (Liao 2012), i.e. areas that can

store or convey water without incurring damage.

To write: Hur risk kan beskrivas genom hazard, exposure, vulnerability

To write: Risk som en kombination av sannolikhet och konsekvens

To write: Filosofin om risk – Johannes Perssons bok

To write: Risk som en avvikelse från den önskade utvecklingen

To write: Koppling till hållbarhet

Future uncertainties and strategies for adaptation

Nothing written yet, only a list with references to use:

Refsgaard et al. (2013) reason that there are three kinds of uncertainty: epistemic

uncertainty, uncertainties related to ambiguity and aleatory uncertainty.

Hallegatte (2009) proposes different strategies to handle large uncertainties:

• No-regret: The solution is beneficial even in absence of climate change.

Multiple problems are solved with such a solution, like energy saving or

urban green.

• Reversible strategies: Flexible solutions that can be adjusted depending on

the outcome. Waiting to urbanise is an example, which is much easier than

first constructing and then retreat.

• Safety margin: Make a little bigger while designing new infrastructure. Be

overpessimistic.

• Build resilience (soft strategies): Creation of institutes that can manage

risks, insurance schemes to cover losses, early warning systems, etc.

17

• Reduce decision-making time horizons: Construct cheap houses that is

made to be rebuilt after some time.

From fail-safe to safe-to-fail strategy (Ahern 2011)

Payo et al. (2015) about lock-in effects

Robust decision making

Flood risk management

To write: Paper1 serves as a background for the thesis. Describe how the other pa-

pers relates to flood risk management by presenting them on Figure XXmanage.

To write: Different aspects of flood risk management. Reduce hazard, exposure and

vulnerability.

Figure XXmanage. The three systems involved in urban flooding. (A) The hydro-

logical system; (B) The impact system; (C) The management system.

Urban flood management can be described with a system-analytical approach (Fig-

ure XXmanage) where three major systems are included: 1) the hydrological system

(where the water physically flows through the urban space, intendedly or not), 2)

the impact system (all parts of the society that are affected detrimentally during a

flood event), and 3) the management system (the features of the society that deals

with flood management to decrease the effect of a flood event). (Paper1) During an

event the impact system is affected by processes governed by the hydrological sys-

tem. These negative effects are to be reduced by the management system, both on

the long term and on the short term, by reducing the societies vulnerability to flood-

ing. Processes in the management system can also impact the hydrological system,

18

especially on the long term, by construction, control and improvement of the urban

drainage system and watercourses, and changes in elevation in the urban landscape.

Such changes will have an impact on the severity of coming flood events.

To write: Knowledge of the social system and its vulnerabilities is still weakly de-

veloped, even though it is a key element of the social response to a flood and of the

urban dynamics (Hall et al. 2003).

In Europe, EU Water Framework Directive (WFD) (Directive 2000/60/EC) and the

Floods Directive (FD) (Directive 2007/60/EC) have forced the countries to work

more extensively with the urban waters. The WFD commits all member states to

achieve good qualitative and quantitative status of all water bodies, such as water-

courses, lakes and marine environment as well as ground water. The FD calls for

actions to mitigate flood risk by first making risk assessments in two steps and then

develop flood risk management plans. Implicitly the FD focuses on riverine floods.

However, pluvial floods, also constitute a formidable threat to cities around the

world. In Sweden, one of the first outcomes of the FD implementation was flood

risk mapping of all major rivers (REF), but also pluvial flood risk gains increasing

attention (REF). Together, these directives call for new ways to manage urban

stormwater, preferably with a holistic view including both water pollution and flood

risk mitigation, however this is not explicitly spelled out in the directives (REF).

To write: Implementation of the Floods Directive in Sweden (Thorsteinsson & Lars-

son 2012).

In Paper1 five challenges related to flood risk management from a transdisciplinary

view were identified: 1) To build flood resilience cities, while not forgetting to meet

other climate change related impacts such as water scarcity, drought, and heat waves

in the municipal planning. 2) To jointly consider the water, energy, land use, trans-

portation, and socioeconomic nexus from a multi-stakeholder perspective. 3) To use

flexible flood protective measures in the view of uncertainty regarding future cli-

mate. 4) To solve questions regarding responsibilities and improved communication

between stakeholders and authorities. 5) To secure critical infrastructures. Blue-

green infrastructure was identified as a key concept to meet challenge number 1 and

3 above.

Study area

Malmö in southern Sweden is at the centre of this work. For the analysis of flood

mechanisms and characteristics on city scale (Paper2), Malmö inside the Outer Ring

is used as study area. To study flooding and blue-green infrastructure (Paper3), the

study area was focused on one district: Augustenborg Eco-City, an area that was

retrofitted with stormwater control measures in early 00s. For the interview studies

of transition to blue-green infrastructure regime (Paper4) and for the framework de-

velopment (Paper5), the study area was zoomed out to include municipalities and

water utilities in the region of Scania, including Helsingborg, Lund, Tomelilla, and

Simrishamn, besides Malmö.

20

City of Malmö

The city of Malmö, Sweden, was selected as study site since, in the Scandinavian

context, Malmö is a large city, where there have been several flood events in recent

years. Like in many other cities, the more densely built areas have sewer systems

where stormwater and sewerage are drained with one single pipe (combined sys-

tem), leading to high risk of basement flooding. The sewer system in Malmö is rep-

resentative for Scandinavian cities, with a mix of combined and separate sewers.

Good quality data on flood extent, precipitation, topography, sewerage system, etc.

are available. Malmö is also well known within the field of urban hydrology, as the

city was an early starter in the work with integrated water management

(Niemczynowicz 1999, Stahre 2008).

Augustenborg Eco-City

In Augustenborg, a 31 hectares residential area in Malmö, stormwater is controlled

by blue-green infrastructure consisting of detention in ponds and areas for tempo-

rarily flooding, infiltration on green roofs, lawns and parking, as well as slow

transport in swales, ditches and canals. The stormwater drainage was disconnected

from the old combined sewers that are only used for wastewater now. The retrofitted

blue-green infrastructure was constructed by the end of the 1990s by VA Syd (utility

company) and MKB (housing company) as a part of the project called Eco-City

Augustenborg. For a detailed description of the blue-green infrastructure in Au-

gustenborg, see Villarreal et al. (2004). About 3,000 people live in the area in 3–6

storeys apartment blocks built in 1948–1952. The area continues to be developed

and in 2016 a 14-storeys building with eco-friendly profile named Greenhouse was

finished.

Municipalities and water utilities in Scania

For the two interview studies, the study area was zoomed out to include municipal-

ities and water utilities in the region of Scania. Swedish municipalities are legally

responsible for stormwater management on public land. The practical responsibility

is frequently delegated to public water utilities which often are co-owned by several

municipalities. In the first interview study (Paper4), on transition to blue-green in-

frastructure regime, municipal and water utility company bureaucrats in Malmö and

Helsingborg was interviewed. The two municipalities use two different water utility

companies, VA-syd (Malmö) and NSVA (Helsingborg), to provide stormwater

21

management and water treatment services to a total of 730,000 persons (Figure

XXskane). Malmö (330,000 inhabitants) and Helsingborg (140,000 inhabitants), are

the two largest cities in the region and have a long experience of working with blue-

green solutions.

Figure XXskane. Municipalities covered by the water utility companies in the study,

NSVA (dark grey) and VA Syd (light grey). Göra figur som visar alla study areas

för samtliga artiklar?

Interviews for the framework development (Paper5) was done with five municipal

and water utility company bureaucrats in Malmö, Lund, Tomelilla and Simrishamn,

and one GIS researcher. In this study, two of the interviewees worked with GIS data

collection, management and support (the researcher and one of the bureaucrats).

Methodology

One of the main purposes of this work is to analyse the physical mechanisms and

characteristics of urban, pluvial flooding (Paper2). Through this analysis, the spatial

distribution of flood risk related to these mechanisms and characteristics are evalu-

ated. According to Crichton’s Risk Triangle (e.g. Crichton 2008), risk comprises

hazard, exposure, and vulnerability. The flood claims are used as a measure of spa-

tial extent and severity of different flood events, which can be related to all three

sides of the Risk Triangle. The vulnerability and exposure of individual households

are not assessed as the study focuses on city-scale patterns.

Insurance claims as a measure of flooding

How insurance claims are used to analyse flood damage during historical flood

events. Risk is a perceived measure (REF Becker book), and in this work flood

damage through registered insurance claims is used as a representation for flood risk

in different areas.

Insurance claim data have been used for various other studies to analyse the relation

between rainfall and pluvial flooding in Denmark (Zhou et al. 2013; Spekkers et al.

2013b), and in the Netherlands (Spekkers et al. 2013a, Bouwens et al. 2018).

One catastrophic event might lead to many claims. For statistical analyses of insur-

ance data, where independent random variables are needed, single claims must be

aggregated into common losses for each event (Smith and Goodman 2000). How-

ever, in this study, where the hydrological process behind the losses is investigated,

23

the induvidual claims provides essential information about the spatial extent of the

damage.

Flow path analysis

In Paper2, the minor and the major drainage systems of Malmö were investigated.

The minor system was based on a sketch of the main sewers by VA Syd (water

utility company), while the major system was derived from a simple analysis of the

topography where the flow from an eight-direction flow model (Jenson and

Domingue 1988) is accumulated into each cell. No validation of the analysis was

done. The main sewer system follows the main flow paths in the major system to a

high degree. Old maps from Malmö was studied in relation to the derived minor and

major system. The main pipelines are found along the stretch of Hylliediket in west-

ern Malmö, Södervärnsstråket in southern Malmö, and the Riseberga Brook in east-

ern Malmö.

Selection of nearby areas for comparison

In Paper3, flood risk in Augustenborg was compared with flood risk in five nearby

areas, Lindgatan, Lönngården, Norra Sofienlund, Södra Sofienlund, and Persbort.

The areas were selected as they are similar to Augustenborg in several ways, like

land use, building coverage, time of urbanisation, and original sewer system (com-

bined). Stormwater control measures have only been implemented in Augustenborg,

while the other areas still mainly have combined system, which make them suitable

for comparison with Augustenborg in this study. Lindgatan is an exception, as the

combined system has been reconstructed with a separate system here.

Interviews

Semi-structured interviews with staff in water utility companies and municipal wa-

ter offices. Interviews used both to better understand the barriers and drivers for

implementation of blue-green infrastructure and to develop a framework for data

use in city planning with blue-green infrastructure. For the first purpose, water en-

gineers and planners where interviewed, and for the latter purpose, water engineers,

ecologists, planners, and GIS experts where interviewed.

24

The interviewed persons work at different departments, as environmental admin-

istration and planning office, and have a varied academic background. For both

studies, a majority though have a background in natural and technical science. In

total, 20 persons were interviewed in the first study and 6 persons in the second.

Data

Data and material used in the study are presented shortly here. Further details are

found in the publications.

To write: Paper1, paper2, paper3…

To write: For Paper4 and Paper5, empirical material was collected through inter-

views, as described in section 3.3. Interviews.

Results

The main findings are here presented shortly. Further details are found in the publi-

cations. As Paper1 serves as a background for the thesis, no results from this article

is presented here.

Mechanisms and characteristics of urban, pluvial

flooding (Paper2)

Pluvial flooding is the most common kind of flooding in Malmö. Only a very few

flood claims have been registered during high sea level caused by storms and there

is only one watercourse in Malmö (Risebergabäcken/Sege å). Three severe, pluvial

flood events are presented in the study: 5 July 2007 with 150 and 169 flood claims

to VA Syd and LF Skåne respectively, 14 August 2010 with 210 and 148 flood

claims, and 31 August 2014 with 2,109 and 2,649 flood claims within the study area.

These flood events were all caused by heavy rainfall distributed over the entire city.

The 2010 and 2014 events were both intense and with a quick development, while

the 2007 event were less intense, but with a long period of rainfall before (Figure

XXrain). The 2014 event was heavier than a 100-year event for durations between

3 and 16 h (average for all stations in Malmö).

26

Figure XXrain. Accumulated rainfall (SMHI station A) for the three flood events in

Malmö on 5 July 2007, 14 August 2010, and 31 August 2014. The accumulation

curve is centred on the peak rainfall (maximum 15 min intensity) and the volume is

accumulated for 20 days before the peak and 1 day after.

There is a relation between large-scale topography and flooding in Malmö. Areas

within 100 m from the major system are more than twice as affected by flooding,

compared to areas further away. During the severe flood events in 2010 and 2014,

areas close to the major system was even more effected by flooding (3.0–4.2 times),

compared to areas further away. During such downpours, runoff is quickly directed

towards low-lying areas, both through the pipe system and by overland flow. In

Malmö, like probably in most other places, the trunk sewers (minor system) are

located under the main overland flow paths (major system), as they follow the to-

pography. The spatial distribution during these two, highly intensive rainfall events

(2010 and 2014) were different than during other events, including the 2007 event,

with more flood claims clustered around the trunk sewers.

The combined system is more exposed to flooding than the separate system. Even

if only 31% of the urban land in Malmö is connected to the combined system, 70%

of the flood claims are reported from these areas. During the 2010 and 2014 events,

the combined system was 3.8–4.2 times more affected by flooding compare to the

separate system. Similar figures are found if all flood events are included. The 2007

event shows a different pattern: the combined system areas were only slightly more

than twice (2.3 times) as severely affected by flooding during this event, compared

to the areas with separate system. One reason why the 2007 event differs from the

other events might be the difference in flood causality, where continuous rainfall

during the preceding weeks saturated the ground with water. Flooding during this

event was therefore less related to type of drainage system. The dataset is biased as

27

more people live in areas with combined system. However, the difference in re-

ported flood claims still exists when adjusted for bias.

Locally, some flooding is caused by breakdown of the system, e.g. when a sewer

pump stops pumping due to system error. On the one hand, the phenomenon with

local breakdowns could be seen as unique incidents that are not likely to happen

during future flood events. On the other hand, and in reality, it seems inevitable that

a few of these unique incidents happen during every flood event.

Flood risk reduction with blue-green infrastructure

(Paper3)

Little data is available from before implementation of the blue-green infrastructure

in Augustenborg. However, the data available indicate decreased flood risk after

implementation.

Flood reduction after stormwater retrofit in Augustenborg was evaluated by com-

parison with the five nearby areas (Lindgatan, Lönngården, Norra and Södra So-

fielund, and Persborg) with similar age, land use, and imperviousness as Augusten-

borg. All five areas have combined sewer systems, corresponding to what Augusten-

borg had before the blue green stormwater retrofit. The flood magnitude (number of

flooded properties per hectare) was more than 10 times smaller in Augustenborg

compared to the other areas both during the extreme 2014 event and during the other

events in 2007–2015. The difference was confirmed with a bootstrap analysis and

found significant (99% bootstrap confidence interval).

The flood event on 31 August 2014 is considered as extreme, as more than 80% of

the flood claims (2007–2015) were reported this day. The event makes it possible

to evaluate the blue-green infrastructure under extreme conditions. In Augusten-

borg, 116 mm was measured, and most of the rainfall (100 mm) fell within 3.5

hours. This was the biggest rainfall event since measurements started in Malmö in

the late 1800s and led to severe flooding in most of the city as well as in neighboring

villages and in some parts of Copenhagen, Denmark. For durations longer than two

hours the rain event exceed the 100 year return period. Compared to the five nearby

areas without blue green infrastructure, approximately 10 times less properties were

flooded in Augustenborg during the event. Lindgatan, which was the least flooded

area, apart from Augustenborg, was 6.4 times more flooded than Augustenborg,

while the most heavily flooded area, Södra Sofielund, was 18.4 times more flooded

than Augustenborg.

It should be noted that the number of reported flood claims was also low in Au-

gustenborg before the blue green infrastructure was implemented. However, it

28

seems that the number of reported flood claims has increased in general, while this

is not the case for Augustenborg.

Barriers and drivers for implementation of blue-green

infrastructure in Swedish municipalities (Paper4)

From the interviews with 20 practitioners, four types of drivers for implementation

of blue-green infrastructure were identified, where the focus on ecosystem services

and climate change were mentioned by all or almost all of the respondents respec-

tively. Eight types of barriers were identified, where economy, roles and responsi-

bilities, and lack of knowledge also were mentioned by almost all of the respond-

ents. In addition, legislation and municipal organisation were mentioned as barriers

by ¾ of the respondents. The barriers are closely related to the current stormwater

management regime as understood from the theory, where the whole system, in-

cluding technical, legal, organisational and other aspects, target the needs in the

current regime. It was found that the economy of, the responsibilities within and the

organisation of Swedish municipalities and their water utility companies are di-

rected to fulfil the goals of the current, pipe bound stormwater management, rather

than widespread implementation of blue-green infrastructure.

All of the interviewees mentioned ecosystem services as a driver. Their suggestion

for the most important service is however diverse and includes services as recrea-

tional value, delay and treatment of stormwater, biodiversity, and cultural services

like aesthetics. The second most mentioned driver was climate change, which was

mentioned by all but one interviewee. The interviews show that there are awareness

and knowledge about that the climate is changing and that the actors need to adapt

urban areas to be able to handle the increased precipitation.

The most mentioned barrier is economy, as maintenance costs are uncertain, and the

financial structure of Swedish municipalities does not support blue-green infrastruc-

ture. It is however interesting that economy also was mentioned as a driver by a

third of the interviewees. They argue for instance that replacement of old pipes will

be expensive, and that blue-green infrastructure can be a less costly alternative in

many cases. Some also mention that it will be costly if flood mitigation related to

climate change are done with conventional methods.

In the studied municipalities, there are several players with different skills,

knowledge and training involved in the stormwater management chain, from the

strategic and overall planning to detail planning for building permits and to private

individuals. In the interviews the lack of clarity of roles and responsibilities emerged

as an issue. Almost all interviewees also mentioned lack of knowledge as a barrier.

29

There is widespread awareness of the idea behind blue-green infrastructure in Swe-

den, but still knowledge about questions regarding for instance design, inclusion of

ecological expertise, and maintenance are lacking.

Legislation and municipal organisation were also mentioned as barriers by many

practitioners. They claim that the current legislation in Sweden and the municipal

organisation do not support blue-green infrastructure in the planning process. The

lack of legal support increases the uncertainty for involved stakeholders and as each

municipal department has their own budget, interest and responsibilities, coopera-

tion between them are difficult. Without such cooperation, widespread implemen-

tation of blue-green infrastructure is not possible.

A framework for city planning with blue-green

infrastructure and nature-based solutions (Paper5)

From the brainstorming sessions, several perspectives on data availability and data

management in the urban spatial planning of blue-green infrastructure and nature-

based solutions were discussed. Three main areas of problems were identified: the

multiple need of data, lack of GIS data, and problems related to data management.

The latter includes technical issues, like incompatibility between database struc-

tures, and human resources issues, like low priority in the municipal organisation,

lack of knowledge among practitioners and competition, prestige and difficulties to

cooperate between departments. Data is collected and processed on both national,

regional, and local level and in relation to the responsibility of the authority. There-

fore, the collected data seldom comprise a comprehensive ground for planning blue-

green infrastructure and its elements.

In the study, nine categories of data needed in the planning process of blue-green

infrastructure were identified: cadastral, technical, geological, biological, environ-

mental, risks, social, administrative, and meteorological.

From the interviews, six themes of problems related to GIS data in the planning

process were found: 1) lack of data, 2) data access and rights, 3) lack of knowledge,

4) data storage tools not adapted to users’ needs, 5) organisation of data storage, and

6) strategic data use.

A framework for strategic use of data to aid the development of blue-green infra-

structure was proposed (Figure XXframe). Three boxes lie outside the main flow in

the figure: citizen’s needs, the fact that values change over time and scientific eval-

uations and support. Appropriate data to develop new NBS, or maintain current

ones, is essential to ensure a strategic development to benefit both people and nature

(biodiversity) itself. Hence the step evaluation and data collection, is essential and

30

can be seen as both the starting and end point of the framework. It is here essential

that the evaluation and data collection step includes links to broader municipal goals

to ensure a relevant collection of data. It is also essential to keep track of time related

changes in data needs. A structured and continued evaluation of implemented struc-

tures is essential to assess their success. However, if the collected data is not stored

in a proper way, access to it may in reality be limited. Moreover, as urban areas

change quickly, data maintenance is essential to ensure its relevance and quality.

This step has to include both data maintenance in a more technical meaning as well

as information on how collected data has been collected.

As the main purpose of the data discussed here is to be used in further analysis, there

is a need for both tools and structures for proper use once data is collected. If dif-

ferent types of models are used it may be important to save and store analyses of

different scenarios in a structured way. The collected data and performed analysis

thereafter has to serve as a basis for planning which need some type of visualisation

of the data at a scale that is useful for spatial planning but also form smaller project.

Figure XXframe. Framework for improved information flows in urban planning of

blue-green infrastructure.

To write: Better description of figure in caption and bodytext.

Discussion

The discussion is under progress. No text is finished yet!

Analysis

Runoff is the governing hydrological process for urban flooding. However, the

study from Malmö shows that soil saturation through long-lasting rainfall impact

flood extent in some occasions (Paper2).

32

Measures to decrease flood risk and increase resilience

Measures used to mitigate urban, pluvial flooding, such as conduit, detention, lev-

elling of building and landscape.

Sea level rise will make storm surge more common in the future.

Redfern et al. 2016: Hydrological behaviour of urban surfaces are complex, depend-

ing on age, slope, maintenance, etc. Roads infiltrate more than often expected, up

to 50–60% of rainfall, while green space, when compacted in different ways, can

generate up to 60–70% runoff when newly established and 5–30% after some years.

Redfern et al. 2016: Precise definition of urban surfaces are important to better

model hydrological processes in urban areas. It is a priority to develop methods for

upscaling of small-scale results to city-scale hydrological behaviour.

Redfern et al. 2016: When studying effects of improved infiltration, realistic infil-

tration rates from already established urban surfaces must be assumed, as older road

pavement often infiltrate more than expected.

Urban trees are sometimes mentioned as an important feature in flood risk reduction

(Ahern 2013), as a large part of precipitation is taken up by the root network. This

feature is probably of minor importance during pluvial floods with quick progress,

like the 2010 and 2014 events in Malmö, but during pluvial floods with slow pro-

gress, like the 2007 event in Malmö, urban trees might play a role by reduction of

soil water content.

Future stormwater control

While fluvial flooding is projected to decrease in southern Sweden due to less snow

accumulation during winter months (REF Rojas), pluvial and coastal flooding are

projected to increase due to increased extreme rainfall during summer months (REF

Ohlsson) and rising sea level (REF) respectively.

Need for increased capacity in sewer if current design level should still be met.

All projections of future climate are highly uncertain, meaning that design of flood

defence as well as stormwater management must be done with care. Implications

for design…

Mission impossible to use quick conduits for drainage. The capacity in the system

will also be too low at some point. Crucial to detend stormwater control. Need to

elevate the land accordingly.

Need to reduce pollution from urban spaces. Polluter pays principle

33

Need for space. Inner yards in Copenhagen.

Transition towards increased resilience

Barriers and drivers that were found

Tratalos et al. (2007) raise the question whether it is possible to build compact cities

with dense urban environments and still maintain urban habitats and biodiversity,

and the resulting ecosystems services.

The transition theory, as described in section 2.5, aims to describe the full transition

from one regime to a new regime through a regime shift. In the case of mainstream-

ing blue-green infrastructure, such a transition is probably not realistic and might

not even be desired. A continuation of the continuous development described by

Brown et al. (2009). While there are successful examples of implemented blue-

green solutions in dense urban areas, complete removal of existing pipe-bound sys-

tem is probably unnecessary. In many cities, like in Malmö and Copenhagen, main

sewers are led through or close to the city centres. Extensive blue-green infrastruc-

ture can more easily be constructed outside the most innercity areas. Because of

direct linkage, blue-green infrastructure in upstream areas outside the city centre can

reduce stress on piped infrastructure in downstream areas inside the city centre. But,

for mainstreaming of blue-green infrastructure to a more extensive implementation

both in retrofitted and newly developed urban areas, a regime shift is essential. The

current regime, with pipe-bound stormwater management, is too stable to allow

widespread implementation of blue-green infrastructure.

Discuss what is needed for a transformation to take place.

How to work with data in municipalities and water utility companies.

Ambiguity and conceptual confusion

Det sokratiska perspektivet:

• Begreppsbildning – Viktigt att ha klara begrepp, risk annars att man inte

kommer framåt

• Kunskapsstabilitet – Vad vet man och vad vet man inte? Vad är kvaliteten

i den kunskap vi tror oss ha? Hålla ett vakande öga på vår kunskaps gränser

och begränsningar.

34

• Felbarhet – Vad kan vi göra fel? Medvetenhet kring svagheterna i de lös-

ningar vi presenterar. Hur påverkar felbarheten användningen av den kun-

skapen vi tror oss ha? Kan teorier vara sanna? Eller bara användbara?

Further research

Relation between rainfall and flooding

During the study of flood characteristics and mechanism, analyses of the data ma-

terial indicated that sub-daily rainfall durations are governing for severe precipita-

tion in Malmö. This relationship is worth to spend more time to investigate, as the

rainfall patterns have implications for design standards and flood prevention.

Responsibility, policy changes, and updated guidelines

In relation to climate change adaptation and development of urban environment.

Mainstreaming of blue-green infrastructure in urban planning

Blue-green infrastructure has been shown to be efficient to locally control storm-

water (Sun et al. 2014) and reduce flood risk (Sörensen 2016, Paper3, byta till an-

nan) and these nature-based solutions are therefore widely promoted (Københavns

Kommune 2012, European Commission 2015, Bai et al. 2018). However, the down-

stream effects of large-scale implementation have not yet fully been understood.

There are a few studies, but they are often based on modelling with vague or sym-

bolic parameterisation of important hydrological features (Stovin et al. 2013,

Viavattene & Ellis 2013, Sun et al. 2014) or without any calibration and validation

(Siekmann & Siekmann 2015, Locatelli et al. 2015). As IPCC conference steering

committee recently claimed in Nature, more knowledge on different measures effect

on urban water balance is needed (Bai et al. 2018). They claim that performance of

green infrastructure for heat wave and flood risk reduction should be investigated,

and that new standards and designs should be developed (ibid.). Here, one major

challenge is to mainstream implementation of blue-green infrastructure (Liao et al.

2017).

More…

Conclusions

To write: A few, main conclusions based on the objectives.

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mate Change and Urbanization Impacts. Water 6, 976–992. doi:10.3390/w6040976

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water

Review

Re-Thinking Urban Flood Management—Time for aRegime Shift

Johanna Sörensen 1, Andreas Persson 2, Catharina Sternudd 3, Henrik Aspegren 4,Jerry Nilsson 5, Jonas Nordström 6, Karin Jönsson 4, Misagh Mottaghi 4, Per Becker 7,Petter Pilesjö 2, Rolf Larsson 1, Ronny Berndtsson 1,8,* and Shifteh Mobini 9

1 Water Resources Engineering, Lund University, Lund SE-221 00, Sweden; johanna.sorensen@tvrl.lth.se (J.S.);rolf.larsson@tvrl.lth.se (R.L.)

2 GIS Centre/Physical Geography and Ecosystem Science, Lund University, Lund SE-221 00, Sweden;andreas.persson@nateko.lu.se (A.P.); petter.pilesjo@gis.lu.se (P.P.)

3 Architecture and Built Environment, Lund University, Lund SE-221 00, Sweden;catharina.sternudd@arkitektur.lth.se

4 Water and Environmental Engineering, Lund University, Lund SE-221 00, Sweden;Henrik.Aspegren@vasyd.se (H.A.); karin.jonsson@chemeng.lth.se (K.J.);misagh.mottaghi@chemeng.lth.se (M.M.)

5 Faculty of Culture and Society, Malmö University, Malmö SE-205 06, Sweden; jerry.nilsson@mah.se6 AgriFood, Economics Centre, Lund University, Lund SE-220 07, Sweden; jonas.nordstrom@agrifood.lu.se7 Risk Management and Societal Safety, Lund University, Lund SE-221 00, Sweden; per.becker@risk.lth.se8 Center for Middle Eastern Studies, Lund University, Lund SE-221 00, Sweden9 VA Syd, Malmö SE-211 20, Sweden; Shifteh.mobini@tvrl.lth.se* Correspondence: ronny.berndtsson@tvrl.lth.se; Tel.: +46-46-222-8986

Academic Editor: Peter J. CoombesReceived: 7 June 2016; Accepted: 2 August 2016; Published: 4 August 2016

Abstract: Urban flooding is of growing concern due to increasing densification of urban areas,changes in land use, and climate change. The traditional engineering approach to flooding isdesigning single-purpose drainage systems, dams, and levees. These methods, however, are knownto increase the long-term flood risk and harm the riverine ecosystems in urban as well as rural areas.In the present paper, we depart from resilience theory and suggest a concept to improve urban floodresilience. We identify areas where contemporary challenges call for improved collaborative urbanflood management. The concept emphasizes resiliency and achieved synergy between increasedcapacity to handle stormwater runoff and improved experiential and functional quality of the urbanenvironments. We identify research needs as well as experiments for improved sustainable andresilient stormwater management namely, flexibility of stormwater systems, energy use reduction,efficient land use, priority of transport and socioeconomic nexus, climate change impact, securingcritical infrastructure, and resolving questions regarding responsibilities.

Keywords: urban flooding; resilience; climate change adaptation; blue-green urban solutions

1. Introduction

Urban flooding problems are increasing due to numerous reasons. Urbanization is an acceleratingtrend. At present about 54% of the global population live in cities [1] and by 2050, almost two thirdsof the world’s population will live in urban environments [2]. Thus, urban areas are growing andin many cases, they are becoming denser [3]. Many cities are striving to reduce their negative,environmental impact and densification of existing urban areas has become the dominating urbanplanning strategy in order to meet a rapid urbanization with limited expansion on agriculturalland [4,5]. The large proportion of impermeable surfaces makes built-up land more vulnerable to

Water 2016, 8, 332; doi:10.3390/w8080332 www.mdpi.com/journal/water

Water 2016, 8, 332 2 of 15

flooding than the surrounding environment. Moreover, the risk of being flooded due to sea levelrise or river discharge is threatening 15% of the world’s population [2]. Recently, severe floodinghit highly developed cities like Prague, Dresden, and several other cities (2002), Bern and severalother cities (2005), New Orleans (2005), Copenhagen (2010, 2011, and 2014) (Figure 1), and New York(2012), as well as areas like Queensland (2010), South-western England (2013–2014), and the FrenchRiviera (2015). The societal consequences are severe. In Europe only, the average cost of flood damagesbetween 2000 and 2012 has been estimated to about 4.9 billion euros per year. It is estimated that thisfigure may increase to about 23.5 billion per year by 2050, i.e., with almost 400% [6].

Water 2016, 8, 332  2 of 14 

flooding than the surrounding environment. Moreover, the risk of being flooded due to sea level rise 

or  river discharge  is  threatening 15% of  the world’s population  [2]. Recently,  severe  flooding hit 

highly developed cities like Prague, Dresden, and several other cities (2002), Bern and several other 

cities (2005), New Orleans (2005), Copenhagen (2010, 2011, and 2014) (Figure 1), and New York (2012), 

as well as areas like Queensland (2010), South‐western England (2013–2014), and the French Riviera 

(2015). The  societal  consequences  are  severe.  In Europe only,  the  average  cost of  flood damages 

between 2000 and 2012 has been estimated to about 4.9 billion euros per year. It is estimated that this 

figure may increase to about 23.5 billion per year by 2050, i.e., with almost 400% [6]. 

 

Figure 1. Pluvial  flooding on  the 31st of August 2014. Photos were  taken at  three different  flood 

affected locations in Copenhagen (photo by Johanna Sörensen). 

The traditional engineering approach to manage urban drainage is by combined (sewage water 

and  stormwater  in  the  same  pipe)  or  separate  pipe  systems.  In  semi‐urban  catchments,  urban 

drainage systems may be combined with dams,  levees, and other  types of storage and detention 

facilities  to cope with  floods. However, during recent decades alternative ways  to manage  floods 

have evolved since traditional methods often harm the riverine ecosystems in urban as well as rural 

areas and increase the long‐term flood risk [7,8]. Alternative methods relate to resilience theory and 

address  the  city’s  capacity  to  mitigate  flooding  in  particularly  sensitive  urban  areas,  tolerate 

controlled  flooding  on  assigned  areas,  and  to  re‐organize  in  case  of  damage.  This means  that 

adaptive, multifunctional infrastructure in combination with water sensitive urban design are seen 

as means  to  reinforce  resilience  against  climate  change  [9–11]. However,  incorporation  of  these 

measures into decision‐making and ways to handle integrative and multi‐criteria aspects in the legal 

and organizational system are still to a great extent undeveloped. In general, a design framework 

integrating technical, social, environmental, legal, and institutional aspects is crucial [12]. Introducing 

such a framework is faced with barriers that are largely socio‐institutional rather than technical [13]. 

Sustainable and  resilient water management  thus needs  to  involve water  supply access and 

security, public health protection, as well as flood protection in densely built urban areas with many 

types  of  important  urban  infrastructure  [10].  In  view  of  the  above,  it  is  clear  that  urban water 

management systems need to become integrated elements in a multifunctional urban environment. 

Increasingly urgent and complex problems have  to be solved by  the city, where  the water sector 

management  systems  should  be  developed  in  close  collaboration with  regional  and municipal 

planning authorities. 

The traditional thinking is that resilient societies bounce back from the state they were in before 

a devastating event. However, lessons learned, from for example the Hurricane Katrina in 2005, show 

that this may not the case. Ten years after the catastrophe, the area is still suffering from reduced 

long‐term population and low economic activity [14]. Thus, instead of viewing the drainage design 

as a static process and to cope with floods of a certain recurrence, a contemporary interpretation of 

urban resilience needs to encompass a more flexible and adaptive approach to flood management. A 

flexible flood management system may be defined as measures for a given level of flooding, but with 

an integrated ability to modify it later [15]. Urban resilience should be viewed as an adaptive process 

where  the  society  continuously  learns how  to  cope with  changing  socioeconomic  conditions and 

Figure 1. Pluvial flooding on the 31st of August 2014. Photos were taken at three different floodaffected locations in Copenhagen (photo by Johanna Sörensen).

The traditional engineering approach to manage urban drainage is by combined (sewage waterand stormwater in the same pipe) or separate pipe systems. In semi-urban catchments, urban drainagesystems may be combined with dams, levees, and other types of storage and detention facilities to copewith floods. However, during recent decades alternative ways to manage floods have evolved sincetraditional methods often harm the riverine ecosystems in urban as well as rural areas and increase thelong-term flood risk [7,8]. Alternative methods relate to resilience theory and address the city’s capacityto mitigate flooding in particularly sensitive urban areas, tolerate controlled flooding on assignedareas, and to re-organize in case of damage. This means that adaptive, multifunctional infrastructurein combination with water sensitive urban design are seen as means to reinforce resilience againstclimate change [9–11]. However, incorporation of these measures into decision-making and ways tohandle integrative and multi-criteria aspects in the legal and organizational system are still to a greatextent undeveloped. In general, a design framework integrating technical, social, environmental, legal,and institutional aspects is crucial [12]. Introducing such a framework is faced with barriers that arelargely socio-institutional rather than technical [13].

Sustainable and resilient water management thus needs to involve water supply access andsecurity, public health protection, as well as flood protection in densely built urban areas with manytypes of important urban infrastructure [10]. In view of the above, it is clear that urban watermanagement systems need to become integrated elements in a multifunctional urban environment.Increasingly urgent and complex problems have to be solved by the city, where the water sectormanagement systems should be developed in close collaboration with regional and municipalplanning authorities.

The traditional thinking is that resilient societies bounce back from the state they were in beforea devastating event. However, lessons learned, from for example the Hurricane Katrina in 2005,show that this may not the case. Ten years after the catastrophe, the area is still suffering from reducedlong-term population and low economic activity [14]. Thus, instead of viewing the drainage designas a static process and to cope with floods of a certain recurrence, a contemporary interpretation ofurban resilience needs to encompass a more flexible and adaptive approach to flood management.A flexible flood management system may be defined as measures for a given level of flooding, but with

Water 2016, 8, 332 3 of 15

an integrated ability to modify it later [15]. Urban resilience should be viewed as an adaptive processwhere the society continuously learns how to cope with changing socioeconomic conditions and urbanland use as well as a changing climate. Since the urban space and flooding are complex, it is necessaryto adopt a systems-analytical approach. Figure 2 outlines the three systems involved in urban floodproblems, namely (1) the hydrological system; (2) the impact system; and (3) the management system.For a systematic approach to flood resilience, the dynamic character of all three systems needs tobe considered.

Water 2016, 8, 332  3 of 14 

urban land use as well as a changing climate. Since the urban space and flooding are complex, it is 

necessary  to adopt a systems‐analytical approach. Figure 2 outlines  the  three systems  involved  in 

urban  flood  problems,  namely  (1)  the  hydrological  system;  (2)  the  impact  system;  and  (3)  the 

management system. For a systematic approach to flood resilience, the dynamic character of all three 

systems needs to be considered. 

 

Figure 2. The three systems involved in urban flooding. (A) The hydrological system is the terrestrial 

part of the hydrological cycle with both natural and man‐made components; (B) The impact system 

is the part of urban society that may be affected in a detrimental way by a flooding event; (C) The 

management  system  is  the  part  of  urban  society  that  deals with  floods  in  order  to  decrease  the 

detrimental effects of flood events. 

A new adaptive approach to urban water management has to be integrated among stakeholders 

and authorities and by using sustainability criteria.  It should secure a higher  level of resilience  to 

climate change and water services, while at the same time enhancing attraction and social inclusion 

of urban environments. In this regard, the complex function of urban areas needs to be weighed into 

the design process. In view of the above, the objective of this paper is to bring forward the concept of 

urban flood resilience into a context of sustainability and risk management. We elaborate on these 

concepts according to the concept graph in Figure 3 and point out areas where an updated approach 

serves to cope with changing risks and increase urban resilience through integrated flood management. 

 

Figure 3. Outline of concept chart for  improved urban flood resilience followed  in this paper. The 

marked area in the middle of the chart denotes sustainable stormwater management solutions. 

2. Resilience in Flood Management 

Figure 2. The three systems involved in urban flooding. (A) The hydrological system is the terrestrialpart of the hydrological cycle with both natural and man-made components; (B) The impact systemis the part of urban society that may be affected in a detrimental way by a flooding event; (C) Themanagement system is the part of urban society that deals with floods in order to decrease thedetrimental effects of flood events.

A new adaptive approach to urban water management has to be integrated among stakeholdersand authorities and by using sustainability criteria. It should secure a higher level of resilience toclimate change and water services, while at the same time enhancing attraction and social inclusion ofurban environments. In this regard, the complex function of urban areas needs to be weighed intothe design process. In view of the above, the objective of this paper is to bring forward the concept ofurban flood resilience into a context of sustainability and risk management. We elaborate on theseconcepts according to the concept graph in Figure 3 and point out areas where an updated approachserves to cope with changing risks and increase urban resilience through integrated flood management.

Water 2016, 8, 332  3 of 14 

urban land use as well as a changing climate. Since the urban space and flooding are complex, it is 

necessary  to adopt a systems‐analytical approach. Figure 2 outlines  the  three systems  involved  in 

urban  flood  problems,  namely  (1)  the  hydrological  system;  (2)  the  impact  system;  and  (3)  the 

management system. For a systematic approach to flood resilience, the dynamic character of all three 

systems needs to be considered. 

 

Figure 2. The three systems involved in urban flooding. (A) The hydrological system is the terrestrial 

part of the hydrological cycle with both natural and man‐made components; (B) The impact system 

is the part of urban society that may be affected in a detrimental way by a flooding event; (C) The 

management  system  is  the  part  of  urban  society  that  deals with  floods  in  order  to  decrease  the 

detrimental effects of flood events. 

A new adaptive approach to urban water management has to be integrated among stakeholders 

and authorities and by using sustainability criteria.  It should secure a higher  level of resilience  to 

climate change and water services, while at the same time enhancing attraction and social inclusion 

of urban environments. In this regard, the complex function of urban areas needs to be weighed into 

the design process. In view of the above, the objective of this paper is to bring forward the concept of 

urban flood resilience into a context of sustainability and risk management. We elaborate on these 

concepts according to the concept graph in Figure 3 and point out areas where an updated approach 

serves to cope with changing risks and increase urban resilience through integrated flood management. 

 

Figure 3. Outline of concept chart for  improved urban flood resilience followed  in this paper. The 

marked area in the middle of the chart denotes sustainable stormwater management solutions. 

2. Resilience in Flood Management 

Figure 3. Outline of concept chart for improved urban flood resilience followed in this paper.The marked area in the middle of the chart denotes sustainable stormwater management solutions.

Water 2016, 8, 332 4 of 15

2. Resilience in Flood Management

As mentioned above, resilience is a key notion in sustainability science and contemporary urbanflood management. For this purpose, we give a brief account of this concept in relation to risk below.

2.1. Concepts of Risk and Resilience

Although risk is a contested concept with numerous definitions [16], most characterizations havethree aspects in common [17], namely (1) The assumption that the future is uncertain [18,19] andthat any future event is possible to influence [20]; (2) the uncertain future has a potential impact onhumans [18,21], or can at least be so perceived [22]; and (3) risk is defined in relation to a preferredoutcome [20,23,24]. Thus, risk is a potentially negative deviation from a preferred expecteddevelopment over time. This definition may at first appear as merely complicating more conventionalapproaches to risk, such as a combination of probability and consequence or of events and consequencesand their associated uncertainties, but even so may serve its purpose [16].

The notion of resilience is usually used to describe (1) the ability of a system to “bounce back” toa single equilibrium [25,26]; (2) a measure of robustness or buffering capacity before a disturbanceforces a system from one stable equilibrium to another [27,28]; or (3) a system’s ability to adapt inreaction to a disturbance [29]. It has been suggested that human beings have the ability not only toreact to disturbances but also to anticipate and learn from them [17]. Resilience can be regarded as apurely descriptive concept in relation to systems behavior, or if it is normative in the sense of relatingoutcomes to human values and objectives. Both approaches have merits, but if resilience is to haveany meaning in relation to risk and sustainable development, which are both inherently normativeconcepts, it becomes equally normative [17].

2.2. Flood Resilience

Urban flood risk management aims at assessing and reducing flood risk, as well as preparingfor effective response to, and recovery after, actual floods, with the purpose of minimizingdisturbances, disruptions, and associated costs in relation to a city’s preferred development over time.Thus, resilience is the capacity of a system, such as a city, to continuously develop along a preferredand expected trajectory [17], while remaining within human and environmental boundaries [30].This approach to resilience is suitable when focusing on the sustainable development of cities, whichentails human beings with preferences and expectations for their future as well as agency to strive tomeet them. City authorities develop visions and plans for the future use of urban areas. Plans mayspan over years and even decades, during which the city changes more or less continuously andmost often significantly due to purposeful and proactive human activities, also often reducing theapplicability of all three main approaches to resilience previously listed. If a city’s resilience instead isits capacity to continuously develop along its preferred expected trajectory, then this resilience is anemergent property determined by the city’s ability to anticipate, recognize, adapt to, and learn fromvariations, changes, disturbances, disruptions, and disasters that may cause harm to what humanbeings value [17]. Sustainable development thus means to manage risk and resilience is the capacityfor doing so in an uncertain, ambiguous, complex, and dynamic world.

2.3. Time Perspective on Flood Management

Long-term strategies are needed to facilitate cost-effective and rapid implementation ofintegrated flood management [31]. The aim is fast recovery from flooding and restoration to goodliving conditions. Sustainability should not only be achieved economically, but also socially andenvironmentally [32]. Cities are challenged by climate change, and according to the EuropeanEnvironmental Agency report [33] immediate action is needed since delaying adaptation actionswill be much more costly in the long-term. Climate change will not only affect the economy but alsoincrease the number of possible hazardous events to citizens. When planning for climate change,

Water 2016, 8, 332 5 of 15

however, also other related climate change effects such as, water scarcity, drought, and heat waves,need to be considered in the municipal planning. Cities should thus, not wait for a larger flood eventor a large-scale catastrophe to act. Instead, city planners need to study front-runner cities that aredealing with flood challenges such as New York, Copenhagen, and Rotterdam. Learning from theexperiences of others, and ourselves is of crucial importance and saves energy, resources, and time [34].Thus, it is a long-term process to achieve a flood resilient city, and it is vital to ensure synergy betweenurban development and urban drainage strategies [35]. It is dangerous to make long-term decisionsbased only on experiences of a recent severe flood event. Doing so might lead to lock-in effects whereirreversible decisions are made [36].

2.4. Flood Management Strategies

After a series of catastrophic floods in Europe, the EU Flood Directive was ratified in 2007 [37].The Flood Directive gives two design levels, namely the 100-year event, and the “worst case scenario”.Implicitly the Flood Directive focuses on riverine floods. However, pluvial floods, i.e., floodinggenerated locally by an overload of the urban drainage system by extreme rainfall, also constitutea formidable threat to cities around the world. Since traditional urban drainage systems rely onunderground pipes, they have, in order to avoid huge dimensions, typically been designed to copewith rainfall of 10 year recurrence period or less. More extreme events are deliberately allowed togenerate inundation of selected areas such as streets, infrastructure, and building basements. Even withdesign level of 100 year recurrence period, the risk of exceeding critical conditions during a period of50 years is 40%. On top of that, the uncertainty associated with recurrence periods based on existing,limited data is quite large [38]. The EU Flood Directive highlights that, irrespective of the recurrenceperiod chosen, there is always a non-negligible probability of system failure. Unfortunately, it remainsto make this an accepted public fact and a component of the strategic thinking among all stakeholdersincluding especially the general public.

As climate change continues and the sea level rises [39], concerns regarding coastal floodingare growing. Three different strategies have been suggested [40,41], i.e., to retreat (slowly movebuildings to higher elevations), to defend (secure areas with measures like dikes and floodgates), or toattack (build on the water, with buildings and infrastructures that can endure the water). From anenvironmental perspective, retreat or attack is most suitable, while defend is found less advisable.From an economical perspective, defend or attack is most suitable if there are high assets in the area,and defend will minimize the construction and maintenance costs. From a social perspective, the retreatmight be a good solution [41]. Mathur, A. et al. [42] discussed flooding and sea level rise for Mumbai(India) and suggested that the sea should be seen as a friend rather than an enemy from which tobe protected. Further, the islands of Mumbai should more correctly be called estuary and MithiRiver a river rather than a part of the sewage system. They argued that the change in naming andunderstanding are important for how we reflect upon the nature as well as the city and that thisinfluences how we plan the city and prepare ourselves for flooding.

In general, coastal flooding is different from riverine (from river) and pluvial (from intenserainfall) flooding with respect to physical planning. Cities are often experiencing a combination ofriverine and pluvial floods. The pluvial flood type is generated locally and the result of exceedanceof natural infiltration and drainage as well as exceedance of the capacity of the urban drainagesystem [43]. On the contrary, riverine floods are usually generated at a much larger rural catchmentscale. Consequently, the flood problem for riverine cities may often be a result related to scale andupstream rainfall-runoff processes. Consequently, upstream flood management will also affect thedownstream water level and discharge. In this regard sustainable flood management for urban areasneeds to consider larger, often rural catchments that discharge nearby or inside urban neighborhoodsas well as direct stormwater runoff from impermeable areas. This can be seen as a scale problem whereboth quantitative and qualitative aspects of runoff need to be considered. Nevertheless, the problem issimilar when it comes to organizational strategies and the understanding of resilience and risk.

Water 2016, 8, 332 6 of 15

3. Integrated Approach to Urban Planning and Design

To manage floods in a sustainable way it is necessary to apply a holistic viewpoint and employ anintegrated approach for the different functions that a modern city entails.

3.1. Water Management beyond the Traditional Pipe System

Continuous urbanization will result in increasing nutrient and contaminant emissions ofwatersheds, putting human health and ecosystems in danger [44,45]. Due to the absence of trans-scalethinking, drainage and flood protection systems mostly rely on expensive and inflexible undergroundsolutions. High-intensive rainfall is causing more frequent overloading of pipes resulting in flooding ofpublic and private property [46]. As most cities still are using combined sewage systems for drainage,more frequent overflow of untreated sewage may be expected in the future [46]. At the same time,urban areas are getting denser, and thus less space will be available for underground infrastructuresincluding extensive use of drainage pipes. Developing the underground water infrastructure will thusbe even more costly in the future. Urban transition should instead lead towards less and slower surfacerunoff, which requires more soil and surface infiltration. Accordingly, applying surface solutions andevolving the drainage systems are essential steps for the reduction of flood impacts [47]. Utilizing urbanareas as integrated parts of the drainage system provides promising opportunities.

Geldof, G.D. [48] suggested the Three Point Approach (3PA) as a tool for how to move fromonly focusing on design standards for rainfall events that occur with a return period of 1 in 10 years(first point) to including extreme rainfall events (second point), and at the same time consider theimpact on every-day life (third point). Fratini, C.F. et al. [49] found this tool useful in discussionswith stakeholders. Rather than seeing the 3PA as going from a one-point to a three-point approach,the present flood management could be developed from a single-purpose view with a one-pointapproach to a multi-disciplinary view with a full spectra approach. This means that the whole range,from the everyday system and processes in the city to the functionality during the most extremeevents, is incorporated. The whole system can be integrated and treated in unison, including alsoextremes. It is no longer appropriate to focus separately on the water issue solely when planning waterinfrastructure. For economical as well as environmental reasons, an integrated approach is needed.New large-scale single-purpose construction projects, such as huge sewerage tunnels in old combinedsewerage systems, have been strongly criticized, for example in Philadelphia [50,51], London [52]and Copenhagen. Integrated flood management calls for solutions with multiple purposes, whichhas a valuable function every day, not only once in 50 or 100 years. With climate change and rapidurbanization, there is also a need to increase the capacity of the stormwater system. At the sametime, since urban areas are becoming more complex including more and more high-tech and sensitiveinfrastructures, the economic value is increasing leading to larger flooding sensitivity. Therefore, moreflexible systems are needed that can adapt to future changes.

3.2. Integrated Approaches to Flood Management

Designing open water management solutions in the urban landscape is a multi-disciplinarytask that requires a combination of scientific and artistic approaches and a new kind of interactionbetween green and blue assets is called for [53]. Different mechanisms for infiltration, storage,transport, evapotranspiration, and treatment are usually applied in surface solutions [54]. In blue-greeninfrastructure, the urban greenery and water management are combined in order to protect the urbanlandscape and its ecological and hydrological values [55,56]. In successful examples, blue-greeninfrastructure not only mitigates flood impacts [57] and improves adaptation to climate change, butalso increases the quality and living conditions of urban environments in terms of improved heatalleviation, increased biodiversity, and better air quality. It may even have the potential to provide forfood and energy production, improve local economy, and benefit social life [58]. Ecological urbanismmakes it possible for both water flow and urban landscape to act as mutual drivers and at the same

Water 2016, 8, 332 7 of 15

time, values are added to the public urban space. Due to historical reasons, most cities have todaymainly piped drainage and a flood control system for pluvial flooding [13,59]. Transfer towardsblue and green solutions will be slow and many challenges regarding responsibility, economy, andmaintenance are yet to be solved.

Increased frequency and intensity of heavy rainfall are affecting private and public stakeholdersand municipal authorities. Compact solutions and efficient land use are called for in both new areasand redevelopment in urban areas. Densely built urban areas should provide easy and convenientaccess to a multitude of functions such as retail, service, and public transport, thereby contributingto reduced energy use and CO2 emissions [5]. This means that a wide range of everyday activities iscarried out simultaneously in densely built urban environments. The value of urban land is high andthus urban space has a multitude of functions (Figure 4). Strategies to address water challenges atearly stages and implement integrated site-specific urban drainage solutions in all urban projects areessential [35].

Water 2016, 8, 332  7 of 14 

and redevelopment in urban areas. Densely built urban areas should provide easy and convenient 

access to a multitude of functions such as retail, service, and public transport, thereby contributing 

to  reduced energy use and CO2 emissions  [5]. This means  that a wide range of everyday activities  is 

carried out simultaneously in densely built urban environments. The value of urban land is high and thus 

urban space has a multitude of functions (Figure 4). Strategies to address water challenges at early stages 

and implement integrated site‐specific urban drainage solutions in all urban projects are essential [35]. 

(a)  (b)

Figure 4. Floodable areas with multifunctional use. (a) A pond in Augustenborg Eco‐City, Malmö. 

During heavy rainfall, the area can store water up to the stone edge near the trees to the right; (b) The 

Water  Plaza  in  Rotterdam.  The multifunctional  basins  prevent  surrounding  streets  from  being 

flooded. The plaza is also used for performances, skating, studying, and group meetings. 

Several critical considerations need  to be  taken when  introducing  flood preventing measures 

above ground to assure that retention ponds, permeable surfaces, and open swales are adding to, 

rather than subtracting from, the experiential and functional quality of everyday living environments 

for urban dwellers (Figure 5). For instance, sustainable urban environments need to prioritize non‐

motorized  travel modes  such  as walking, bicycling,  and  the use of public  transport  [60]. Special 

attention needs to be given to vulnerable and less mobile groups such as children, elderly people, 

and  those with  physical  or  visual  impairment.  It  is  therefore  crucial  to  include  designers with 

expertise  in urban  landscaping and social structures  in the design process, to make sure that new 

designs meet the requirements for all users. With multipurpose solutions, increased complexity in 

the design process will follow. Water planning and urban planning integration are thus keys to flood 

resilience. The focal points should be at improving the spatial and economical values of the use of 

water  in  the  city,  protecting  the  city  against  sea  level  rise  and  river  discharge,  and  increasing 

resilience to stormwater. Solutions should consist of the combination of planning, technology, and 

design. Accordingly, merging  different  urban  projects  and  taking  advantage  of  various  sectors, 

working groups, and experts are necessary [61]. 

(a) (b) (c) 

Figure 5. (a) Blue‐green infrastructures in Rotterdam; (b) London; and (c) Amsterdam. 

Figure 4. Floodable areas with multifunctional use. (a) A pond in Augustenborg Eco-City,Malmö. During heavy rainfall, the area can store water up to the stone edge near the trees to theright (photo by Johanna Sörensen); (b) The Water Plaza in Rotterdam. The multifunctional basinsprevent surrounding streets from being flooded. The plaza is also used for performances, skating,studying, and group meetings (photo by Misagh Mottaghi).

Several critical considerations need to be taken when introducing flood preventing measuresabove ground to assure that retention ponds, permeable surfaces, and open swales are adding to,rather than subtracting from, the experiential and functional quality of everyday living environmentsfor urban dwellers (Figure 5). For instance, sustainable urban environments need to prioritizenon-motorized travel modes such as walking, bicycling, and the use of public transport [60].Special attention needs to be given to vulnerable and less mobile groups such as children, elderlypeople, and those with physical or visual impairment. It is therefore crucial to include designers withexpertise in urban landscaping and social structures in the design process, to make sure that newdesigns meet the requirements for all users. With multipurpose solutions, increased complexity inthe design process will follow. Water planning and urban planning integration are thus keys to floodresilience. The focal points should be at improving the spatial and economical values of the use ofwater in the city, protecting the city against sea level rise and river discharge, and increasing resilienceto stormwater. Solutions should consist of the combination of planning, technology, and design.Accordingly, merging different urban projects and taking advantage of various sectors, workinggroups, and experts are necessary [61].

Water 2016, 8, 332 8 of 15

Water 2016, 8, 332  7 of 14 

and redevelopment in urban areas. Densely built urban areas should provide easy and convenient 

access to a multitude of functions such as retail, service, and public transport, thereby contributing 

to  reduced energy use and CO2 emissions  [5]. This means  that a wide range of everyday activities  is 

carried out simultaneously in densely built urban environments. The value of urban land is high and thus 

urban space has a multitude of functions (Figure 4). Strategies to address water challenges at early stages 

and implement integrated site‐specific urban drainage solutions in all urban projects are essential [35]. 

(a)  (b)

Figure 4. Floodable areas with multifunctional use. (a) A pond in Augustenborg Eco‐City, Malmö. 

During heavy rainfall, the area can store water up to the stone edge near the trees to the right; (b) The 

Water  Plaza  in  Rotterdam.  The multifunctional  basins  prevent  surrounding  streets  from  being 

flooded. The plaza is also used for performances, skating, studying, and group meetings. 

Several critical considerations need  to be  taken when  introducing  flood preventing measures 

above ground to assure that retention ponds, permeable surfaces, and open swales are adding to, 

rather than subtracting from, the experiential and functional quality of everyday living environments 

for urban dwellers (Figure 5). For instance, sustainable urban environments need to prioritize non‐

motorized  travel modes  such  as walking, bicycling,  and  the use of public  transport  [60]. Special 

attention needs to be given to vulnerable and less mobile groups such as children, elderly people, 

and  those with  physical  or  visual  impairment.  It  is  therefore  crucial  to  include  designers with 

expertise  in urban  landscaping and social structures  in the design process, to make sure that new 

designs meet the requirements for all users. With multipurpose solutions, increased complexity in 

the design process will follow. Water planning and urban planning integration are thus keys to flood 

resilience. The focal points should be at improving the spatial and economical values of the use of 

water  in  the  city,  protecting  the  city  against  sea  level  rise  and  river  discharge,  and  increasing 

resilience to stormwater. Solutions should consist of the combination of planning, technology, and 

design. Accordingly, merging  different  urban  projects  and  taking  advantage  of  various  sectors, 

working groups, and experts are necessary [61]. 

(a) (b) (c) 

Figure 5. (a) Blue‐green infrastructures in Rotterdam; (b) London; and (c) Amsterdam. Figure 5. (a) Blue-green infrastructures in Rotterdam (photo by Misagh Mottaghi); (b) Amsterdam(photo by Misagh Mottaghi); and (c) London (photo by Johanna Sörensen).

3.3. Importance of Planning for Sensitive Infrastructure

Flood events pose special threats to society through the effects on infrastructure. Large quantitiesof water may flood buildings and cut off roads. Flooded buildings with sensitive equipment such aselectrical and IT systems may have devastating societal effects. Moreover, since sensitive infrastructuresystems usually are connected and interdependent, effects may cascade to other systems and overa much larger area than the initially affected one [62,63]. Additionally, the consequences may occurinstantly or show up later making it very difficult to assess the consequences that a flood event mayhave on society as a whole [64,65]. This is serious since the infrastructure is often critical for society’sfunction to work properly and deliver basic services and supplies to its inhabitants, such as freshwater and electricity. This is especially true for urban environments with no or few alternativesto the failed infrastructure. Moreover, vital societal functions, such as hospitals, may not tolerateinterruptions in water supply, electricity, as well as transportation. Hence, it is paramount that effectiveplanning measures for protecting the infrastructure are developed and implemented. Considering thedamages flooding causes worldwide there is still much to do when it comes to protecting the sensitiveinfrastructure from being damaged or affected by flooding.

From the above, it is clear that planning is essential for protection of vital infrastructure whendealing with disaster risk management. Not only flooding, but also other potential threats to afunctional society, have to be taken into account when designing and building infrastructure. We haveto make sure that water, transportation, energy, and other important infrastructures are protectedin non-normal situations, like during flood events. A way to prepare the society is to simulatedifferent scenarios by applying disturbing factors. Using modern technology, like up to date spatialplanning techniques, makes it possible to integrate a large number of societal threats and optimizesolutions [66]. Solutions can also be based on different priorities, such as cost-benefit, time, orenvironment [67]. For this purpose, adequate and high quality data are a necessity to make simulationstrustworthy, as even small errors in these datasets will highly influence the results [68]. The scale andaccuracy challenge makes the planning site specific, and does not allow for spatial generalizations.Coombes, P.J. [68] noted that an appropriate policy framework is required that integrates land andwater management with design processes at spatial scales from local to regional and that also appliesto urban renewal and asset renewal or replacement choices. However, integrative aspects may as welllead to competing scales of issues and inertia of existing systems may severely challenge innovativefacets of solutions [69].

Water 2016, 8, 332 9 of 15

4. Flood Management with Multiple Stakeholders

4.1. Roles and Responsibilities

The water supply and water sector is arguably the local activity that is first to be exposed by achanging climate and increased flood risk. However, the water sector cannot be expected to handle thecomplex problems by itself only. To find cost-effective solutions, public and private actors at the localand national level must cooperate and share the responsibility to reduce the negative effects of flooding.For the water sector, this would call, e.g., for measures to integrate water management with a widerplanning system, such as land-use planning and development of transport systems, and decentralizedblue-green solutions to handle stormwater. Future effects of climate change are also expected to varyconsiderably, due to, e.g., variation in local climate as well as variation in natural and social conditions.These characteristics make adaption to climate change and flood control a local task [70]. There is,thus, a need for differentiated and flexible measures. For example, giving responsibility to differentsectors at the local level can motivate them to seek cross-sector cooperation where they find thisrelevant [70]. For this to function, stricter central government regulations may be necessary. Adoptinga highly decentralized organization is not without problems. [71,72] studied the effects of introductionof a revised system of funding flood management schemes in England, the ‘Partnership Funding’ [73].The results suggested, e.g., that the economic efficiency did not increase. Instead, higher costs resulteddue to longer decision-making practices. In addition, the scheme increased social inequality, whererural middle-class groups with local capacities, such a networks, skills, and cultural capital, gainedfrom the funding scheme. Hovik, S. [72] concluded that there is not necessarily a contradiction betweenstrong vertical links and a strengthening of horizontal cooperation in a system of network governance.Although in the resilience literature decentralized and deregulated risk management systems havebeen suggested, often as part of a neoliberal policy agenda [74–76], there is nothing in the concept itselfthat disqualifies all levels to take active part. In other words, just because individuals, households,and communities become involved, the state is not restrained from a central role in a resilient city.A characteristic of the 20th century was development of utility services that did not require the citizens’active behavior. With regard to solid waste, citizens need to and are willing to interact with this systemon a wider basis. It may well be that we now see changes also with regard to water, where the citizenshave to interact and be part of the system. For this to take place, support by legislation is required.The Danish law on sky burst management may well be the first example of this. The concept requiresan environment where different techniques are allowed to co-exist as different possible ways to solvemultifunctional goals.

4.2. Multi-Stakeholder Planning

To facilitate multi-stakeholder planning, it is first necessary to identify all relevant stakeholders ina given area and create forums in which they can communicate their specific interests and needs toeach other [77]. There is also a need to increase the stakeholders’ knowledge and understanding aboutflooding as phenomena as well as how flooding may affect specific urban locations. The stakeholdersneed to work with and see the result of reliable flow models that can visualize how flooding maydevelop at these locations. The stakeholders also need to increase their understanding on howflooding may affect the infrastructure in these places and how the effects may spread from onesystem to another. This calls for solutions where it is possible to illustrate and visualize how exposedthe infrastructure is to flooding and collectively analyze what the effects flooding may have oninfrastructure and society. The analysis can be refined by clarifying the sensitivity of the differentinfrastructures, their interdependences, and the functions they support. An exciting way to performthis analysis may be to use social media and techniques in virtual reality (VR) or augmented reality(AR). A common denominator is to improve the communication between stakeholders before, during,and after flood events.

Water 2016, 8, 332 10 of 15

A major challenge concerns the implementation of effective measures. Although the infrastructureis connected, the responsibility, ownership, and competence are often separated, adding to thedifficulties to obtain a holistic perspective and a general understanding among the stakeholders.This is further complicated by the fact that information about infrastructure may be sensitive andtherefore cannot be openly communicated. Moreover, the divided responsibility makes it difficultto implement cost effective measures since cost and effect may be separated for different actors andsystems. Hence, there is a need to encourage formation of joint priorities and objectives among allstakeholders. For a holistic flood risk planning to function, the planning process must be imbuedby a high degree of communication and collective learning. Collective learning can be defined as“ . . . a broad term and includes learning between dyads, teams, organizations, communities, andsocieties” [78]. It stresses “ . . . characteristics such as relationships, shared vision and meanings,mental models and cognitive and behavioral learning” [78].

Zhou, Q. [12] provided a general summary of the capacity of various models and software interms of water quantity and quality simulation, sustainable drainage device modeling, and spatialplanning. It is important that the multi-stakeholder planning involves a presentation and motivationof selected models for the quantitative and qualitative modeling. A general concern, however, for thesemodels is the lack of a shared interface/platform for integrated use. Many models are specialized foronly one or a few aspects of SUDS (Sustainable Urban Drainage System) and therefore the simulationis often performed in isolation and thus only partially reveals all effects of SUDS. For a detaileddiscussion of models see [12]. A possible way to better integrate hydrological and hydraulic modelresults into multi-stakeholder planning is the use of Geographic Information Systems (GIS).

4.3. Information Sharing between Stakeholders

Geographic Information Systems (GIS) is an effective tool for building databases and analyzingspatial data, and may be of great help for accomplishing learning across organizational boundaries.GIS enables numerous types of analyses based on, e.g., proximity and network. In addition,the effective visualization capability of a GIS makes it highly suitable for learning and communicationactivities. As a result different forms of collective GIS-approaches have emerged, e.g., communitymapping and participatory GIS. GIS is also used and developed for improving (flood) risk management.GIS has, e.g., been used for identifying interdependencies between local infrastructures [65], modelingand simulation of infrastructure elements interdependencies [79], and modeling urban surface waterbalances [80].

Shared responsibility and access to databases and analysis results are essential for successfulplanning as discussed above. One way of implementing this is by the use of a spatial data infrastructure(SDI) that is available on the Internet [80]. Integrated systems and integrated spatial data infrastructures(see e.g., [81–83]), are essential to make information available and create possibilities to includevarious types of data from different stakeholders. SDI makes it possible for data providers, includingparticipatory data collection made by the broad society, to contribute in building information databases,to be used for planning as well as for awareness and protective measures. Web-based solutions makeit possible for responsible parties (like municipalities) to retrieve data and information on a detailedlevel and for the public to get up-to-date information when needed. All stakeholders are also able tofeed the system, through database writing permission set by the authorities [84,85]. This means thatparts of the system are publicly available, while other parts are restricted to planners and officials only.

5. Discussion and Conclusions

In the above, we elaborated on concepts related to urban flood resilience and pointed out severalareas where the society needs to change the thinking to reach our goal: an integrated flood managementsystem that can cope with changing risk by increasing urban resilience. In addition to an active civilsociety, effective urban water governance is also required. It is crucial to realize that cities are urbansocio-ecological systems where multiple stakeholders can jointly develop multiple-purpose solutions to

Water 2016, 8, 332 11 of 15

the complex problem of flood prevention in densely built urban areas. There are several challenges thatstill call for a solution, and the more important ones are summarized below. Transdisciplinary researchhas the potential to identify obstacles, learn from successful examples, promote the development ofnew processes, and to support progress in the mentioned areas.

1. Climate change and related impacts

Future flood protective measures should be climate change resilient. However, also other relatedclimate change effects such as, water scarcity, drought, and heat waves, need to be considered in themunicipal planning.

2. Water, energy, land use, transportation, and socioeconomic nexus

The urban water system has traditionally been regarded as a stand-alone system. To developflexible, resilient, and multipurpose flood protection systems, the water, energy, land use,transportation, and socioeconomic nexus need to be jointly considered from a multi-stakeholderperspective (see also e.g., [86]).

3. Flexibility of different kinds of stormwater systems

Flexibility of flood protective measures is paramount. Urbanization as well as the changing use ofurban areas imply that flexibility in measures against floods becomes very important. Flexibility isalso needed in view of uncertain future climate change impact.

4. Unresolved questions regarding responsibilities and improved communication betweenstakeholders and authorities.

Sustainable flooding and resilience thinking to flood prevention need better integration amongstakeholders and authorities managing flooding. Flooding, like other sudden events, may change thecity from one state to a new, different one (especially for catastrophic events). This understandingopens up for new approaches to urban planning. In this process, the responsibility of different sectorsof city and planning authorities needs to be clarified.

5. Securing critical infrastructures

Important societal sectors may be highly dependent on certain infrastructure and different flowsof supplies, e.g., the health care sector. Methods and tools to clarify the infrastructures’ vulnerabilityto flooding can involve simulations, which are efficient in visualizing and have predictive capability;integrated databases; broad participation of many stakeholders with varying interests; and collectivelearning efforts that enhance information sharing. There is a need to reflect carefully on these andother methods and tools, and consider how they can be implemented effectively in the flood riskmanagement work.

Acknowledgments: This research was funded by The Swedish Research Council Formas under contract942-2015-149. Financial support was also provided by Sweden Water Research, Göteborg City, Höje å WaterCouncil, the Skåne Region, and Länsförsäkringar Skåne.

Author Contributions: All authors contributed equally in writing the manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

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Journal of Hydrology 555 (2017) 51–67

Contents lists available at ScienceDirect

Journal of Hydrology

journal homepage: www.elsevier .com/ locate / jhydrol

Research papers

Pluvial, urban flood mechanisms and characteristics – Assessment basedon insurance claims

https://doi.org/10.1016/j.jhydrol.2017.09.0390022-1694/� 2017 Published by Elsevier B.V.

⇑ Corresponding author.E-mail address: johanna.sorensen@tvrl.lth.se (J. Sörensen).

Johanna Sörensen ⇑, Shifteh MobiniWater Resources Engineering, Lund University, Lund, Sweden

a r t i c l e i n f o

Article history:Received 26 April 2017Received in revised form 19 September2017Accepted 20 September 2017Available online 6 October 2017This manuscript was handled by G. Syme,Editor-in-Chief

Keywords:Pluvial floodingExtreme precipitationUrban floodingInsurance claimsFlood mechanisms

a b s t r a c t

Pluvial flooding is a problem in many cities and for city planning purpose the mechanisms behind pluvialflooding are of interest. Previous studies seldom use insurance claim data to analyse city scale character-istics that lead to flooding. In the present study, two long time series (�20 years) of flood claims fromproperty owners have been collected and analysed in detail to investigate the mechanisms and charac-teristics leading to urban flooding. The flood claim data come from the municipal water utility companyand property owners with insurance that covers property loss from overland flooding, groundwaterintrusion through basement walls and flooding from the drainage system. These data are used as a proxyfor flood severity for several events in the Swedish city of Malmö. It is discussed which rainfall charac-teristics give most flooding and why some rainfall events do not lead to severe flooding, how city scaletopography and sewerage system type influence spatial distribution of flood claims, and which impacthigh sea level has on flooding in Malmö. Three severe flood events are described in detail and comparedwith a number of smaller flood events. It was found that the main mechanisms and characteristics offlood extent and its spatial distribution in Malmö are intensity and spatial distribution of rainfall, dis-tance to the main sewer system as well as overland flow paths, and type of drainage system, while highsea level has little impact on the flood extent. Finally, measures that could be taken to lower the flood riskin Malmö, and other cities with similar characteristics, are discussed.

� 2017 Published by Elsevier B.V.

1. Introduction

Extreme rainfall leads to pluvial flooding in many cities, affect-ing many people (Houston et al., 2011). Extensive urban and sub-urban growth in combination with insufficient sewer systems(Swan, 2010) as well as climate change (Semadeni-Davies et al.,2008a,b) aggravates the problem. Pluvial flooding is becoming lessaccepted as society is becoming more reliant on highly developed,power supply dependent technology and as cities are becomingmore densely populated. Property owners pay service fees andexpect that the drainage system shall work properly (Schmittet al., 2004). Where flooding earlier was seen as an unforeseenevent that cities could not afford to prevent from happening,awareness is rising that cities can be better prepared for floodingby using flood warning systems, by implementing measures thatcan lower peak runoff through retention, and by leading excessflow to less vulnerable places. It is however expensive to increasethe drainage capacity in cities, even when multiple-purpose solu-tions, e.g. blue-green infrastructure, are used. For strategic reasons,

it is therefore necessary to identify in which environments rains ofdifferent characters, or combinations of rain and high receivingwater levels, contribute to flooding and what the characteristicsof flood-threatened areas are. Thus, it is our objective to investigatethe mechanisms and characteristics of urban, pluvial flooding.

Several studies have sought mechanisms behind urban flooding.Akukwe (2014) collected data from questionnaires and interviews.Two thirds of the flooding in Port Harcourt Metropolis, Nigeria,could be explained by human-induced factors like unplannedurban development and poor drainage capacity, which can beinfluenced by city planners, while the biggest factor, rainfall, can-not. While information from the public can be used to identifyproblems in the drainage system, the physical drivers behind plu-vial flooding can barely be understood by questionnaires and inter-views solely. ten Veldhuis et al. (2009) analysed data from acomplaint register, which might give a better understanding ofthe mechanisms. They found that the most important contributionto flooding in Haarlem, the Netherlands, was gully pot blockage(79% of reported complaints), while the contribution of heavystorm events were much lower (5%). Torgersen et al. (2015) usedinsurance data to study the relation between extreme rainfalland urban flooding and found that long-lasting, but less intensive,

52 J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67

rainfall lead to more flooding than shorter, intensive rainfall inFredrikstad, Norway.

Studies on causality of fluvial flooding are more prevalent, asthere are more data available from rivers and more methods of anal-ysis developed than for urban, pluvial flooding. There are interestinglessons to learn both about climatological (Glaser et al., 2010) andhydrological (Berghuijs et al., 2016) drivers as well as the effect ofhuman activities (Zhang et al., 2014). Every river catchment isunique and flood frequency patterns must be understood indepen-dently (Glaser et al., 2010). Pluvial flooding acts on a different scalethan fluvial flooding, where mechanisms and characteristics arefunctioning differently. Consequently, research is needed to investi-gate mechanisms and characteristics also for pluvial flooding.

In the present study of pluvial, urban flooding, two long timeseries (�20 years) of flood claims in Malmö, Sweden, have beencollected and analysed. The data come from a municipal water util-ity company and from an insurance company that covers propertyloss from overland flooding, groundwater intrusion through base-ment walls and flooding from the drainage system. The floodclaims are used as a proxy for flood severity for different events.As the addresses for all claims are included in the datasets, theyare suitable for spatial analysis. The flood claim data give a goodmeasure of flood extent during both minor flood events and moresevere flood events, and in combination with climatic data andcharacteristics of urban drainage, topography, etc., the mecha-nisms of urban, pluvial flooding are analysed and discussed.

Insurance claim data have recently been used for various otherstudies, e.g. statistical analysis to determine distribution ofextreme events (Smith and Goodman, 2000), and to explain floodvariability related to sewer flooding with rainfall data in Aarhus,Denmark (Spekkers et al., 2013b). Spekkers et al. (2013a) analysedthe relation between flood claims and rainfall extremes for theNetherlands. They were able to explain some of the spatial vari-ance in flood magnitude with spatial distribution of the rainfall,while a considerable fraction could not be explained. Zhou et al.(2013) tried to find a relation between hourly and daily rainfalland flood related costs, but could only explain some of the floodrelated costs variance by rainfall intensities. Spekkers et al.(2013b) suggest that further research is needed where factors thatmay potentially influence the severity of flooding, such as topogra-phy, building and household characteristics, urban drainage char-acteristics and spatial distribution of rainfall are evaluated. Someof these parameters, i.e. topography, urban drainage and spatialdistribution of rainfall, are analysed and discussed in the presentarticle together with other parameters, such as sea level and his-torical existence of watercourses in the urban landscape.

The main objective of the present study is to analyse mecha-nisms and characteristics of urban, pluvial flooding on city-scale.It is discussed which rainfall characteristics result in most floodingand why some extreme rainfall events do not lead to severe flood-ing. The relation between topography and flood extent under dif-ferent rainfall characteristics are analysed, and the influence ofhigh sea level is also discussed.

The detailed research questions are

� Which are the most important mechanisms and characteristicsthat influence flood extent and its spatial distribution?

� Can any spatial patterns of flooding be identified?� Is the spatial pattern consistent for different rainfall events withsimilar distribution?

� What are the consequences of extreme rainfall with differentspatial and temporary patterns?

In the following sections, we first introduce the study site,Malmö, Sweden, and the data used in the study. Second, we pre-sent the methodology by which the data are analysed. Then, the

relation between flooding and different mechanisms and charac-teristics, i.e. rainfall, sea level, topography, and drainage systemtype, are analysed and discussed. After these sections, three severeand recent flood events are described and discussed. A flood hazardmap from City of Malmö is discussed. Finally, after a summary ofthe most important results, potential strategies for Malmö anddata reliability are discussed.

2. Study site and data

The city of Malmö, Sweden, was selected as study site since, inthe Scandinavian context, Malmö is a large city, where there havebeen several flood events in recent years. Like in many other cities,the more densely built areas have sewer systems where stormwa-ter and sewerage are drained with one single pipe (combined sys-tem), leading to high risk of basement flooding. The sewer systemin Malmö is representative for Scandinavian cities, with a mix ofcombined and separate sewers. Good quality data on flood extent,precipitation, topography, sewerage system, etc. are available.Malmö is also well known within the field of urban hydrology, asthe city was an early starter in the work with integrated watermanagement (Niemczynowicz, 1999; Stahre, 2008).

As the focus is urban, pluvial flooding, only urbanised areas ofMalmö were included in the study. A boundary of the study areawas drawn, restricted by the outer ring road in south and east,by the municipal border in northeast and by the coastline in northand west (Figs. 1 and 3).

In the following section, data are described that relates tourban, pluvial flooding, including meteorological factors (e.g. rain-fall depth and duration), hydrological factors (e.g. topography andwatercourses) and factors related to the drainage system (e.g. typeof system).

2.1. The urban landscape

Malmö is the third biggest city of Sweden with a population of320,000 (SCB, 2015), situated in the very south of the country(Fig. 1). The city of Malmö went through a rapid urbanisation frommid-19th century until the 1970s and again after the 1990s. Landbetween smaller villages that were located outside Malmö in1940, during the first period of urbanisation, is today (2017) urba-nised. Places like Limhamn, Bunkeflo, Fosie and V. Skrävlinge arenow well-integrated parts of Malmö and the population has dou-bled since 1940. The urban part of Malmö has an area of 7681 hec-tares, where approximately half of it is impermeable (SCB, 2016).

2.2. Geology and watercourses

Malmö is situated in a flat landscape where the highest eleva-tion is 37 m above sea level. The bedrock consists of limestone,which generally is covered with clayey till (Länsstyrelsen Skåne,2016). The Riseberga Brook runs from south to north in the easternpart of the city connecting to the Sege Brook short before it reachesthe sea north of Malmö. The two brooks can be seen in some of theoldest maps of Malmö (Provincial map, late 1600s; Biurman, 1752).In addition to these, presently open brooks, a watercourse in west-ern Malmö is shown on the old maps, reaching from Bunkeflo insouthwest to Slottsstaden. A map from 1940 shows that (probably)the same watercourse was still noticeable at this time, as well as awatercourse reaching from Mariedal/Kulladal in south to Pil-dammsstaden. None of these watercourses is visible in the urbanlandscape anymore, as they have been drained in pipes. A canalsurrounding the inner part of the city tracks back in history to atleast late 17th century and is culturally an important landscapefeature in the city. Many combined sewerage overflows (CSOs) pol-lute the canal during rainfall. There are no lakes in Malmö.

Fig. 2. Location of rain gauges. Malmö main sewer system and wastewatertreatment plant (WWTP) is also shown.

Fig. 1. Malmö and the neighbouring cities of Lund, Helsingborg, and Copenhagen(Denmark). Vedbæk (Denmark), and Höllviken are also marked on the map as theyare mentioned in the results section. The red square marks where Malmö is situatedin northern Europe (small map). The study area is also marked (main map).

Table 1Values for coefficients in Eq. (1) according to Hernebring et al. (2015) based on datafrom 1980 to 2013, Malmö.

k11 k12 k21 k22 k31 k32

6.555 0.38 �0.7319 �0.0157 �2.1386 �0.1958

J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67 53

2.3. Climate, precipitation and sea level

Situated in northern Europe, Malmö has a temperate climate.Intense rainfall is most common during late summer (Gustafssonet al., 2010), when humid air from the sea reaches the warm land,while stormy weather with extreme waves and water levels ismost common during autumn and winter (Hanson and Larson,2008). The maximum hourly rainfall is 26.1 and 53.4 mm, for 10and 100 years return period respectively (Hernebring et al.,2015). The mean annual precipitation is 605 mm.

The main flood events appear primarily during the late summermonths (July–August). In Malmö, snowmelt is not an importantmechanism behind flooding.

Rainfall data were collected from SMHI (Swedish Meteorologi-cal and Hydrological Institute) and VA Syd, the local water utility.SMHI has one station in Malmö (Station A) and VA Syd has eightstationary stations (MA01 Turbinen Pumping Station, MA02, Lim-hamn Strandgatan, MA03 Augustenborg Ystadvägen, MA04 Dju-padalsskolan, MA05 Bulltofta Water Work, MA06 Hammars ParkPumping Station, MA07 Bellevue Vikingagatan and MA08 HöjaHögatorpsvägen). Fig. 2 shows the locations of these stations. Sealevel data were collected from the SMHI station Klagshamn, 10km south of Malmö, and the SMHI station Barsebäck, 18 km northof Malmö, from November 1995 until December 2014.

SMHI measures volume of precipitation during 15 min while allVA Syd stations use tipping buckets with 0.2 mm bucket. Data fromVA Syd were converted to 5 min precipitation volume before fur-ther analysis. In 2015, Hernebring et al. updated the precipitationstatistics for Malmö based on data from 1980 to 2013. The statis-tics is based on the formula (Eq. (1)) suggested by Hernebring(2006), which is often used by practitioners in Sweden for designand modelling, where i intensity (l/s/ha) can be calculated from trrainfall duration (min), T return period (year), and six coefficientsk11–k32. The coefficients for the equation are given in Table 1.Hernebring et al. (2015) fitted a Log-Pearson type III distribution,assuming stationarity, to observed precipitation with durationbetween 5 min and 4 days.

i ¼ eðk11þk31=tr Þ � Tðk12þk32=trÞ � tðk21þk22�lnðTÞÞr ð1Þ

1 Semi-separate system is a separate system where the stormwater pipes areconnected to the combined system downstream. The stormwater is thereby led to thewastewater treatment plant due to lack of a local recipient.

2.4. Drainage system

The urban drainage can be divided into major and minor sys-tems, where the minor system reflects the constructed drainagesystem underground, while the major system reflects flow on

streets, in channels and streams as well as detention basins aboveground (Bengtsson et al., 1993). In Fig. 3, the major and minor drai-nage systems of Malmö are shown. The minor system is based on asketch of the main sewers by VA Syd (water utility company),while the major system was derived from a simple analysis ofthe topography where the flow from an eight-direction flow model(Jenson and Domingue, 1988) is accumulated into each cell. No val-idation of the analysis was done. The main sewer system followsthe main flow paths in the major system to a high degree. The mainpipelines are found along the stretch of Hylliediket in westernMalmö, Södervärnsstråket in southern Malmö, and the RisebergaBrook in eastern Malmö.

The current main drainage pipes closely follow the open water-courses like Hylliediket, Södervärnsstråket, and Pildammsdiketthat can be seen on a map from 1940. No parts of these water-courses are open anymore, except for the ponds Pildammarna,which are not connected to the drainage system. These water-courses do however still exist as major overland flow paths,according to the simple analysis of the topography.

Data on drainage system type (combined sewer, separate sewerand semi-separate1 system) were provided by VA Syd: one datasetfrom 2016 and one older dataset from an unknown year (probably2000–2010). Areas developed during early urbanisation are still con-nected to the combined sewer system today, except for the mostcentral area close to the canals, which was constructed with separatesystem. Until the 1950s, mainly combined sewer systems, with sew-erage, stormwater, and groundwater drainage in the same pipe, werebuilt in Malmö. From 1950s, separated sewer and stormwater sys-tems were built in newly developed districts. In Malmö city, 31%of the urban land has combined system, 5% has semi-separate sys-tem and 64% has separate system (Fig. 3). Within the inner ring road,where the city is more densely built, 40% of the area has combinedsystem, 7% has semi-separate system and 53% has separate system.

Fig. 3. Major and minor drainage systems of Malmö. The major system (overlandflow paths, dashed line) is here calculated by a simple analysis of the topography,while the minor system (grey line) consists of the main sewer pipes. Areas withcombined sewer system are shown in dark grey, semi-separate in mid-grey, andseparate in light grey. The study area is marked with a thin, dashed line. Thewastewater treatment plant (WWTP) is also shown.

54 J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67

Several areas with semi-separate system has been reconstructed toseparate system during recent years. In total, 4400 hectares are con-nected to the drainage system within the inner ring road, and further2600 hectares between the inner and the outer ring road (mostlyseparate system). According to Swedish law, the pipe system shouldbe designed so that flooding occurs less than once every ten yearsover a longer period. This corresponds to a rainfall volume of 16mm in 15 min, 26 mm in one hour, and 30 mm in two hours inMalmö (Hernebring et al., 2015). The system is nevertheless, accord-ing to Swedish design standard, designed to handle stormwater frommore extreme events in some places (Svenskt Vatten, 2016).

As the topography of Malmö is flat, pumping is needed in thesewerage system. The catchments of Turbinen and Rosendal, thetwo main pumping stations, include both separate and combinedsystems. Because of inflow from the combined system, they areaffected by rainfall. The portion of combined sewerage is biggerin Turbinen catchment compared to Rosendal catchment, andtherefore rainfall influences the flow more here. The pump capac-ity of Turbinen pst is 2.5 m3/s, Rosendal pst 2.7 m3/s, Spillepengenpst 1.6 m3/s, and from the harbour 1.5 m3/s. All these stations areconnected to the wastewater treatment plant Sjölunda and thetotal maximum capacity of the pressurised system is 8 m3/s. Intotal, the utility company owns 134 smaller and bigger pumpingstations.

There are several detention basins in Malmö for both stormwa-ter and combined sewerage. The total volume is �52,000 m3,where 9500 m3 of the volume is found in the combined system,13,500 m3 is found as open basins and 29,000 m3 as undergroundbasins in the separate stormwater system (VA Syd, 2009). The mainfunction of detention in the combined system is to reduce CSO tothe recipients.

2.5. Flood claims and reports

The general practice of insurance in Sweden is home insurance(‘hemförsäkring’), which a majority of homeowners have. Thehome insurance typically covers personal property, personal liabil-ity, and legal expenses, including flood related damage on theproperty. There are three categories of flood damages: via drainagesystem, overland flow into the building, and groundwater intru-sion. Some home insurances cover all three types, while othersonly cover leakage and surcharge of the drainage system. Flood

insurance is excluded from home insurances in some geographicalareas with higher flood risk. It is not possible for homeowners tosign a separate flood insurance in Sweden.

Persons and companies affected by a flood event can make floodclaims both to their insurance company and to the water utilitycompany. If the drainage system is found to be insufficient, thewater utility company covers the deductible for property owners.The water utility company also collects flood reports from otherproperty owners and covers claims from insurance companies.For the present study, insurance claims and reports of floodedproperties have been collected from the insurance company Länsförsäkringar Skåne (LF Skåne) (received 2015–03-24) and thewater utility company, VA Syd (received 2016–03-14). The LFSkåne dataset contains 9704 reported flooded properties in theregion Scania, Southern Sweden during a period of 28 years(1987–2015), where the first claim from Malmö is from March1994. LF Skåne covers 40–50% of the private insurance market inthe region. From this dataset, 3776 flood claims are from propertieswithin the study area and a thorough quality control was made toensure the right address of all claims. The VA Syd dataset includesin total 5868 claims and reports from Malmö for the periodbetween June 1990 and March 2016 (26 years), whereof 5502 arefrom within the study area. Both datasets contain flood dates andtherefore no adjustments of the dates are needed (see Spekkerset al., 2013b). There is no information in the two datasets thatmakes it possible to distinguish between floods of different kinds,like pluvial, fluvial or coastal flooding, to identify what part of thebuilding that was flooded, or to identify the direct cause of theflooding (e.g. through drains, pipes, windows, etc.).

In general, there is consistency between the data from LF Skåneand from VA Syd. The LF Skåne dataset includes insurance claimsfrom each affected insurance holder while the VA Syd datasetincludes maximum one claim or report per property connected tothe sewerage system. There are, as expected, more reports fromVA Syd for most events. The biggest difference between the datasetsare expected during coastal flooding, as it makes no sense for prop-erty owners to report such floods to the utility company, VA Syd. Thespatial patterns of flood claims from most of the severe flood eventsare found concurring for the two datasets. However, for the event onthe 26 August 1996, VA Syd received more than 900 claims, while LFSkåne only received 30 claims. LF Skåne has increased its number ofcustomers in Malmö during recent years, which can explain why thenumber of LF Skåne flood claims are much lower compared to floodclaims from VA Syd until 2006, while the number are more equalfrom 2007. For this reason, the dataset from LF Skåne is mainly usedfrom 2007. After 2007, the number of customers has increased with8% (2007–2014). In addition, the number of buildings connected tothe sewerage system, and thus VA Syd customers, might haveincreased somewhat within the time period of the study. Theincrease is thus limited as most buildings are constructed wherean old building has been torn down.

2.6. Topographic data and street maps

Data on topography in a 2 � 2 m grid were retrieved from theSwedish official digital elevation model (Ny Nationell Höjdmodell,NNH). The vertical error of the elevation model is 5 cm and thehorizontal error is 25 cm on average for open, hard surfaces(Lantmäteriet, 2015). Street maps were received from the Swedishnational mapping, cadastral and land registration authority.

3. Methodology

In this study, the main purpose is to analyse the physical mech-anisms and characteristics of urban, pluvial flooding. Through thisanalysis, the spatial distribution of flood risk related to these

J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67 55

mechanisms and characteristics are evaluated. According to Crich-ton’s Risk Triangle (e.g. Crichton, 2008), risk comprises hazard,exposure, and vulnerability. The flood claims are used as a measureof spatial extent and severity of different flood events, which canbe related to all three sides of the Risk Triangle. The vulnerabilityand exposure of individual households are not assessed as thestudy focuses on city-scale patterns.

Some of the registered flood claims include information on eco-nomic losses for the property owners. While Zhou et al. (2013)used such information as a measure of flood severity, the informa-tion was excluded in this study. It might be misleading to use theinformation as a proxy for flood severity, as the losses are relatedto the economic value of the insurance holders’ belongings. Thus,high-income areas might seem to be more easily flooded, indepen-dent on the actual flood risk in these areas. Furthermore, the infor-mation tells little about the person’s vulnerability to flooding.

The relation between rainfall and flooding are analysed, fol-lowed by an analysis of the impact from high sea level, topography,and type of drainage system. Three severe flood events and theconsequences for the city are then described. At last, results froma flood hazard model are compared with flood claim data.

3.1. Selection of events

Characteristics and mechanisms affecting flood extent and spa-tial distribution was assessed from a wide range of events. Firstly,three recent, severe flood events in Malmö were chosen fordetailed analyses: 5 July 2007, 14 August 2010, and 31 August2014. All of these flood events were caused by heavy rainfall. Therewere also severe flood events on 26 August 1996 and 9 August1999, but these events were excluded, as little information onthese events were available. Secondly, the biggest flood events,according to the VA Syd dataset, were selected for comparison.From these events, flood events that were highly localised to onepart of the city, like the flood event on the 3–4 August 2014, wereselected and analysed in a separate category. Thirdly, big rainfallevents were selected for analysis of rainfall events that did not leadto severe flooding. The eight rainfall events with most precipitationvolumes for 1 h, 6 h and 1 day were found and added to the list ofevents. The final selection of flood and rainfall events are presentedin Table 2 together with precipitation during the most intense 1, 6,and 24 h. Return periods for the 6-h precipitation volume, calcu-lated with Eq. (1), are also presented. According to Swedish designstandards, the sewerage systemmust have the capacity to handle a

Table 2Selected events, number of flooded properties reported to LF Skåne and VA Syd, maximumevent), return period for 6-h precipitation, and category of event: Severe flood events (a), Odid not lead to severe flooding (d). Flood claims reported to LF Skåne before 2007 are sho

Date Flood claimsfrom LF Skåne[no]

Flood claimsfrom VA Syd[no]

Max. 1-hprecip.[mm]

2001-08-07 (3) 1 15.92002-08-03 (12) 75 14.62003-05-24 (35) 105 8.22003-07-18 (30) 105 19.12006-08-26 (26) 121 17.22007-07-05 169 150 10.32007-07-22 1 1 9.22008-08-24 0 0 1.32010-08-14 148 210 20.82011-06-06 26 128 13.92011-06-23 4 1 10.82012-09-25 3 4 10.02014-08-04 67* 230 17.52014-08-31 2649 2109 30.1

* Includes claims from 2014-08-03 as the rain fell around midnight.

10-year rainfall event, which corresponds to �40 mm precipitationduring 6 h (Hernebring et al., 2015).

The selected events were categorised and assessed in fourgroups (Table 2): a) Severe flood events, i.e. the 2007, 2010 and2014 events (flooding in more or less all of the city), b) Other bigflood events (flooding of properties spread over bigger areas ofthe city), c) Highly localised flood events (big flood events thathit limited areas of the city), and d) Big rainfall events that didnot lead to severe flooding.

3.2. Rainfall analysis

Maximum rainfall volumes for each day were calculated fordurations from 15 min to 12 days with a moving window of thecorresponding duration. The total rainfall volume within a windowwas dated according to the last time step of the window, where themaximum 1-h rainfall can fall any time from 23.05–00.05, startingthe day before, until 23.00–24.00 during the date of interest.Intensity-duration curves were prepared and compared withintensity-duration frequency (IDF) curves from Hernebring et al.(2015), as mentioned earlier. Confidence intervals (95%) were con-structed with the assumption that the mean precipitation for eachduration is normally distributed.

3.3. Spatial analysis of flood claims

For spatial analyses of the flood claim data, a 50 � 50 m gridwas positioned over the city and each grid cell was assigned floodedor not flooded for every flood event, where flooded was assigned tocells with one or more flood claims reported for an address withinthe cell and not floodedwas assigned to cells with no reported floodclaims. Similarly, each grid cell was assigned one of the systemtypes separate, semi-separate or combined, given by the maximumcover of drainage system type in the cell, as well as distance tothe major flow paths. In total, there are 35,746 cells, where�13,200 has separate system, �5800 has semi-separate system,�9700 has combined system, and �7200 are outside the areas con-nected to sewerage system (according to the old dataset, seeSection 2.4).

The relation between drainage system type and flooding wasanalysed for different rainfall events. It is assumed that buildingsconnected to the combined system is more vulnerable to floodingthan buildings connected to the separate and the semi-separatesystem. Toilets and sinks are often connected by gravitational flow

precipitation during 1 h, 6 h, and 1 day (average of all stations functioning during thether big flood events (b), Highly localised flood events (c), and Big rainfall events thatwn in brackets, as these are considered more uncertain.

Max. 6-hprecip.[mm]

Max. 1-dayprecip.[mm]

Return periodfor 6-h precip.[years]

Category

22.3 24.3 1.4 d22.8 34.4 1.5 b8.2 14.3 0.04 c30.0 32.0 3.9 b20.1 20.2 1.0 c39.5 70.8 10.2 a31.1 37.9 4.4 d7.2 39.0 0.03 d50.8 55.3 24.3 a15.0 15.0 0.3 c28.5 31.0 3.3 d12.2 15.7 0.2 d19.1 27.3 0.8 b83.0 96.0 134.5 a

Table 3Number of flooded properties reported to VA Syd and LF Skåne for the biggest eventsand suggested cause of the flood events. Flood claims reported to LF Skåne before2007 are shown in brackets, as these are considered more uncertain.

Date Flooded properties [no] Cause

VA Syd LF Skåne

2014-08-31 2109 2649 Rain1996-08-26 882 (30) Rain*

2014-08-04 230 67** Rain2010-08-14 210 148 Rain2007-07-05 150 169 Rain1999-08-09 144 (7) Rain*

2003-05-24 105 (35) Rain (local)2003-07-18 105 (30) Rain2011-06-06 128 26 Rain (local)2006-08-26 121 (26) Rain (local)2002-08-03 75 (12) Rain

* Precipitation data are only available from VA Syd station MA01 Turbinen pstand at SMHI station A. At Turbinen station, 38.6 mm and 22.0 mm were measuredon the 26 August 1996 and 9 August 1999, respectively, while only 7.6 mm and 0.3mm were measured at SMHI station A on the 26 August 1996 and 9 August 1999,respectively. In both cases, the flood claims were reported from large parts of thecity, indicating that the floods were caused by heavy rainfall. Sea level was low inboth Klagshamn and Barsebäck.** Includes claims from 2014-08-03 as the rain fell around midnight.

56 J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67

(no pumping) to the sewers, making the basements vulnerable tobackwater flow from the combined sewerage pipe during heavyrainfall.

The effect of catchment scale topography on flooding was anal-ysed as the relation between distance to the major system andreported flood claims per hectare. Cells without buildings wereexcluded from the calculations as flood claims only are reportedfor properties. There are roughly the same number of addressesper hectare near the major flow paths as further away from them(762 claims per hectare <100 m from the major flow paths and716 claims per hectare >100 m away). VA Syd has approximatelyone customer for each address, indicating that the dataset is notbiased in relation to distance to the major system. The spatial dis-tribution of LF Skåne’s customers is unknown, but there is no rea-son to believe that they should live closer to the major system thancustomers from other insurance companies.

3.4. Description of flood events and their effects on the city

The three severe flood events (2007, 2010, and 2014) weredescribed in terms of how they affected the city, including the sew-erage system. The main source of information was the insuranceclaims, information collected through personal contact2 with VASyd, the utility company in Malmö, and information from unpub-lished reports written by VA Syd after the flood events. The spatialdistribution of flood claims was analysed together with data aboutthe main sewer system and the topography of the city. The topogra-phy of flooded areas as well as a map from 1940, showing originalpath of watercourses before urbanisation of Malmö, were studied.

4. Results and discussion

4.1. Relation between rainfall and flooding

The main cause behind the biggest flood events in Malmö isheavy rainfall (Table 3). The events in the table includes 76% and83% of all claims reported by VA Syd and LF Skåne, respectively.As the flood claim data do not indicate causality for each floodevent, this was derived from study of precipitation data (IDFcurves), spatial pattern of the flood claims, and sea level data. Afew flood events were caused by highly localised rainfall, asmarked in the table, where the spatial extent both of the floodingand the rainfall were limited. Most flood events are noted in latesummer months (July, August), which indicates that they arecaused by convective rainfall. Out of the 64 events with at least 5flood claims reported, 77% happened during June, July or August(June 16%, July 23%, August 38%). It cannot be assured that all floodclaims during a certain day have the same cause, as there mighthave been a functional failure in the sewerage system at the sametime.

During the six most intense hours of the 2014 event, approxi-mately 3.2*106 m3 of precipitation fell over Malmö city(Hernebring et al., 2015). The minor, ordinary drainage systemcould only handle a small part of this (separate system: 0.2*106

m3; combined system: 0.25*106 m3), while 1.2*106 m3 infiltratedduring these 6 h and about half of the total volume ran as overlandflow to low-lying areas (1.5*106 m3). It took approximately fourdays before all low-lying areas were drained from inundated water(ibid.). The analysis shows that such extreme events cannot behandled only via pipe engineering, but has to be dealt with usingappropriate spatial planning and urban design.

2 The second author worked for VA Syd for several years, hereunder during andafter the 2014 event, where she made investigations of the pipe system (on site,through reports from property owners and insurance companies, as well as bymodelling).

4.1.1. Severe flood events (a)In recent years, there were three severe flood events: 2007 (5

July), 2010 (14 Aug) and 2014 (31 Aug), see Table 2. They wereall caused by heavy rainfall distributed over the entire city. Ascan be seen in Fig. 4a, the measured rainfall during the three eventshad a return period between 10 and 100 years for most durations.The 2007 event stands out with shorter return period for durationsbelow 6 h, and the 2014 event was heavier than a 100-year eventfor durations between 3 and 16 h. These three, severe events,and the effects of them, are discussed in detail in Section 4.5.

4.1.2. Other big flood events (b)Three other big flood events were selected (3 Aug 2002, 18 July

2003, and 4 Aug 2014), see Table 2. These events flooded big partsof the city. The damages were, however, not as widespread as dur-ing the three severe flood events in 2007, 2010 and 2014. All thebig events had reached their highest return period for shorter dura-tions (up to 4–6 h), see Fig. 4b. From this figure, it can also be seenthat these floods were caused by short, intense rainfall after severaldays with more or less persistent rainfall. The event on the 4 Aug2014 was, according to VA Syd flood claim data, as extensive asthe severe 2007 and 2010 events, while the data from LF Skåneindicates a flood extent less than half. The reason for the differenceis unknown.

4.1.3. Highly localised flood events (c)Three highly localised flood events were selected, see Table 2.

An event on 24 May 2003 affected areas with combined systemmostly in the western parts of Malmö. The same areas were againhit on 26 August 2006, while an event on 6 June 2011 mostlyaffected areas with combined system in eastern parts of the city.As expected, most rainfall was measured at stations close to themost affected areas. On 24 May 2003 and 26 August 2006, thehighest intensities were measured at Limhamn and Turbinen inwest, while the highest intensities on 6 June 2011 were measuredat SMHI A and Höja in east (for location of precipitation stations,see Fig. 2). In Fig. 4c, average precipitation from the two stationswith highest intensities are shown (‘‘Top 2”). The rainfall on 26August 2006 was more intense than for the two other events(Fig. 4c). The lowest intensities were measured on 24 May 2003.

2011-06-23

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Fig. 4. Intensity-duration curves for all nine rain gauges in Malmö, average forstations in function with confidence interval (95%) reflecting the spatial variationbetween the gauges. Note the different scale in figure a. Comparison with 10-, 50-and 100-year rainfall statistics from Hernebring et al. (2015). a) Rainfall that led tosevere flooding in more or less the entire city. b) Rainfall events that led to floodingof properties spread over bigger areas of the city. c) Rainfall events that led toflooding in limited areas of the city. d) Big rainfall events that did not lead to severeflooding. Figure c includes average precipitation from the two stations with highestintensities (‘‘Top 2”), as these events were highly localised.

3 Personal communication with Helén Nilsson at Länsförsäkringar Skåne.

J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67 57

There were, however, no significant difference in number of floodclaims.

4.1.4. Big rainfall events that did not lead to severe flooding (d)Five big rainfall events that did not lead to flooding were

selected for comparison (7 Aug 2001, 22 July 2007, 24 Aug 2008,23 June 2011, and 25 Sep 2012), see Table 2. These events wereall selected as they were ranked high on the list of events with highintensity for the 1-h, 6 h and 1-day durations. It is not possible to

make a clear distinction in rainfall intensity between these eventsand the three bigger flood events (3 Aug 2002, 18 July 2003, 4 Aug2014), as can be seen in Fig. 4d in comparison with Fig. 4b. Theintensities for shorter durations (<8–10 h) seem to be slightlylower, but the difference is small. However, the spatial distributionis more even for the events that did not lead to flooding, indicatingthat the bigger flood events had higher intensities in some parts ofthe city. Therefore, even if the registered rainfall is similar on aver-age, there might have been areas with more extreme precipitationduring the events in category b (other big flood events) that werenot captured by the metres.

4.2. Relation between sea level and flooding

As mentioned before, SMHI measures sea level 18 km north and10 km south of Malmö. The closest station is Klagshamn, south ofMalmö, but as the water level can differ at least up to 150 cmbetween the stations, data from both stations are useful for Malmö.The difference in level is an effect of the Saltholm-Malmö High, alarge-scale, first order mounded succession across the straitbetween Malmö and Copenhagen (Hansen, 1943; Bjerager et al.,2010). The water level in the strait is mainly governed by winddirection and stormy weather in the North Sea, leading to higherflood risk in most of Malmö when the water level is high in Bar-sebäck. During storms, waves might rise several metres higherthan the actual water level (Sweco, 2017). There is little influenceof tide.

There are 230 hectares of land in Malmö below +1 m, but nobuildings, roads or other infrastructure. Below +2 m there are intotal 580 hectares of land, including 50 hectares of housing andindustrial land, and 12 km streets and 3 km railroad. Only half ametre higher, below +2.5 m, there are in total 1400 hectares ofland, including 100 hectares of the city centre and 450 hectaresindustrial land. (Malmö Stad, 2008)

Out of the ten days with the highest water level measured atKlagshamn, flooding was only reported on one day, namely onthe 20 January 2007 (Fig. 5). On the same day, about 20 mm of rainfell over Malmö. Two flood claims were made to LF Skåne and therewere six claims or reports to VA Syd. All claims, except for one,came from properties close to the sea in Limhamn. It can be notedthat the sea level was even higher in Barsebäck the day before, buton this day, no flooding was reported.

Out of the ten days with the highest water level measured atBarsebäck, flooding was reported on five days, namely on the 1November 2006, when one property close to the canal was flooded,on the 27 and 28 November 2011, when four properties close tothe canal were flooded, and on the 6 and 7 December 2013, duringthe storm named ‘‘Sven” by SMHI. In Fig. 5 it can be noted that outof the 30 biggest flood events, there was only one day with high sealevel, namely on the 5 December 2013, also during storm Sven. Onthis day, 17 flood claims were made to the insurance company LFSkåne, while there were no claims or reports to the utility companyVA Syd. From the latter fact, it can be assumed that there was noflooding related to the sewerage system during the storm Sven.All of the flood claims to LF Skåne, except one, came from propertyowners along the coast.

Only for a few events during the period of this study, stormyweather seems to have caused flooding. This is expected, as onlyareas of minor importance are located lower than the water levelsmeasured during recent storms. The storm Sven was the only floodevent with more than 15 claims that were caused by storm. A fewstreets along the coast and harbour were also flooded. Storms haveso far caused much more wind related damage3, but with slightly

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Fig. 5. Daily maximum sea levels in Klagshamn (x-axis) and Barsebäck (y-axis)between November 1995 and December 2014 in light grey. Maximum sea levelduring the 30 biggest flood events are marked with dark grey. Maximum sea levelon the 1 November 2006 (cross), the 19 and 20 January 2007 (square), the 27 and 28November 2011 (plus), and the 5, 6 and 7 December 2013 (ring, the storm Sven) arehighlighted as they are of a certain interest (mentioned in the text).

Table 4Number of flood claims per 100 hectares that has been reported within a distance of100 m from the major system on 5 July 2007, 14 August 2010, and 31 August 2014 inMalmö (based on data from LF Skåne and VA Syd). Only grid cells (50 � 50 m) with abuilding are included in the calculations.

All events in thedatasets (20 years)

2007-07-05 2010-08-14 2014-08-31

<100 m 343 11 20 214>100 m 153 5 5 72

Fig. 6. a) Flood claims reported by LF Skåne and VA Syd during two of the threesevere flood events in Malmö (2010 and 2014). b) Flood claims reported by LF Skåneand VA Syd during all other events, except for two severe flood events (2010 and2014).

58 J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67

higher water level during Storm Sven, the railway tunnel underMalmö would have been flooded (Sweco, 2017). During the studiedperiod, coastal flooding did not affect many property owners, com-pared to pluvial flooding, in Malmö. However, there are examplesof more severe flooding from sea under current climate, like ‘‘theChristmas Storm” in 1902, with levels higher than +2 m (Sweco,2017). With rising sea level in the future, coastal floods are expectedmore frequently (SMHI, 2007).

4.3. Relation between topography and flooding

Areas within 100 m from the major system are more than twiceas affected by flooding, compared to areas further away (Table 4).During the severe flood events in 2010 and 2014, areas close to themajor system was even more effected by flooding (3.0–4.2 times),compared to areas further away. During such downpours, runoff isquickly directed towards low-lying areas, both through the pipesystem and by overland flow. In Malmö, like probably in mostother places, the main sewers (minor system) are located underthe main overland flow paths (major system), as they follow thetopography. The pattern of flood claims during extreme rainfallcan be noted on Fig. 6a, where claims from the 2010 and 2014events are shown. While Fig. 6a shows claims from the two severeflood events in 2010 and 2014, Fig. 6b shows claims from all otherflood events, including the severe, but slower, event in 2007. Theseclaims are not as clearly localised along the major system, butthere are still concentration of flood claims in some areas.

The number of flood claims per 100 hectares, as shown inTable 4, were calculated only for grid cells (50 � 50 m) where thereis a building, as only claims from buildings are included in the twoflood claim datasets.

4.4. Relation between drainage system type and flooding

Areas with combined or semi-separate system were moreaffected by flooding than areas with separate system for all flood

events in total as well as the three severe flood events indepen-dently (Table 5). During the 2010 and 2014 events, the combinedsystem was 3.8–4.2 times more affected by flooding compare tothe separate system. If the ‘‘old pipe system” (see Section 2.4) isused for the analysis, instead of the pipe system according to the2016 dataset, the combined system was as much as 6.3–6.8 timesmore affected by flooding compared to the separate system duringthese events. Similar figures are found if all flood events areincluded, indicating that the combined system is more exposedto flooding than the separate system. 70% of areas with separatesystem (in 2016) had a semi-separate or combined system before(according to older dataset). Many of these areas have been

Table 5Number of flood claims that has been reported per 100 hectares in areas with separate, semi-separate and combined sewerage system for the flood events on 5 July 2007, 14August 2010, and 31 August 2014 in Malmö (based on data from LF Skåne and VA Syd). System type as reported in 2016. Only grid cells (50 � 50 m) with a building are includedin the calculations.

Pipe system System area [ha](with building)

Total number offlood claims (all)

Flood claims per 100 hectares

All events in thedatasets (20 years)

2007-07-05 2010-08-14 2014-08-31

Separate 2654 1869 69.6 3.8 3.2 39.8Semi-separate 311 870 278.9 17.0 13.2 184.4Combined 1879 6523 347.0 8.7 12.3 165.6Total 4845 9278* 181.7 6.3 7.0 93.2

* 16 flood claims are reported from areas not connected to the sewerage system.

J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67 59

affected by flooding, which makes interpretation of the resultsdifficult.

The 2007 event shows a different pattern when it comes toflood exposure in combined system areas compared to separatesystem areas. The combined system areas were only slightly morethan twice (2.3 times) as severely affected by flooding during thisevent, compared to the areas with separate system. Almost thesame result is found when the old pipe system is analysed instead(2.4 times). One reason why the 2007 event differs from the otherevents might be the difference in flood causality, where continuousrainfall during the preceding weeks saturated the ground withwater. Flooding during this event was therefore less related to typeof drainage system.

The number of flood claims per 100 hectares, as shown inTable 5, were calculated only for grid cells (50 � 50 m) where thereis a building, as only claims from buildings are included in the twoflood claim datasets. There are more people living in areas withcombined system compared to areas with separate system (1.7times more addresses), which makes the data biased. However,the number of reported claims to VA Syd in combined system com-pared to separate system areas are even higher. There are 8.3 timesmore claims to VA Syd from areas with combined system com-pared to areas with separate system. The bias in the LF Skåne data-set related to combined/separate system is probably similar as forthe VA Syd dataset.

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Fig. 7. Accumulated rainfall (SMHI station A) for the three flood events in Malmö on5 July 2007, 14 August 2010, and 31 August 2014. The accumulation curve iscentred on the peak rainfall (maximum 15 min intensity) and the volume isaccumulated for 20 days before the peak and 1 day after.

4.5. Three severe flood events

In this section, an overview of three severe flood events inMalmö is given (5 July 2007, 14 August 2010, and 31 August2014). The overview is followed by descriptions of the conse-quences of the events.

During the rainfall event on the 5 July 2007, 89 mm rain fellbetween midnight and 19.45 in the evening (SMHI, station A).The ground was already almost saturated due to recurring rainfallduring the weeks before. This led to flooding all around Malmö and169 flood claims were registered at LF Skåne and 150 at VA Syd.Flooding was reported to have occurred in locations representedby 245 cells in the 50 � 50 m grid (from now on notated ‘‘x cellswere flooded”). Several areas outside Malmö, like the neighbouringcity of Lund (Fig. 1), were also flooded on the 5 July 2007, but thebiggest rainfall volume was measured in Malmö.

On the 14 August 2010, 66 mm rain fell between 06.00 and19.30 (SMHI, station A), out of which �50 mm fell during the last3 h. The number of flood claims sent to LF Skåne was 148 and toVA Syd 210. 270 cells were flooded. The rainfall event also led tosevere flooding in northern Copenhagen and 99 mm rainfall wasmeasured in Vedbæk (Fig. 1) (Olesen and Siewertsen, 2010).

The severe flooding on the 31 August 2014 is the biggest flood-ing event in Malmö so far, with 10 times more flooded cells com-pared to the 2007 and 2010 events. The flooding affected all

parts of the city. A total volume of 101 mm fell between earlymorning (02.15) and late evening (21.30) (SMHI, station A). Duringthe most intense hours (4.30–7.30), 71 mm fell. In total, 2649 floodclaims were reported to LF Skåne and 2109 to VA Syd, spread over2388 cells. Many properties within the same cells were flooded.The rainfall was even heavier south of Malmö compared to the cityof Malmö. The maximum rainfall was measured in Höllviken(Fig. 1), Vellinge municipality (168 mm in total, 118 mm during90 min). The rainfall covered a large area including Copenhagen,Denmark (Fig. 1). More details on the rainfall is found in a reportby DHI (2014a).

The maximum sea level during the three biggest flood events(2007, 2010, and 2014) was less than 0.5 m and thus not highenough to affect the flooding.

Accumulated rainfall for the three flood events are shown inFig. 7. During a period of 20 days before the 2007 event, about145 mm had fallen, while about 90 mm had fallen before the2010 event, and about 40 mm before the 2014 event. This is shownas the initial value on the left hand side. It can also be observedthat the rainfall during the 2007 event was more evenly dis-tributed in time, while the 2010 and 2014 events had more distinctpeaks of rainfall. The 12 days before the 2014 event was almost dry(12 mm). Intensity-duration curves for the three flood events areshown in Fig. 4a together with IDF curves with return period of10, 50 and 100 years from Hernebring et al. (2015). The 2014 eventwas the most intense for all durations less than 4 days (�100 h),except for the 2 h duration, where the 2010 event was slightlymore intense. For durations longer than 4 days, the 2007 eventwas the most intense. The 2010 event was more intense than the2007 event for durations shorter than 8 h.

Table 6Consequences of the severe flood events in Malmö in 2007, 2010, and 2014. Effect on areas with combined system, compared to areas with separate system, spatial distribution offlooding, and effects on the pipe system, pumping stations, and detention basins and recipients.

Flood event Flood magnitudewithin combinedsewer area,compared toseparate sewerarea

Spatialdistribution offlooding

Effects on pipe system* Effects on pumping stations* Effects on detention basins andrecipients*

2007-07-05 2.3 times more Evenly spreadover the city

A lot of extraneous waterfrom intrusion ofgroundwater due to highgroundwater table

Spillepengen pst overloaded.Led to flooding of properties atSegevång and Valdemarsro

Problems with manual flood gate controlat Toftanäs basin led to flooding ofsurrounding buildings

2010-08-14 3.8 times more More severealong mainpipe network

Several main pipes in theseparate sewerage systemwere filled with extraneouswater

Spillepengen pst overloaded No reports

2014-08-31 4.2 times more Almost theentire city.More severealong mainpipe network

Overland flooding in severalparts of the city. Inundatedwater entered separatesewerage pipes throughbasements

Turbinen pst was partlyflooded and CSO could notfunction as intended. Thisprobably exacerbated floodingin upstream areas

Most basins were overloaded, which ledto inundation above/around them. Highwater levels in Sege Brook led todecreased CSO capacity and therebyflooding upstream

* Source: Unpublished reports written by VA Syd after the flood events.

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In Table 6, an overview of the consequences of the three severeflood events are given. These consequences are presented in detailin the following Section 4.5.1–4.5.3.

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Fig. 8. Flood event 2007, Malmö. a) Cells where flooding was reported are markedwith black dots. Rainfall volume for 1h (light blue circles), rainfall volume for 6h(mid-blue circles) and rainfall volume for 1d (dark blue circles). Areas of specialinterest like Kronprinsen (KrP), Toftanäs (Tof), Segevång (Seg), Valdemarsro (Val),and Spillepengen pumping station (Sp pst) are marked on the map. b) Rainfallvolumes during and before the 2007 event in Malmö measured at eight rain gauges.

4.5.1. Consequences of 2007 flood eventIn Fig. 8a, cells where flooding was reported during the 5 July

2007 are marked with black. The flooded cells are spread all overMalmö, with higher density in areas with combined system. Therainfall was most intense in eastern Malmö, but the spatial varia-tion was small. As can be seen in Fig. 8b, all durations under fourhours represent less than a one in ten years event. During the mostintense hour that day, �10 mm rain fell. For durations longer thanone day, the return periods were longer – from ten years up to fiftyyears for eight days duration. Since the soil was almost saturatedafter the long, rainy period, the infiltration capacity was reducedand the groundwater table was high. This led to worse floodingthan would normally be expected from a 1-day rainfall with simi-lar intensity as during the 2007 event. Many properties inupstream parts of the system were affected.

During the investigations after the 2007 flood event, it wasfound that approximately 50%4 of the basement flooding was dueto groundwater intrusion through the foundation walls because oflong-lasting rainfall. The number of reported flooded propertiescaused by overload of the pipe system was small in relation to theincident. Some areas, like Kronprinsen (Fig. 8a), a high-rise buildingin central Malmö, were affected by big amounts of extraneous waterinto the separate sewerage pipes. Although the high-rise building isconnected to the separate system, it is situated close to the down-stream part of the combined system, just 700 meter south of Tur-binen pumping station. An investigation after the flood eventshowed huge amounts of extraneous water in the separate seweragepipes at Kronprinsen as well as in other areas. The extraneous waterprobably came from intrusion of groundwater into the pipe due tohigh groundwater table.

In Toftanäs in eastern Malmö (Fig. 8a), the manual control of afloodgate was covered with water during the long-lasting rainfalland the open detention basin could therefore not be drained, asit was impossible to open the gate. Because of this, many buildingsin the surrounding area were flooded. After that, the control gatewas redesigned and reconstructed.

4 Unpublished reports written by VA Syd after the flood events.

A couple of buildings in Segevång were severely flooded withbig damage (Fig. 8a). The area is connected to a separate systemand located 1000 m upstream of Spillepengen pumping station.

The locations of the rain gauges are found on the map in Fig. 2. (For interpretation ofthe references to colour in this figure legend, the reader is referred to the webversion of this article.)

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The pumping station was overloaded during the rainfall event dueto reduced capacity of the pumps in combination with heavy loadfrom the combined sewerage system. One of the pumps was notworking and another pump was stopped during the critical phaseof the rainfall due to an error in the control system. This led toflooding of properties at Segevång, and consequent claims foraround 1.5 MSEK (�160 k€).

The failure of Spillepengen pumping station also led to floodingof at least 30 houses in Valdemarsro (Fig. 8a), an area from the1920s, with combined sewerage system, located northeast of thecity centre.

4.5.2. Consequences of 2010 flood eventThe flooding on 14 August 2010 was spread over all of Malmö,

but some areas were more severely flooded than others (Fig. 9a).One highly affected area was Södervärn, an area where there usedto be a small watercourse. Along a similar watercourse in anotherpart of Malmö, Limhamn, 25–30 properties were reported flooded(Fig. 9a). Heavy rainfall in combination with wet soils after a rainyAugust led to high amounts of extraneous water in the separatesewerage system. Still, the flooded areas were more concentratedalong flow paths of the major system, compared to the 2007 event.

There was no clear relationship between local rainfall measuredat the different rain gauges and the number of flooded properties

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Fig. 9. Flood event 2010, Malmö. a) Cells where flooding was reported are markedwith black dots. Rainfall volume for 1 h (light blue circles), rainfall volume for 6 h(mid-blue circles) and rainfall volume for 1d (dark blue circles). Areas of specialinterest like Södervärn (SV), Limhamn (Lim), Regementsgatan (Reg), Roskildevägen(Ros), Turbinen pumping station (Tu pst), Söderkulla (SK), Annetorpsvägen (Ann),Sövdeborgsgatan (Söv), Erikslust (ErL), Vittskövlegatan (Vit) and Gyllebogatan (Gyl)are marked on the map. b) Rainfall volumes during and before the 2010 event inMalmö measured at eight rain gauges. The locations of the rain gauges are found onthe map in Fig. 2. (For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

nearby. For instance, the rainfall volume was small at Bellevue(Fig. 9a, third station from the left), compared to other stations,while areas close to this rain gauge were still severely flooded.Rainfall for all stations are presented in Fig. 9b.

From the reported basement floods, some areas can be distin-guished. Along the Södervärn pipeline, which has a separate sys-tem, large volumes of extraneous water caused flooding in thearea of Regementsgatan and Roskildevägen (Fig. 9a) along the east-ern branch of the main sewerage pipe toward Turbinen pumpingstation. This main pipe ran full all the way from Söderkulla tothe Turbinen pumping station (�4500 m).

In western Malmö, the main separate sewerage pipes (diame-ter: 500 & 600 mm) ran full all the way from Annetorpsvägen toTurbinen pumping station (�4500 m), probably due to misconnec-tion or leaking pipes. This caused flooding of 30 basements alongthe pipeline. The combined system in the area also got flooded.Around 30 houses, which were connected to the combined pipebetween Sövdeborgsgatan and Erikslust, were flooded (Fig. 9a),since some parts along the main pipeline in Vittskövlegatan–Gyllebogatan are undersized.

The Spillepengen pumping station was overloaded due to aproblem with the inlet pipe just like during the 2007 event. Thisproblem was resolved after the 2010 event.

4.5.3. Consequences of 2014 flood eventAlmost the entire city was severely affected by the flood event

in 2014. The rainfall was heavy all over Malmö and most intensein the central areas (Fig. 10a and b). Like both the 2007 and the2010 events, the flooding in 2014 affected more or less all areasof the city. The spatial pattern of flooding during the 2014 eventwas more similar to the 2010 event than the 2007 event, as someareas were clearly more affected than others. The flood claimscame from property owners in more or less the same areas as dur-ing the 2010 event, although many more properties were affected.The severely flooded area in western Malmö (Limhamn) followsthe former open watercourse, like in the 2010 event, indicatingthat stormwater rapidly flows towards the former open water-course during intense rainfall. This was also true for the floodedareas around Södervärn (in the centre of Fig. 10a).

A large part of the basement flood reports (67%) came from thecombined system, which is not designed to handle such a heavyrainfall during a short time. Compared to the 2007 and 2010 event,several parts of Malmö were flooded not only in basements, butalso affected by overland flooding, which was the situation atRönneholm, Holma, Söderkulla, Heleneholm, Lindängen and Sofie-lund (Fig. 10a). Most detention basins, both open and underground,were overloaded which led to local inundation above and aroundseveral of them.

At some places, inundated water entered separate seweragepipes through basement floor drains in areas with separate systemas well as through small holes in manhole covers. From Söderkullato Turbinen pumping station (Pildammsdiket), and fromRosenvång to Turbinen pumping station (Hylliediket), the assess-ments showed that the water level in the manholes of the separatesewerage system had reached the ground, or near the ground. Thisled to basement flooding mainly in Djupedal, Ribersborg, Sofielund,Fågelbacken and Slottsstaden (Fig. 10a). Kronprinsen and the mainhospital were also flooded from the separate sewerage system.

The main pumping station, Turbinen (Fig. 10a), was partlyflooded during the heavy rainfall. Problems with an inlet gatecaused a huge pressure on the station and problems with a gatefor combined sewerage overflow (CSO) led to flooding of thepumping hall. The reduced pumping capacity probably had a minoreffect on the flooding situation in the upstream system, as thepumping station is designed for 2.5 m3/s and the inflow was extre-mely high. The inflow is unknown, but if half of the upstream area

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Fig. 10. Flood event 2014, Malmö. a) Cells where flooding was reported are markedwith black dots. Rainfall volume for 1 h (light blue circles), rainfall volume for 6 h(mid-blue circles) and rainfall volume for 1d (dark blue circles). Areas of specialinterest like Södervärn (SV), Limhamn (Lim), Turbinen pumping station (Tu pst),Söderkulla (SK), Heleneholm (Hel), Lindängen (Lind), Sofielund (Sof), Rosenvång(RoV), Djupadal (Dju), Ribersborg (Rib), Fågelbacken (Fåg), Slottsstaden (Slo),Kronprinsen (KrP), and the main hospital Skånes Universitetssjukhus (SUS) aremarked on the map. b) Rainfall volumes during and before the 2014 event in Malmömeasured at nine rain gauges. The locations of the rain gauges are found on the mapin Fig. 2. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

Fig. 11. Flood hazard map (upper left, a). Comparison of flood claims and hydraulicoverland flow simulation (maximum depth) for the 2007 (upper right, b), 2010(lower left, c), and 2014 (lower right, d) events. Streets and buildings are removedfrom the figure to ensure integrity for affected individuals.

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contributed to the flow, it was�150 m3/s during the six hours withmost intense rainfall. The problems with reduced CSO capacity wasprobably a bigger problem, as it serves as a security during heavyinflow. Moreover, the inflow to the pumping station was obviouslylimited by pipe and inlet capacity upstream in the system. It tooktwo days for the water level to decrease, and another two daysbefore the pumping system operated at normal capacity. No prob-lems were reported from any of the other pumping stations duringthe 2014 event.

Some areas in Rosendal catchment got flooded due to highwater level in Sege Brook (north-eastern Malmö), which decreasedthe CSO capacity. The high water level might even have causedbackflow from the brook into the sewerage system.

In summary, most flood claims came from areas with combinedsewer system in central Malmö. The flooding was most severealong the main pipelines following former open watercoursesand in some areas the flooding was aggravated by local, technicalproblems.

4.6. Flood hazard modelling

The City of Malmö made a thematic supplement to the compre-hensive master plan, ‘‘Plan for water in Malmö”. During this work,a flood hazard model was prepared to investigate the conse-

quences of extreme rainfall in central Malmö. This two-dimensional, hydrodynamic model includes overland flow andinfiltration, but not pipe flow. Instead, the simulated rainfall (a100-year event according to statistics on local data) is reducedwith the 10-year rainfall intensity, corresponding to the drainagecapacity of the piped system. The model is developed in Mike 21(software by DHI) with a 4 � 4 m rectangular grid based on Cityof Malmö’s lidar data. (DHI, 2014b) In this study, the model is com-pared with flood claim data to discuss how modelling results andhistorical data can be used separately or together.

Fig. 11 shows maximum inundation depth from an overlandflow simulation for a 100-year rainfall event (a). On top of themodel simulation, the minor (main sewers) and major system(overland flow paths) are shown, as well as flood claims from VASyd and LF Skåne for the 2007 (b), 2010 (c), and 2014 (d) events.In the northwestern corner, the overland flowmodel indicates highflood risk. The minor and major systems pass this area. The over-land flow model gives a good indication of where intense rainfallevents lead to severe flooding (Fig. 11c and d). However, the modelcannot predict the spatial extension of flooding during longer rain-fall events, like the 2007 event (Fig. 11b), probably because of lim-itation in interaction between runoff, infiltration and subsurfaceflow dynamics. The model points out many, small areas where

Fig. 11 (continued)

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overland flooding might happen. Most of these areas are not indi-cated as problematic by the flood claim data, which might haveone of two causes. Either information is missing, as no insuredproperties are located in the area, or the model incorrectly indi-cates higher flood risk than expected. It is difficult to point out con-tinuous areas with especially high risk from the flood hazardmodel solely. These areas, with higher exposure to flooding, aremore clearly visible from the flood claim data. It must be kept inmind that the flood claim data show affected properties, oftenflooded from the sewerage system, while the flood hazard modelshows flooding from overland flow. The model can show streets,railroads, etc. with flood risk, which are not included in the floodclaim data, while the model does not show basements that mightget flooded as they are connected to overloaded parts of the pipesystem.

The present datasets contribute with valuable information toflood hazard mapping. For cities that have been saved from severefloods, and therefore have little data, flood hazard mapping can bemade e.g. through two-dimensional flood inundation modelling ofoverland flow (Fewtrell et al., 2008) or through coupled modellingof major and minor system (Maksimovic et al., 2009). However,modelling should be carried out with caution if data are unavail-able for validation (Mark et al., 2004; Mignot et al., 2006). In gen-eral, hydraulic modelling can give a rough estimate of the effects ofextreme events. Beven (2008) divides model construction intothree parts: 1) a perceptual model, which is how we understand

the physical system, including the full complexity of the system,2) a formal model, which is the mathematical description of thesystem, and 3) a procedural model, which is the model that willbe run on the computer using numerical solution techniques. Allthree parts of model construction are associated with problemswhen the model is used to simulate extreme events. Extremeevents are uncommon, and therefore the modeller’s understandingof how the system behaves during extreme situations is limited.Inundation models are still often used to predict pluvial floodingin cities (Lekuthai and Vongvisessomjai, 2001; Mark et al., 2004;Zerger and Wealands, 2004; Mignot et al., 2006; Fewtrell et al.,2008; Maksimovic et al., 2009; Schmitt and Thomas, 2009). Mea-sured data to calibrate models for spatial distribution of inundationis often missing (Hunter et al., 2007), especially data from extremeevents. Only a few studies have used insurance data for calibrationand validation. Chen et al. (2009) validated a GIS-based, urbanflood inundation model for a small area in Memphis, Tennessee,US, with inundation depths in flooded rooms in the Law SchoolLibrary. Zhou et al. (2013) validated an inundation model for Aar-hus, Denmark, with spatial distribution of flood claims. Both stud-ies conclude that flood claims are valuable for calibration ofinundation models with respect to frequency and location of flood-ing, especially as other kinds of data for calibration are rare. Yuet al. (2016) used crowd-sourced data from a single, extreme stormevent in a similar validation process and found good agreementbetween the model results and reported flooding.

This study has shown that an overland flow model can give agood approximation of flood extent from intense rainfall. However,it does not capture the spatial distribution of flooding after a longperiod of wet weather, where the pluvial flooding is less related tothe minor and major systems. The slower dynamics of such anevent requires a different kind of model, where groundwaterdynamics is better taken into account. It also underestimates theseverity of flooding along major flow paths during intense rainfall.The studies on dual-drainage modelling of urban flooding thatSchmitt et al. (2004) carried out, point out that flow simulationsrequire a highly detailed, physical representation of surface areato be accurate. This kind of data is usually not available, and there-fore restricts the use of model results for practical application. Inthe present study, historical data has been available and useddirectly to describe mechanisms and characteristics related to plu-vial flooding, without support of modelling. However, it is not pos-sible to test future scenarios, e.g. related to climate change(Paludan et al., 2010), or interaction with other systems, e.g. acces-sibility for emergency service (Coles et al., 2017), without a model.For such studies, a model calibrated and validated with measuredinundation data, like flood claims including one or several extremeevents, are preferably used.

4.7. Mechanisms and characteristics of flooding in Malmö

4.7.1. RainfallFlooding in Malmö is mainly pluvial. The rainfall during the

three biggest flood events (2007, 2010, and 2014) was heavy allover the city. Other, minor floods were caused by highly localisedrainfall, where only a certain part of the city was affected. A fewof these events were however severe in the affected area. Theextent of flooding is related to both rainfall intensity and total vol-umes. This was made clear by the flooding on 31 August 2014,which was bigger than all other events in flood severity, as wellas, in rainfall intensity and rainfall volume. However, the correla-tion between flood severity and rainfall is not linear. This has beenshown in other studies as well (Spekkers et al., 2013a,b; Zhou et al.,2013; Zhang et al., 2014; Berghuijs et al., 2016). ten Veldhuis et al.(2009) showed that the most important contribution to flooding inHaarlem, the Netherlands, was gully pot blockage (79% of reported

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complaints), while the contribution of heavy storm events wasmuch lower (5%). The present study shows that the situation inMalmö is different, with rainfall as the dominating driver for flood-ing. ten Veldhuis et al. do not explain why gully pot blockage issuch a big problem in Haarlem. The results from Torgersen et al.(2015) showed a correlation between extreme rainfall and extentof urban flooding in terms of insurance claims. The most costlyevents occur during the long intensive rainfall rather than shorterrain, which is different from Malmö. Zhou et al. (2013) discussedpossibilities to pursue the relation between precipitation and plu-vial flooding in depth. Such possibilities were noted during thisstudy as well. As pluvial flooding is caused by short, very intensedownpours or convective storms, the spatial distribution of theprecipitation is highly uncertain and might represent a major chal-lenge for such analyses (Niemi et al., 2017). Spekkers et al. (2013a)show that rainfall alone cannot explain flood severity. Therefore,other mechanisms and characteristics are important to understandflood extent. This study contributes to a better understanding ofsuch factors.

Bengtsson and Milotti (2010) have made a detailed descriptionof extreme precipitation in Gustafsson et al. (2010) have identifiedthe atmospheric patterns behind extreme summer precipitation insouthern Sweden. Climate change will lead to more urban, pluvialfloods in Sweden in the future, as examined by Semadeni-Davieset al. (2008a,b) and Berggren et al. (2012).

4.7.2. TopographyThe severity and the location of flooding within Malmö are,

apart from the rainfall, governed by the topography: the flatground and the flow paths towards and along old watercourses.The most severe flood events were caused by rainfall evenly dis-tributed over the whole city. From this study, it is clear that formerwatercourses that have been integrated in the underground pipesystem, will act as aboveground watercourses during severe floodevents. Therefore, it is not recommended to develop new urbanareas above such watercourses. The spatial pattern of flood claimscorrelates well with the main pipelines. Overland flow follows flowpaths towards the old watercourses and thus towards the mainpipelines. During some large rainfalls, like the 2014 event, manyparts of Malmö are unaffected by flooding. Areas close to the mainsewers and the major overland flow paths (<100 m) are more thantwice as affected by flooding as upstream areas. For rainfall eventslike the 2007 event, when consistent rainfall during a long periodmade the soil wet and the groundwater level increased, and forsmaller events, the flooding is more evenly spread over the city.In this case, also upstream areas are affected and the concentrationof flood claims from property owners along the main sewers is lessnoticeable.

4.7.3. Drainage system typeAnother decisive factor behind flooding is the type of sewer sys-

tem. In general, for all flood events, areas with combined seweragesystem are more severely flooded and more vulnerable to base-ment floods than areas with separate sewerage system. On aver-age, the combined system was five times more flooded. Thereason for this is the direct connection between sewer and instal-lations in basements in the combined system. In most places, thesystem is not designed for more than a 10-year event. As manyhouse owners do not have a backwater valve to protect their base-ments, the risk of getting flooded in areas with combined systemare much higher than in areas with separate system, where surfacerunoff or groundwater intrusion in principle is a prerequisite forflooding. Combined and separate systems are equally threatenedby overland flooding, and during extreme events, even the separatesewerage pipes can get flooded due to inflow from inundatedareas. On top of the higher flood risk in areas with combined sys-

tem, the wastewater in the combined systemmakes the health riskhigher compared to flooding from separate stormwater pipes. Asan example, an infection from rats during clean-up of a floodedbasement lead to a man’s death in Copenhagen after a severe floodin 2011. It is well known among sewage engineers that combinedsewers are more vulnerable to extreme rainfall. However, this hasnot been confirmed in a comprehensive scientific study prior to thepresent study due to lack of detailed, spatial data of good quality.Semadeni-Davies et al. (2008b) showed that both urbanisationand climate change would exacerbate current problems with com-bined sewers. By disconnection of stormwater from the sewers,these effects can be diminished. SUDS and stormwater connectionscan e.g. reduce CSO to very low levels, both for the present andfuture climate (ibid.).

4.7.4. Technical breakdownsLocally, some flooding is caused by breakdown of the system,

e.g. when a sewer pump stops pumping due to system error. Onthe one hand, the phenomenon with local breakdowns could beseen as unique incidents that are not likely to happen during futureflood events. On the other hand, and in reality, it seems inevitablethat a few of these unique incidents happen during every floodevent. They should therefore be expected and seen as an indirectcause of flooding also on city scale.

4.8. Potential strategies for Malmö

The city of Malmö is most exposed to flooding along the majorflow paths from the outer areas into the city centre. These areas arealigned with former watercourses, where the main paths of boththe minor (pipe system) and the major (overland) system arefound. If some particularly vulnerable buildings could be removedfrom parts of these low-lying stripes of land associated with highrisk, and the pipes could be reconstructed with open canals for sep-arate stormwater, then the flood risk could be substantially low-ered (Jha et al., 2012). Proper overland flow paths are oneimportant factor to reduce flooding (Torgersen et al., 2015). Theflood risk would also become more visible for citizens and cityplanners with an open canal system. Such open canals are expen-sive to establish. Nevertheless, they might have multiple functionssince they contribute to a dynamic and beautiful urban landscape,even though these effects are difficult to ‘‘prove”, according to Wildet al. (2011), due to lack of peer-reviewed publications. It mightalso be fruitful to increase downstream capacity by strategic main-tenance before late summer when flood risk increases, as they dobefore monsoon season in Mumbai, India (Sörensen and Rana,2013) and as suggested for Fredrikstad, Norway (Torgersen et al.,2015).

To lower the flood risk along the major sewers more exten-sively, the velocity of the stormwater must be slowed down. Thiscan be achieved through partial separation of the centralisedstormwater system in combination with construction of big andsmall structures for detention in upstream areas. Separation ofmajor pipes and/or restoration of natural drainage channels, ‘‘day-lighting”, are very expensive, as already mentioned. Therefore, it ispreferable to separate locally in upstream areas instead, ratherthan close to the main sewers. The city of Malmö has good experi-ence from separation of combined sewer systems. An example isthe 31 hectares area of Augustenborg, where stormwater was dis-connected from the combined system and instead handled by openstorm drains, a so-called Sustainable Urban Drainage System(SUDS). This area was less affected by flooding during the 2014event, compared to similar areas with conventional drainage(Sörensen and Emilsson, 2017). Still, a few basements in the areawere flooded, as overland flow along the streets could make itsway directly into those basements. Sörensen and Emilsson (2017)

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showed that local, often small elevation differences can play animportant role in directing the overland flow away from buildings.Besides aesthetical, ecological and health benefits with SUDS, it hasbeen shown that flood risk reduction by SUDS retrofitting can befeasible from an economic point of view (Ossa-Moreno et al.,2017). The work to decrease pluvial flood risk can be used as a cat-alyst to improve green infrastructure and urban spaces (Malmöstad, 2016).

Another way forward is to introduce insurance policies withstrong incentives for house owners to install a backwater valve,as suggested by Ladson and Tilleard (2013). If all buildings withbasements have a backwater valve, the risk of basement floodingwould be considerably lower. This solution is rather cheap andeasy to implement, compared to separation of combined sewers.However, separation of stormwater from the combined systemhas valuable side effects, like decreased combined sewer overflow(CSO) and physical and environmental improvements by improvedblue-green infrastructures. Implementation of one solution shouldnot hinder implementation of others.

Besides concrete measures, organisational changes are needed.The City of Malmö and VA Syd work together on a strategy, anaction plan, and an emergency plan to manage pluvial flooding.They suggest organisational changes for municipal departmentsto cooperate better and to incorporate flood risk in all physicalplanning (Malmö stad, 2016). As a part of this work, it is importantto assess if groups that are more vulnerable need better protection,information and other measures. Such a study has been conductedfor Greater Manchester, UK, where areas associated with povertyand cultural diversity correlates with areas susceptible to flooding(Kazmierczak and Cavan, 2011). In Malmö, the socio-economicalsegregation and social polarisation has increased between 1992and 2006, leading to bigger differences between richer and poorerareas (Wallin, 2010). It is therefore relevant to make a similarstudy in Malmö, as Kazmierczak & Cavan did in Greater Manch-ester. The new plans also incorporate measures to increase thepublic awareness of urban flooding, which is an important successfactor in any flood management scheme.

4.9. Data reliability

Flood claim data are found to be a useful source of informationto evaluate flood risk in urban areas. In this study, data from boththe utility company (VA Syd) and an insurance company (LF Skåne)were used. The two datasets are slightly different, as the insurancecompany collects data for each insured household that wasflooded, while VA Syd has approximately one customer per con-nection to the water and waste water systems. It was found thatthe spatial distribution of flood claims in the dataset from the util-ity company VA Syd does not differ significantly from the datasetfrom the insurance company LF Skåne. As LF Skåne, being an insur-ance company, can make claims to VA Syd, some claims might beidentical. However, some citizens might not have a proper insur-ance, probably due to low income, and it is probable that citizenswithout insurance are more common in some areas. There are alsoother insurance companies beside LF Skåne not included in thestudy. Precaution must therefore be taken if decisions are madebased on the dataset. Both datasets give information on floodedbuildings. No information is available on flooded roads, parks,etc. Despite this lack in the data, it is plausible that the datasetsgive a good representation of flood severity and spatialdistribution.

Like other datasets (e.g. Spekkers et al., 2012), damage causesare poorly defined in the two databases. Therefore, thoroughexamination of the records were required and done before furtheranalyses. There might however, still be misinterpretations regard-ing the causality behind some of the flood events, especially for

highly localised flood events for which precipitation data mightbe lacking in spatial resolution.

Douglas et al. (2010) emphasise that historic flood data must becollected by a single body and made available to flood risk manage-ment agencies. Such data are available in Sweden through thewater utility companies or the municipal sewer department, butit is not clear whether the data can be made available for otheragencies outside the water utility company or municipality. Infor-mation on flooding of individual properties are available due to theSwedish principle of public access to official records.

Rainfall data from both VA Syd and SMHI as well as sea leveldata from SMHI are considered reliable. The same applies for dataon topography, which is from a national, good quality dataset, anddata on sewerage system types, urban areas, streets, watercourses,etc.

4.10. Conclusions

Flooding in Malmö, Sweden is mainly pluvial, although a fewflood claims related to high sea level have been reported afterstorms. Densification of urban areas and climate change areexpected to aggravate the problem. The spatial pattern of floodingin Malmö, a typical Scandinavian city, differs between floodingcaused by short (1–10 h), intense rainfall events, and floodingcaused by consistent rainfall during several days. For the moreintense rainfall events, most flood claims are reported from areasclose to the main sewers. Both the minor system (pipe system)and the major system (overland flow paths) rapidly lead water tothese areas. For floods caused by long-term rainfall, the floodclaims are more evenly spread and not as strongly related to thetopography. Another factor governing the spatial distribution offlooding is the drainage system type. In general, areas with com-bined system are more affected by flooding, compared to areaswith separate system. Even if only 31% of the urban land in Malmöis connected to the combined system, 70% of the flood claims arereported from these areas. Finally, the spatial pattern of the precip-itation affects the spatial pattern of flooding. For the three biggestrainfall events in Malmö, however, heavy rainfall was measured atmore or less all nine stations.

In conclusion, the main mechanisms and characteristics offloods extent and its spatial distribution in Malmö are intensityand spatial distribution of rainfall, distance to the main sewer sys-tem as well as overland flow paths, and type of sewer system. Thesea level has little influence on flooding.

Acknowledgements

The authors wish to thank Helén Nilsson from LänsförsäkringarSkåne, and Tomas Wolf, Susanne Steen Kronborg, Stefan Milotti,Lemiss Sebty, and Leif Ståhl from VA Syd for valuable data. TomasWolf did also contribute with detailed information on monitoring,especially the rain gauges. Cooperation with LänsförsäkringarSkåne was originally established by Joanna Theland while she col-lected data for her master’s thesis; data which were used in earlyanalyses for this study. Lars Bengtsson and Rolf Larsson arethanked for their contribution to the development of the idea lead-ing to this paper and for valuable comments to the manuscript. Theauthors also wish to thank Birgitta Sörensen for proofreading. Inaddition, two anonymous reviewers gave very valuable commentsand suggestions.

GIS data on urban areas, streets, watercourses, etc., were dis-tributed by the Swedish national mapping, cadastral and land reg-istration authority: � Lantmäteriet, Dnr: I2014/00579.

This study was supported by a grant from Formas, No. 942-2015-149 (Sustainable Urban Flood Management).

66 J. Sörensen, S. Mobini / Journal of Hydrology 555 (2017) 51–67

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Flood Risk Reduction by Urban Blue Green Infrastructure – an Evaluation using Insurance 1 Data 2

J. Sörensen1, T. Emilsson2 3

Abstract 4 One of many important features of blue green infrastructure, is the ability to lower flood risks by detention 5 of stormwater. As extreme events are rare, few assessments have been made to evaluate the risk reduction. 6 In 2014, Malmö (Sweden) was hit with by extreme precipitation corresponding to a return period of 50–7 200 years which led to severe, pluvial flooding. This event, as well as a number of other large events, gave 8 the opportunity to evaluate the efficiency of the 15 year old stormwater system retrofit in the 9 Augustenborg area (30 hectares) through analyses of flood claim data. Augustenborg and its stormwater 10 control measures was compared with five similar neighborhoods that have conventional sewer systems 11 (combined or separate). The long-term trends of insurance claims showed less flood damage in 12 Augustenborg compared to the nearby areas which indicates a direct effect of the retrofit with stormwater 13 control measures. Even though a few properties were flooded in Augustenborg, it was shown that the 14 retrofitted stormwater system performed successfully during the extreme event in 2014. 15

KEY WORDS: extreme precipitation; pluvial flooding; best management practices (BMP); green infrastructure (GI); low impact development (LID); sustainable 16 urban drainage systems (SUDS) 17

18

1Department of Water Resources Engineering, Lund University, Box 118, S-221 00 Lund, Sweden; Ph +46 46 222 44 87; email: johanna.sorensen@tvrl.lth.se 19 (corresponding author) 20 2Department of Landscape Architecture, Planning and Management, Swedish University of Agricultural Sciences 21 22

1. Introduction 23 The societal consequences of flooding are severe. In the developed world, flooding primarily causes 24 financial loss, but there are also reports of direct mortality and indirect health effects from contamination 25 of drinking water and failing sanitation systems (Kunkel et al. 1999; Schmitt et al. 2016). In Europe, 26 flooding is expected to increase due to climate changes (Jongman et al. 2014). More intense rainfall and 27 more flood events are expected in several regions worldwide (IPCC 2014). In combination with current 28 land use changes due to urbanization, this will increase the problems with urban, pluvial flooding 29 (Semadeni-Davies et al. 2008). 30

There is a need for proactive planning, design, and implementation of a range of solutions to mitigate 31 future problems with urban pluvial flooding (Sörensen et al. 2016). Blue green stormwater control 32 measures (also known under e.g., Best Management Practices (BMP), Sustainable Urban Drainage Systems 33 (SUDS), Water Sensitive Urban Design (WSUD), and Low Impact Development (LID) (see Fletcher et al. 34 2014)) are designed and installed to decrease urban runoff and reduce peak flows, but have during the 35 last years also been put forward as a component of larger concepts, including Green Infrastructure or Blue 36 Green Infrastructure (Lerer et al. 2015) and Nature Based Solutions as defined by the European union 37 (European Commission 2013, European Commission Directorate-General for Research and Innovation 38 2015). These concepts focus on multifunctionality, connectivity, innovation, social factors, co-creation, and 39 involvement as key success factors for implementation of solutions for improved quality of urban spaces 40 and for combating urban flooding, which have very much been at the heart of the development of the city 41 of Malmö and especially the Augustenborg neighborhood. 42

Urban flooding can be categorized in many ways: pluvial flooding, fluvial flooding, sewer system failure, 43 flash floods, groundwater flooding, ice melt flooding, and coastal flooding are often mentioned in the 44 literature (Douglas et al. 2007; Zevenbergen et al. 2010 (p. 42–45); Saul et al. 2011). This study focuses on 45 pluvial flooding, which is caused by extreme local storms when the drainage capacity is exceeded 46 (Maksimović et al. 2009). Urban drainage systems often have a limited hydraulic capacity that is designed 47 to cope with precipitation with, for example, a 10 year return period (Svenskt Vatten 2016). This means 48 that even when design standards are followed, minor and moderate flood events will occur on average 49 every 10 years. 50

There are numerous investigations on the performance of blue green infrastructure under moderate rain 51 events (Villarreal et al. 2004; Shukri 2010; Qin et al. 2013; Liu et al. 2014; Sebti et al. 2014; Liu et al. 2015) 52 but, to our knowledge, no systems installed with stormwater control measures have been evaluated 53 during extreme events, as these events are rare. Earlier research on flood reduction with stormwater 54 control measures has focused on model simulations, including three studies made in Augustenborg 55 (Villarreal et al. 2004; Shukri 2010; Haghighatafshar et al. 2018). 56

Hydraulic modeling can give a rough estimate of the effects of extreme events, but there are some 57 problems related to such modeling. Beven (2008) divides model construction into three parts: firstly a 58 perceptual model, which is how we understand the physical system, including the full complexity of the 59 system; secondly a formal model, which is the mathematical description of the system; and finally a 60 procedural model, which is the model that will run on the computer. Where necessary, approximate 61 numerical solutions are implemented. In all these parts, extreme event modeling is problematic. As 62 extreme events are uncommon, modelers’ understanding of how the system behaves during extreme 63 situations is limited. As little data is available, calibration and validation of extreme events is difficult. 64 Despite this, modeling is a powerful tool to evaluate the hydraulic function of different measures; 65 however, other methods must be used to evaluate flood control effects achieved by blue green 66 infrastructure during extreme events. 67

In this study, we have chosen to go straight to the measured effects of extreme rainfall, and to use 68 insurance claims to evaluate the performance of integrated blue green infrastructure on urban flood 69 reduction. Flood insurance data from a long series of flood events, including one severe event, are used 70 for this evaluation. In Sweden, property losses from overland flooding, flooding of drainage systems, and 71

flooding from groundwater intrusion are typically covered by a person’s or a company’s insurance 72 company. The property owner can make a claim both to the insurance company and to the water utility 73 company. The water utility will cover the deductible from the insurance company. In addition, the water 74 utility company collects flood reports from other property owners and covers claims from insurance 75 companies. Due to this system, spatial data on flooded properties are available. Similar data sets have 76 previously been used in several studies, for example to analyze the distribution of large claims (Smith & 77 Goodman 2000), for validation of urban flood inundation and damage models (Chen et al. 2009; Zhou et 78 al. 2013), to study correlations and links between extreme rainfall, their characteristics, and flood damage 79 (Spekkers et al. 2013 a&b; Torgersen et al. 2015; Sörensen & Mobini 2017), and to analyze buildings’ 80 exposure and the influence of spatial placement variables for vulnerability to lake floods and extreme 81 rainfall (Grahn and Nyberg 2014; Torgersen et al. 2015). However, no such studies on the performance of 82 stormwater control measures during extreme events were found. 83

The extreme rain event that hit Malmö on August 31, 2014 is central to this study. Almost the entire city 84 was affected, and about 3000 properties were reported flooded (SMHI 2014; Lindher 2015; Hernebring et 85 al. 2015; Sörensen & Mobini 2017). Streets, houses, underground garages, and even the hospital were 86 flooded. Several families could not move back to their homes for more than a year. The return period was 87 estimated at 50–200 years. The intense rainfall and the severe flooding following it make this event suitable 88 for evaluation of flood reduction by the blue green infrastructure in Augustenborg. The evaluation is also 89 needed given the current interest in installation of stormwater control measures and nature-based 90 solutions for the mitigation of problems related to urbanization and climate change. Few places have been 91 retrofitted with blue green infrastructure as extensively as this area. It is hard to find any similar 92 evaluation of flood reduction with historical data in the scientific literature. 93

Thus, the present study is an in-depth analysis of flood claims from property owners that were affected 94 by the severe storm in August 2014 as well as by other events during a 20-year period. The function of the 95 blue green infrastructure is evaluated by a combination of insurance data and flood reports in relation to 96 flood events during the 20-year period, including the severe single storm event that hit Malmö in 2014. 97 Flood risk is evaluated by studying the damage during these historical events, which are assumed to be 98 representative for urban pluvial flood events in the dense urban fabric of mid-size cities in temperate 99 climates. 100

Our investigation compares a 30 hectares area retrofitted according to typical principles for design of blue 101 green infrastructure with stormwater control measures (Stahre 2008), to adjacent conventional 102 neighborhoods where no or very few stormwater control measures have been installed. The study uses 103 insurance claims to investigate: 1) the long term trends of these claims in the selected neighborhoods; 2) 104 the direct effect of the retrofit looking at pre- and post-installation data; and in particular 3) the 105 performance of a completely retrofitted neighborhood during an extreme rainfall event. 106

2. Methodology 107

Study area 108

This investigation is set in the southern parts of Malmö, Sweden (Figure 1a) where a large-scale retrofit of 109 an existing neighborhood, Augustenborg, took place in a project called Eco-City Augustenborg in the late 110 1990s (55°34'45"N 13°01'30"E). Eco-City Augustenborg is used in this investigation of flood risk reduction 111 by blue green infrastructure, as the retrofit of the combined drainage system was completed with 112 stormwater control measures as an alternative to an installation of a conventional separated pipe system. 113 To be able to discuss what effects blue green infrastructure has on flooding, Augustenborg was compared 114 to the nearby neighborhoods of Persborg, Lönngården, Lindgatan, Södra Sofielund, and Norra Sofielund 115 (Figure 1b). These areas are similar to Augustenborg in several ways, with the exception that stormwater 116 control measures have not been implemented there. 117

About 3000 people live in the Augustenborg area in 3–6 story apartment blocks built in 1948–1952. Lawns, 118 bushes, and trees were planted around the buildings from the start, but with no direct function for 119 stormwater drainage. The objectives to renew Augustenborg in the late 1990s were to achieve sustainable 120 urban development, to make the buildings energy efficient, to improve solid waste management, to stop 121 basement flooding, to provide renewable energy, to involve residents in the development of their 122 neighborhood, and to make the environment greener. This study mainly focuses on flood reduction by 123 the blue green infrastructure that was implemented during the retrofit of Augustenborg. The 124 multifunctional systems have been investigated elsewhere in relation to many aspects during these 15 125 years as in, for example, citizen involvement (Krantz & Hjerpe 2002), environmental assessment (Ludzia 126 2014), green roof maintenance (Emilsson et al. 2007), water quality (Berndtsson et al. 2006; Ahmed 2010), 127 water runoff quantity (Bengtsson et al. 2004b), blue green infrastructure design and maintenance 128 (Söderblom 2004), and biodiversity (Ohlsson 2001; Ohlsson 2002). The area continues to be developed, 129 and in 2016, a 14-story eco-friendly building named Greenhouse was finished. 130

Augustenborg stormwater system 131

The Augustenborg blue green infrastructure contains flow detention in ponds and areas for temporary 132 flooding, infiltration on green roofs, lawns, and parking, as well as slow transport in swales, ditches and 133 channels (Stahre 2008). The blue green infrastructure was constructed by the end of the 1990s by Malmö 134 Service Administration and Malmö Water (today VA Syd, utility company) and MKB (housing company) 135 as a part of the Eco-City Augustenborg project (Stahre 2008). An existing combined sewerage system was 136 upgraded and separated with soft, vegetated and hard, constructed features for detention and 137 transportation of stormwater. Green areas were lowered to make space for excess water. The capacity of 138 the blue green infrastructure is unknown and it was not designed according to a certain standard. The old 139 combined sewers are still used for wastewater and backwater valves are not installed in basements. Two 140 pictures from the area of the first redevelopment phase are shown in Figure 2. 141

The piped drainage system of Augustenborg is dominated by a combined system (225–750 mm). 142 However, as the blue green infrastructure manages most of the stormwater, the combined system acts 143 almost as a separate system for sewerage only. Through the central part of the area, a stormwater pipe 144 (300–600 mm) constructed in 2002–2003 complements the blue green infrastructure by draining the main 145 street. 146

The southern part of Augustenborg’s blue green infrastructure consists of an upstream, industrial area 147 with 2000 m2 of impermeable paving and 9500 m2 of green roofs. The water is gathered in two small ponds, 148 interconnected with a channel. From the downstream pond, the water is pumped to a small wetland, 149 where a further 3000 m2 of paved surfaces and 1000 m2 of roofs are connected. The discharge is detained 150 in two dry detention ponds. From the ponds, the main channel leads the stormwater toward two wet 151 ponds. The channel is 170 m long and slopes approximately 1%. The full capacity of the channel is 200 l/s. 152 Additional stormwater from 1500 m2 of permeable paving, 3000 m2 of roofs, and 1300 m2 of semi-153 permeable surfaces discharge into the channel. A mini-wetland of 100 m2 is placed along the channel, to 154 which the channel can overflow. (Bengtsson et al. 2004b) The original blue green infrastructure was 155 constructed with a large (750 m2) pond at the end of the channel. This pond was later divided into two 156 ponds with a grassy area in-between and a small brook interlinking the ponds, for aesthetic and 157 maintenance reasons (Söderblom 2004; Stahre 2008). After this double pond, the flow continues via an 158 overflow to a “cube channel.” This is a 100 m long channel with concrete cubes of different heights on the 159 bottom. The channel ends in a public park where the grass area has been lowered to make space for the 160 water in a bioswale type of installation. Through the park, a meandering swale leads the water toward a 161 pond, from where there is an overflow to the municipal stormwater pipe system (Stahre 2008). For further 162 description of the southern part of the blue green infrastructure in Augustenborg, see Villarreal et al. 163 (2004). 164

The northern part of Augustenborg’s blue green infrastructure is constructed more naturistically than the 165 southern part. It consists of a long swale that has partly been designed as a rain garden. Stormwater is 166

collected from buildings through small concrete channels and led to the swale. The uppermost part of the 167 swale is about 200 meters. It leads to an open pond, and from there the flow goes under a street through 168 a siphon pipe. Another 300 meters of naturistic swale leads the water from the siphon pipe to a 60 meter 169 long concrete channel, ending in a small pond. (Stahre 2008) 170

Due to lack of reports on actual usage during the flood event on August 31, 2014, the full capacity of the 171 stormwater control measures in Augustenborg is unknown. However, the size of the most important 172 detention basins was measured and the total storage capacity of ponds and swales in Augustenborg was 173 estimated as up to 1000 m3 during extreme rainfall. In addition, the grass area that has been lowered in 174 the central, public park can probably store up to 3000 or 4000 m3 during extreme rainfall. 175

Description of nearby areas 176

The five nearby areas that are used for comparison can, similarly to Augustenborg, be described as 177 urbanized with mostly medium density to dense urban fabric (Figure 1b). Norra Sofielund has more 178 continuous urban fabric compared to the other areas. Land use is predominant residential with some 179 industrial and commercial areas. Only a few areas are green urban areas or areas for sport and leisure 180 activities. The building coverage is 20% in Augustenborg, compared to 19–30% in the five nearby areas 181 that are used as comparison areas (Table 1). Roads and parking cover between 31% and 42% of the areas, 182 with the least roads and parking in Lönngården and Persborg and the most in Lindgatan. The total 183 approximate imperviousness is 57% in Augustenborg and between 50% and 70% in the nearby areas. 184 Some buildings in Lindgatan and Södra Sofielund were constructed in the 1910s and 1920s, but most of 185 the land was agricultural land until the 1930s when this part of Malmö was urbanized. Augustenborg and 186 later Persborg were not urbanized until late 1940s and early 1950s. All of the areas were constructed with 187 a combined sewer system, which was the dominating system in Sweden until at least the 1950s (Cettner 188 et al. 2012). Later, the sewer systems in some areas were reconstructed with separate stormwater and 189 sanitary sewers. Below follows a description of each of the five nearby areas. Information on buildings 190 come from the city of Malmö cultural and environment department and the County administrative board 191 of Skåne (2001, 2002 & 2004), and information on the pipe system were received from VA Syd. 192

Lindgatan is a part of Augustenborg but was not part of the Eco-City development and is regarded as a 193 separate area in this study. The oldest stormwater pipes in the area are from 1927, while most of the 194 stormwater pipes (600 mm) and the sewerage pipes (200 mm) are from 2001. The system has separate 195 stormwater and sewerage pipes. 196

Lönngården has combined sewers from 1935–1936. The privately owned 4-story buildings are placed 197 perpendicular to the main roads to the north and the south of the area with green yards in-between. Under 198 each of the roads is a 450 mm pipeline, interconnected by 300 mm pipes between the buildings. 199

Norra Sofielund. The northern half of the area consists of 4-story apartment buildings with closed inner 200 yards. The combined sewers (225–450 mm) in this area are from 1935–1936. In the south, the buildings are 201 generally lower and smaller, typically with two stories. This part of the area has a separate sewer system 202 where most pipes were constructed in 1975–1980. The separate sewerage pipes are 225–400 mm and the 203 separate stormwater pipes are mainly 300–600 mm. 204

Södra Sofielund. The first pipes in the drainage system were constructed in 1910, while most of the system 205 is from the 1930s and the 1940s (225–750 mm, combined system). Between 1965 and 1985, the western part 206 of Södra Sofielund, which consists mainly of small, privately owned houses, was separated with a new 207 sewerage pipe (225–1000 mm). The old combined pipes were turned into stormwater pipes. One of the 208 main interceptor sewer pipes, which is a 2000 mm concrete pipe that drains a large area with the combined 209 system, runs through the area. This sewer was constructed in 2000 as a combined stormwater and 210 wastewater pipe. The area used to consist of mainly industrial buildings and open land until the 1930s 211 and 1950s when residential buildings were erected. There are still industrial buildings in the area. The 212 apartment blocks in the area have both closed and half-public courtyards. 213

Persborg was built upon in the mid-1950s by MKB. The meandering placement of the buildings ensures 214 spacious, green yards between the redbrick buildings. The drainage system consists of a combined sewer 215 system constructed in 1954, reaching from south (300 mm concrete pipe) to north (600 mm concrete pipe). 216

Flood claim data from insurance companies and water utility company 217

The flood claim data used in this study mainly consist of two data sets: one from the insurance company 218 Länsförsäkringar Skåne (LF Skåne) (received 03-24-2015), and one from the water utility company, VA 219 Syd (received 03-14-2016). The data set from LF Skåne, the dominating insurance company in the region, 220 contains 9704 reported flooded properties in the Scania Region, Southern Sweden, during a period of 28 221 years (1987–2015), and the data set from VA Syd contains 5868 claims and reports from Malmö and 222 neighboring municipalities for the period between June 1990 and March 2016 (26 years). In general, there 223 is consistency between the data from LF Skåne and from VA Syd, and the material gives a good measure 224 of flood extent from both small and severe flood events. Data from LF Skåne are not considered as reliable 225 before 2007 compared to after 2007, and these data were therefore not used in the study. The data set from 226 the water utility company covers mostly flooding events where water has entered the building from the 227 sewerage system. In addition to the long-term data sets from LF Skåne and VA Syd, data representing the 228 severe flood event on August 31, 2014 were received from Malmö municipal housing company (MKB, 25 229 claims), the insurance companies Lifra (33 claims), IF Skadeförsäkring (list of number of flooded 230 properties for each postal code), and Moderna Försäkring (132 claims), and the municipal property 231 manager Stadsfastigheter (165 claims). The insurance database covers property loss from overland 232 flooding, groundwater intrusion through basement walls, and flooding from the drainage system, and is 233 used as a proxy for flood severity for different events. 234

Rainfall data 235

Rainfall data were collected from SMHI (Swedish Meteorological and Hydrological Institute), which has 236 one station in Malmö (Station A), and VA Syd, which has eight stationary stations. Station MA03 237 Augustenborg Ystadvägen, located in the botanical roof garden in Augustenborg, is the closest station for 238 all study areas (<2 km distance). Data from a mobile station in Söderkulla was also used in the data 239 analysis of the 2014 event. 240

SMHI measures the volume of precipitation each 15 minutes, while all VA Syd stations are tipping buckets 241 with 0.2 mm bucket volume. Data from VA Syd were converted to 5 min precipitation volume before 242 further analysis. For comparison, the updated precipitation statistics from Hernebring et al. (2015) based 243 on precipitation measured in Malmö 1980–2013 are used. 244

Topography data 245

Data on topography in a 2x2 m grid were retrieved from the Swedish official digital elevation model (Ny 246 Nationell Höjdmodell, NNH). The vertical error of the elevation model is 5 cm and the horizontal error is 247 25 cm on average for open, hard surfaces (Swedish national mapping, cadastral, land registration 248 authority 2015). 249

Data analysis and statistics 250

Flood magnitude is defined as the number of flooded properties per hectares (NFP/ha). For comparison, 251 the number of flooded properties per number of addresses (NFP/address) is used, as the number of 252 addresses in an area is (loosely) connected to the number of sewer connections. Each entrance typically 253 has its own address (e.g., Augustenborgsgatan 3A, 3B, 3C, etc.). The number of flooded properties per 254 total number of properties was also considered as a measure for flood magnitude. However, it turns out 255 that the size of the properties in the different areas is inhomogeneous, and the other measures (NFP/ha 256 and NFP/address) therefore give a better picture of the flood magnitude. 257

Thorough quality control was made for all data points in the LF Skåne data set. This data set contains the 258 Swedish unique identifying numbers for parcels (in Swedish: fastighetsbeteckning). All parcel numbers 259 were verified against cadastral data as street addresses were needed to georeference the data points. The 260 online tool Batchgeo was used for georeferencing of the LF Skåne data set, and a random set of the 261 georeferenced data points (~10%) were then manually verified. For the VA Syd data set, this procedure 262 was not needed, as both the flooded addresses and georeferences were given. 263

To study the flood reduction in Augustenborg after the blue green infrastructure was implemented in the 264 district, the flood magnitude during 2007–2015 was compared with the nearby areas Lindgatan, 265 Lönngården, Norra and Södra Sofielund, and Persborg. Insurance claims and reports from both VA Syd 266 and LF Skåne were used in this analysis. 267

A bootstrap resampling technique was used to statistically evaluate long-term differences between flood 268 magnitudes in the retrofitted area Augustenborg in comparison to the five nearby areas. Flood 269 magnitudes were listed for each event in each area, and 1000 random samples were taken from the 270 Augustenborg list and the list of one other area. This procedure was repeated 10 000 times. The 271 comparisons with Augustenborg were done independently for all five nearby areas. As the 2014 event 272 dominates the flood statistics in Malmö, the bootstrap analysis was done both including and excluding 273 this event. Data from both VA Syd and LF Skåne were used for these analyses. 274

Data from VA Syd were used to compare flood claims during one period before blue green infrastructure 275 was implemented in Augustenborg with flood claims after the implementation. The comparison was done 276 for Augustenborg as well as the other, nearby areas to assess similarities and differences in detected 277 changes. 278

Detailed information about the 2014 event is presented in this study to evaluate the function of the blue 279 green infrastructure during an extreme flood event. This event is of particular interest, as such extreme 280 events are uncommon, and no similar event has been reported in the literature on flood reduction by blue 281 green infrastructure before. 282

3. Results 283 In this section, we first present the long-term trends of reported insurance claims in the selected 284 neighborhoods, secondly the direct effect of the retrofit on flood frequency based on pre- and post-285 installation data, and finally the performance of a complete retrofitted neighborhood during an extreme 286 rainfall event. 287

Long term trends of reported insurance claims in the selected neighborhoods 288

Flood reduction after stormwater retrofit in Augustenborg was evaluated by comparison with the five 289 nearby areas. The flood event on August 31, 2014 dominates the flood claims in the evaluated period; 290 during 2007–2015, more than 80% of the flood claims in Augustenborg, Lindgatan, Lönngården, Norra 291 and Södra Sofielund, and Persborg were reported during this single event. Therefore, the 2014 event is 292 considered as an extreme flood event and studied separately from the other, less severe, flood events. As 293 can be seen from Table 2, the flood magnitude (number of flooded properties per hectare) was more than 294 10 times smaller in Augustenborg compared to the other areas both during the 2014 event and during the 295 other events in 2007–2015. 296

297

All flood events in the period, in total 36 events, were evaluated with bootstrap statistics. It was found 298 that the flood magnitude is significantly lower, that is, zero differences are outside the 99% bootstrap 299 confidence interval, in Augustenborg compared to all the nearby areas individually. The test was also 300 performed without the 2014 event, giving the same result. 301

Direct effect of the retrofit on flood frequency based on pre- and post-installation data 302

Augustenborg was more affected by flooding before blue green infrastructure was implemented, with five 303 flood claims reported to VA Syd before and none after (Table 3). However, the number of flood claims 304 reported was also low before implementation. In addition, Lindgatan was less affected by flooding after, 305 which could be explained either by the fact that the area is connected to the same sewer as Augustenborg 306 downstream, or that the combined sewer in the Lindgatan areas was separated in 2001. For the other four 307 areas, either as many or more flood claims were reported during the after period as compared to before. 308 One reason for the increase in two of the areas (Norra and Södra Sofielund) might be heavier rainfall in 309 the after period. It should be noted that the rainfall during the two periods are not necessarily comparable. 310 Another reason for the increase might be degradation of the sewer system. Both these areas have a mix of 311 combined and separate sewers. 312

Reporting to VA Syd is slightly different than claims to an insurance company, as mainly property owners 313 will report to VA Syd and consequently there is at most one report per sewer connection per event. This 314 limits the number of reports to VA Syd from Augustenborg, as the area consist of rental apartments owned 315 by Malmö municipal housing company (MKB). Persborg, an area with similar owner structure, is thus 316 especially useful for comparison. Flooding was not reduced in Persborg between the period before and 317 after the retrofit of Augustenborg. This strengthens the indication of flood reduction by blue green 318 infrastructure in Augustenborg after the retrofit. 319

It is difficult to draw definitive conclusions based on a comparison of flood claims before and after 320 stormwater control measures were constructed as there might have been a change in several other 321 variables such as the tendency to report flooding to VA Syd, rainfall pattern, etc. Unfortunately, no flow 322 measurements were made in Augustenborg before the new system was built. 323

Performance of a completely retrofitted neighborhood during an extreme rainfall event 324

The extreme rainfall event on August 31, 2014 makes it possible to evaluate flood reduction of the blue 325 green infrastructure in Augustenborg. In the following subsections, details on the rainfall event and its 326 consequences in Augustenborg and nearby areas are given. 327

3.3.1.Rainfall on August 31, 2014 328

The rainfall over Malmö on August 31, 2014 corresponded to a rainfall with return period of 50 to ~200 329 years and had a total volume of 60–120 mm (spatial variation). This was the biggest rainfall event since 330 measurements started in the late 1800s and led to severe flooding in most of Malmö as well as in 331 neighboring villages and in some parts of Copenhagen, Denmark. In Augustenborg, 116 mm was 332 measured, and most of the rainfall (100 mm) fell within 3.5 hours (Figure 3). The rainfall was severe, 333 causing damage spread over all Malmö, with slightly more damage in areas with a combined system 334 (Sörensen & Mobini 2017). The maximum rainfall measured at Augustenborg during one hour was 41.6 335 mm. It can be assumed that the intensity was similar in neighboring areas, looking at a mobile station 336 nearby. Bengtsson & Milotti (2010) earlier reported the maximum rainfall during one hour to be 28.6 mm 337 (data from 1996–2007, station M03 Augustenborg) and for a nearby station (Turbinen) they found that 41.0 338 mm could be expected during one hour with a 100 year return period. As can be seen in Figure 3, the 339 return period according to Hernebring et al. (2015) is shorter. Only for durations longer than two hours 340 does the rain event exceed the 100 year rainfall threshold. 341

3.3.2.Flood reduction in retrofitted area on August 31, 2014 342

The flood magnitude, that is, the number of flooded properties per hectare, for August 31, 2014 is shown 343 in Figure 4 as reported by five sources (LF Skåne, VA Syd, Lifra, Stadsfast, and Moderna). It can be seen 344 that the level is lower in Augustenborg compared to the five nearby areas. The highest flood magnitude 345 is found in Södra Sofielund, where one of Malmö’s main sewerage pipes passes through. Table 4 shows 346 that compared to the five nearby areas without blue green infrastructure, in the same part of Malmö, 347

approximately 10 times less properties were flooded in Augustenborg during the event. Lindgatan, which 348 was the least flooded apart from Augustenborg, was 6.4 times more flooded than Augustenborg, while 349 the most heavily flooded area, Södra Sofielund, was 18.4 times more flooded than Augustenborg. Flood 350 magnitude was also calculated as the number of flooded properties per address (Table 4). The same 351 pattern appears, however with smaller differences. It should be noted that the number of reported flood 352 claims was also low in Augustenborg before the blue green infrastructure was implemented (Table 3). 353 However, it seems that the number of reported flood claims has increased in general, while this is not the 354 case for Augustenborg. 355

The flood reports from IF Skadeförsäkringar cannot be interpreted in the same way as the data from LF 356 Skåne, VA Syd, Lifra, Stadsfast, and Moderna, as these reports were received as a list of flooded properties 357 per postal area. In Figure 5, the number of flooded properties for each postal area is shown. It can be seen 358 that there was no reported flooding in the postal area that is fully within Augustenborg. From one area 359 partly within Augustenborg, partly within Södra Sofielund, and partly within Lindvägen, eight flood 360 claims were reported to IF Skadeförsäkringar. Apart from the fact that many flood claims were reported 361 from Södra Sofielund compared to all the other areas, and none from the postal area within Augustenborg, 362 which is consistent with the reports from the five other sources, there is no clear pattern of flood claims 363 from IF Skadeförsäkringar. 364

3.3.3.Flood reports from MKB housing company on August 31, 2014 365

Additional information regarding the effects of the flooding was derived from descriptive, internal reports 366 from the local offices of Malmö’s biggest housing company, MKB, after the flood event on August 31, 367 2014. Not all flooded properties are listed in these reports, and therefore no quantitative comparison is 368 possible, but interesting information can still be derived from the reports. 369

The MKB flood report from Augustenborg reports that several basements were flooded, however none of 370 them severely. One garage was severely flooded on Lantmannagatan. An elevator was out of order on 371 Norra Grängesbergsgatan and the pavement was undermined at Grängesbergsgatan 44. It is worth noting 372 that Lönngården, which was not retrofitted with stormwater control measures, is included in this report, 373 and that the elevator that was flooded is located there. MKB has 33 properties in Augustenborg and 4 in 374 Lönngården. 375

The MKB flood report from Möllan includes MKB properties in the Möllevången (outside the study areas) 376 and Norra Sofielund neighborhoods, where MKB has 51 properties in total. It reports that all basements 377 were flooded with up to 10 cm of black sewage except for six properties. MKB has five properties in Norra 378 Sofielund and two of these were reported as not flooded, meaning that three properties likely were 379 flooded here. Apart from the flooded basements, there were few problems in the MKB properties in Norra 380 Sofielund. 381

The MKB flood report from Persborg reports that almost all of the properties owned by MKB in Persborg 382 were flooded. The basements in 18 properties were flooded. Some garages were also flooded. In total MKB 383 has 25 properties in Persborg. 384

The MKB flood report from Seved reports that there was 3–25 cm of black sewage in all basements of MKB 385 owned properties. In three of them, it was much worse, with 6 meters of water in the most severely flooded 386 basement. In addition, basement apartments were flooded in Seved as well as garages, however not 387 severely. Seved is an area within Södra Sofielund, where MKB has 38 properties in total of which 37 are 388 in Seved. 389

These reports from MKB also describe that some buildings lost electrical power, hot water, and TV 390 connection, that pavements were damaged, and that elevators were broken. No human injuries were 391 reported, but one person was stuck in an elevator in Södra Sofielund and luckily managed to escape by 392 herself as the water reached her chest (Sydsvenskan 2014). In conclusion, most damage in Augustenborg 393

reported internally by MKB regards pavement and other damage outside, while several buildings in the 394 other areas were severely flooded on August 31, 2014. 395

4. Discussion 396

Flood risk reduction 397

Blue green infrastructure, as constructed in Augustenborg, Malmö has been proven efficient in flood risk 398 reduction by the analysis of flood damage during historical flood events registered as insurance claims. 399 Similar results were found through hydraulic simulation of the severe flood event in 2014 400 (Haghighatafshar et al. 2018). Altogether, the land use and stormwater systems are similar in all study 401 areas, except for the blue green infrastructure in Augustenborg, excluding other determinants of flood 402 risks in neighboring areas. What is interesting to note is that Augustenborg was rather green already 403 before the blue green infrastructure was constructed. During extreme rainfall, the green/pervious areas 404 contribute to runoff (Berggren et al. 2013) if they are not constructed for stormwater detention, as they 405 were in Augustenborg after retrofit. From the long-term trends, it can be concluded that it is not only 406 green areas that lower flood risk. The small-scale topography is also of importance for flood risk reduction. 407

Based on findings in previous studies (Villarreal et al. 2004; Shukri 2010), a reasonable explanation for 408 reduced flood damage is a reduction in peak flows and total runoff volumes from blue green structures 409 for stormwater management. For example, in a modeling study of Augustenborg for up to a 10-year 410 design storm, Villarreal et al. (2004) found that the detention and retention capacity of the ponds, in 411 combination with the regulation of the outflows, plays a major role in reducing the flood risk in the area. 412 In addition, it was shown by hydrograph simulations that the system is able to detain all runoff from a 413 10-year event, both under dry and wet initial conditions. Villarreal et al. concluded that the 10-year event 414 could have been handled without risk of flooding with a pipe-bound system instead of the blue green 415 infrastructure, through separation of the combined system. This would, however, have led to major 416 earthworks in the area during construction and increased flow downstream of Augustenborg after 417 reconstruction. The downstream areas would thus have suffered increased flood risk, and the receiving 418 waters would have suffered from erosion or water quality degradation (Villarreal et al. 2004). The results 419 from Lindgatan indicate some proof for this theory, but the data are too weak as the area is small (3.4 ha). 420 Another study compared the blue green infrastructure in Augustenborg with the former (combined 421 sewer) system by modeling, and found a 50% decrease in discharge rate for a heavy rainfall in June 2007 422 (Shukri 2010). The peak runoff was delayed by 5–10 minutes with blue green infrastructure, compared to 423 the combined sewer. For high-intensity rainfall, the piped system reacted with more flooding, compared 424 to the blue green infrastructure. Villarreal et al. (2004) emphasize that storage capacity is of major 425 importance for flood risk reduction in such systems, especially for high intensity events like 10-year 426 events, and Shukri (2010) points to the larger available volume in the system as the main reason why the 427 blue green infrastructure gave a lower discharge rate compared to the piped system. These explanations 428 are reasonable also for flood reduction during extreme events, as shown in this study. 429

Using model simulation, Qin et al. (2013) found that stormwater control measures could reduce flood risk 430 in Guang-Ming New District in Shenzhen, China. Swales, which are a space-efficient technology, have a 431 positive impact on flood risk reduction as they can store big water volumes. Despite this, they recommend 432 green roofs and permeable paving for flood reduction, as these had the biggest effect according to their 433 study. The three mentioned technologies, swales, green roofs, and permeable paving, have different 434 effects on early, mid, and late runoff peaks. Qin et al. therefore recommend a combination of the three 435 solutions to achieve maximal effects for different kinds of rainfall events. Liu et al. (2014) claimed that 436 runoff with 1–2 years return period can be eliminated with blue green infrastructure, while runoff with 437 5–10 years return period can be reduced by 93–97% in an urban community in Beijing. These results come 438 from a model, where no calibration or validation was done. They showed that an integrated solution, 439 where several technologies are used together, is more useful to reduce flood risk in urban areas, compared 440 to single technology facilities. Torgersen (2015) has shown that precipitation events prior to the extreme 441 events have a significant impact on insurance claims after urban flooding. The ability for Augustenborg’s 442

blue green infrastructure to handle large storms during wet initial conditions as modeled by Villarreal et 443 al. (2004) is in line with this previous research. 444

Green roofs were one of the main components during retrofit of Augustenborg, frequently used in 445 branding of the project as green and sustainable. Runoff from a 30 mm sedum-moss roof in Augustenborg 446 was measured during one year by Bengtsson (2004a). Approximately 9–10 mm rainfall was needed to 447 initiate runoff, corresponding to storage at field capacity. It can thus be assumed that the green roofs in 448 Augustenborg had a minor effect on flood reduction during the extreme event in 2014. If the roof is wet 449 when the rainfall starts, runoff during one hour equals the precipitation during that hour. Compared to 450 runoff from black roofs, the runoff from the studied green roofs in Augustenborg is delayed with a time 451 of concentration of up to 20 minutes (Bengtsson 2005). Villarreal (2007) modeled the runoff from 452 Augustenborg’s green roofs with the downstream pond complex. To achieve the same detention for the 453 10 years event, without green roofs, the pond complex needed to be at least 29% bigger than its present 454 size. The thickness of the green roof substrate layer as well as vegetation composition will influence the 455 effective storage depth, but this will have most influence on small and moderate storms (Qin et al. 2013, 456 Stovin et al. 2017). In Augustenborg, close to 14 000 m2 (24%) of the roofs are green roofs which is similar 457 to the green roof scenario (20%) in the Guang-Ming New District model (Qin et al. 2013). However, the 458 green roofs in Augustenborg could only capture ~140 m3 of the 35 000 m3 stormwater that fell over the 459 area during the extreme rainfall in August 2014, if the results from Bengtsson (2004a) are valid also for 460 this event. This can be explained by the thickness of the roof, where the roofs in Guang-Ming New District 461 consists of a vegetated layer, a soil layer (150 mm), and a storage layer (75 mm) with an underlying drain, 462 while the total height of the Augustenborg roofs are 30 mm. It is worth noting that storage capacity of the 463 Guang-Ming roofs was calculated with a simple formula, while it was measured in Augustenborg. 464

Zahmatkesh et al. (2014) modeled the potential effect of blue green infrastructure in Bronx River in New 465 York City. They found the greatest effect from permeable pavement. In Augustenborg, the effect of 466 permeable pavement on flood reduction is probably small. Only limited parts of the pavements are 467 permeable, and as the underlying soil is clayey till (County administrative board of Skåne 2016), deeper 468 percolation is probably very slow. Limited infiltration is only possible in the upper soil layer. 469

Malmö is situated in a flat landscape where the highest elevation is 37 m above sea level. In Augustenborg, 470 the elevation is on average 14.7 meters above sea level. This is slightly higher compared to the other areas 471 (Persborg 14.3 m; Lönngården 12.6 m; Lindgatan 11.1 m; Södra Sofielund 11.1 m; Norra Sofielund 10.2 m). 472 The difference is small and cannot explain the great difference in flood magnitude. Two areas next to 473 Augustenborg on higher elevation, Hindby in the west and Almhög in the south, were heavily affected 474 by flooding on August 31, 2014. These areas are not used for comparison in this study, but they show that 475 small differences in altitude do not influence flood magnitude. For Södra Sofielund, however, the 476 elevation might play a role, as a small stream used to run through this area. This watercourse is now 477 tunneled and constitutes one of the main pipelines in the sewer system of Malmö. Södra Sofielund is 478 situated along the major flow path through this part of Malmö and is thereby at higher risk of flooding 479 (Sörensen & Mobini 2017). Norra Sofielund, the lowest area in this study, was flooded only to half the 480 magnitude of the flooding in Södra Sofielund, indicating that the main sewers have a greater influence on 481 flood magnitude than topography. Topography mainly governs where the main sewers are placed, but 482 not exclusively. 483

Flood risk reduction through stormwater management is not enough to fully protect an area from 484 flooding. The Augustenborg case shows that excess water still can flow into basements and garages 485 straight from the streets down ramps to underground parking garages and down stairs to basement 486 entrances. Water from upstream areas can enter basements in Augustenborg through the combined 487 system, as no backwater valves are installed (Haghighatafshar et al. 2018). To avoid all basement flooding 488 during extreme rainfall, when new flow paths appear on the surface as on August 31, 2014, such 489 constructions must be reconstructed with a barrier that keeps the excess water away from buildings, and 490 backwater valves must be installed. 491

Insurance data as a measure for flood extent 492

Insurance claims and flood reports from housing companies, as used in this study, have some limitations. 493 Firstly, not everyone is covered by insurance according to the data provider at Länsförsäkringar Skåne. 494 People in different housing districts are probably covered differently by insurances, as the coverage might 495 be related to the socioeconomic situation for a family. Even though Sweden is regarded as a country with 496 a high degree of income equality, residential segregation exists in Malmö (the National Board of Housing, 497 Building and Planning 2010). Differences in insurance coverage between the areas compared in this study 498 is, however, probably small, as the socioeconomic status in the areas are even: the six areas all belong to 499 the lower quartile of socioeconomic status (Wallin 2010). Secondly, property owners who only get minor 500 losses might not report these to the insurance company, as the deductible is high for some insurances. 501 Therefore important information, especially on smaller events, might be missing in the data sets. Thirdly, 502 the data sets only represent information from buildings. There is no data from other societally important 503 constructions and functions, like roads and railways, power stations, the harbor, water utilities, and 504 telecommunication installations. As mentioned before, the areas compared in this study are, however, 505 similar in many aspects, and the main objective of this study – to analyze what influences blue green 506 infrastructure has on flooding– is probably not affected by this limitation in the data sets. 507

5. Conclusions 508 Flood risk reduction in Augustenborg was achieved through implementation of an extensive blue green 509 infrastructure. Five areas with similar age, land use, and imperviousness around Augustenborg were 510 selected for comparison of flood magnitude. All five areas have combined sewer systems, corresponding 511 to what Augustenborg had before the blue green stormwater retrofit. Direct effect of the retrofit looking 512 at pre- and post-installation data indicates that Augustenborg was more affected by flooding before 513 stormwater control measures were constructed. We also showed that most of the unretrofitted 514 neighboring areas have been more affected by flooding in recent years as compared to Augustenborg. In 515 the long term, reported insurance claims show that the flood magnitude was more than 10 times smaller 516 in Augustenborg compared to the other areas both during the extreme event on August 31, 2014 and 517 during other reported events in 2007–2015. Our data also show that the system is not perfect, as excess 518 water can reach basements directly from the streets during extreme events. 519

Flood insurance data are found to be a useful source of information to evaluate flood risk in urban areas. 520 The study can also conclude that risk levels for the insurance companies are lower in areas with 521 stormwater control measures, which might increase insurance companies’ interest in such retrofit projects. 522

Acknowledgments 523 The authors thank Helén Nilsson from the insurance company Länsförsäkringar Skåne and Stefan Milotti 524 from the utility company VA Syd for flood claim data. Susanne Steen Kronborg and Tomas Wolf from VA 525 Syd are thanked for valuable data about the Malmö drainage system, rainfall data, etc. Cooperation with 526 Länsförsäkringar Skåne was originally established by Joanna Theland during her master’s thesis and she 527 collected data from several companies and organizations, which were used in this study together with 528 complementary flood claim data received later. Thanks also to Malmö municipal housing company 529 (MKB), Lifra (property manager), IF Skadeförsäkring (insurance company), Moderna Försäkringar 530 (insurance company), and Stadsfastigheter (property manager for Malmö City) for data. Lars Bengtsson 531 and Rolf Larsson are thanked for their valuable comments to the manuscript and Cintia Uvo are thanked 532 for help with the statistical analysis. 533

GIS data on urban areas, streets, etc., were distributed by the Swedish national mapping, cadastral, land 534 registration authority: © Lantmäteriet, Dnr: I2014/00579. 535

This study was supported by a grant from Formas, No. 942-2015-149 (Sustainable Urban Flood 536 Management) and Vinnova project: Testbädd för grönblå urbana lösningar, Dnr: 2016-04254. 537

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Table 1. Calculated building and road coverage as well as details on drainage system in Augustenborg and the five nearby areas used for 663 comparison in this study. 664

Name of area

Area [hectares]

Building coverage [%]

Road coverage [%]

Total approximate imperviousness [%]

Period of construction

Dominating drainage system type

Maximum pipe dimension

Augustenborg

30 20 37 57 1948–1952

Blue green infrastructure Old combined system used as sanitary sewer

750 mm (pipe in combined system)

Lindgatan 3.4 20 42 62 1910s–1960

Separate 600 mm

Lönngården

9.2 19 31 50 1935–1936

Combined 450 mm

Norra Sofielund

25 29 41 70 (1910s) 1935–1936

North: combined South: separate

North: 450 mm South: 600 mm

Södra Sofielund

33 30 38 68 1910s–1950s

Combined West: separate

750 mm West: 1000 mm*

Persborg 11 19 31 50 mid-1950s

Combined 600 mm

* One of the main interceptor sewer pipes lies in this area, a 2000 mm combined stormwater and waste water pipe. 665

666

667

668

Table 2. Flood magnitude (NFP/ha) reported by LF Skåne and VA Syd in Augustenborg and five similar areas for 2007–2015 (both years included) 669 with the severe flood event in 2014 shown separately. 670

Name of area Flood magnitude (NFP/ha) 2007–2015 (except 08-31-2014)

08-31-2014

Augustenborg 0.03 0.37 Lindgatan 0.29 3.20 Lönngården 0.87 6.16 N Sofielund 0.67 2.99 S Sofielund 2.02 8.96 Persborg 0.54 3.57

671

672

Table 3. Flood claims reported in each area before and after stormwater control measures were constructed in Augustenborg. Before is represented 673 by the period 1994–1999 (six years), while after is represented by 2009–2014 (six years). Data from VA Syd was used. 674

Period Augustenborg Lindg. Lönng. N. Sof. S. Sof. Persborg Total Before (1994–1999) 5 25 12 23 44 4 113 After (2009–2014) 0 3 10 39 162 3 225

675

676

Table 4. Number of flooded properties, and flood magnitude for Augustenborg and the five nearby areas on August 31, 2014. 677 Name of area Area

[hectares] Number of flooded properties [NFP]

Flood magnitude

LF Skåne

VA Syd

TOTAL (including Lifra, Stadsfast, and Moderna)

[NFP/ha]* [NFP/ address]

Augustenborg 30 11 0 15 0.5 0.10 Lindgatan 3.4 8 3 11 3.2 0.24 Lönngården 9.2 50 7 59 6.4 0.55 Norra Sofielund

25 46 30 85 3.4 0.19

Södra Sofielund

33 163 129 300 9.2 0.67

Persborg 11 37 3 41 3.7 0.43 * Note that the numbers in this table are slightly higher than in Table 2, as Table 2 only includes data from LF Skåne and VA 678 Syd, while this table also includes data from Lifra Stadsfast and Moderna. 679

680

681

Fig. 1. a) Top corner: Europe (part of) with Malmö marked. Main map: Malmö with study areas marked (black), main sewers 682 (dark, thick lines), and system type (combined sewers in dark gray and separate waste and stormwater in light gray with hatch). 683

b) Study areas, i.e. Norra Sofielund, Södra Sofielund, Lönngården, Persborg, Lindgatan, and Augustenborg. Land use types from 684 Urban Atlas (European Commission 2012) with degree of sealing for urban fabric within the brackets. 685

686

Fig. 2. Channel and pond in Augustenborg in the southern area, which was redeveloped during the first phase of retrofit. The 687 area in the right hand side picture was reconstructed from a wetland to a double pond with a small brook between for aesthetic 688 and maintenance reasons. Photographer: Johanna Sörensen. 689

690

Fig. 3. Maximum precipitation volumes in Augustenborg (M03_Augu) and average precipitation measured in Malmö 691 (Augustenborg, Turbinen, Djupadal, Bellevue, Limhamn, Bulltofta, Hammars Park, Höja, Söderkulla (mobile station), and SMHI 692 station A) on August 31, 2014 for durations between 15 minutes and 12 days rainfall. For comparison, intensity-duration-693 frequency (IDF) curves from Hernebring et al. (2015) are given. For the average rainfall, a 95% confidence interval is given, 694 assuming normal distribution of the precipitation for each duration. 695

696

Fig. 4. Flood magnitude (NFP/ha) for Augustenborg and five similar areas on August 31, 2014. High flood magnitude is shown 697 in dark gray and low flood magnitude in light gray. 698

699

Fig. 5. Number of flooded properties reported to the insurance company IF Skadeförsäkring. The reports only give the total 700 number of flooded properties for each postal area. Therefore, the extents of each postal area are marked. 701

702

703

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Assessment of barriers and drivers for implementation of blue-green solutions in Swedish municipalities

M. Wihlborga, J. Sörensenb*, J. Alkan Olssona

a) Centre for Environmental and Climate Research (CEC), Lund University, Lund, Sweden

b) Water Resources Engineering, Lund University, Lund, Sweden

* Corresponding author, +46 46 222 44 87, johanna.sorensen@tvrl.lth.se

Abstract

Amidst the on-going trend of urbanisation, and the notable effect of climate change, there have been calls for a more sustainable management of stormwater. As part of this move-ment, blue-green measures, with their multifunctional benefits, have been recognised as a sustainable solution and a necessary complement to pipe-bound solutions. The aim of this study is to identify barriers and drivers of the implementation of blue-green measures in a Swedish context in order to increase the understanding of how they could be implemented in a more successful manner. The used methodology is qualitative and based on semi-structured interviews. Through the lens of transition theory and a multi-level perspective, was used to locate the main barriers and drivers as well as to provide an understanding of the interaction or lack of interaction between levels. Today there are many factors that en-courage municipal actors to implement blue-green measures, such as increased need for rec-reation, protection of biodiversity and protection from climate change. The majority of the problems related to the implementation of blue-green solutions however exist within the municipal stormwater management it-self but are also related to other inter-organizational barriers, such as lack of knowledge among politicians, officials, exploiters and civilians, fragmented roles and responsibilities, as well as uncertainty within the long term, economic management of stormwater. Another important barrier for municipal actors is related to the legal structure around stormwater management which, tie the hands of involved actors. Leg-islation therefore needs to be clarified. Also, new norms as well as organisational models related to how to manage urban stormwater must be established, as this can change the per-ception of the problem and facilitate a broader insight into the benefits of blue-green measures.

Keywords: blue-green solutions, sustainable urban drainage systems (SUDS), urban planning, transition theo-ry, multi-level perspective

1. Introduction

More and more people choose to live in cities, which leads to an increasing urban growth (Jha et al. 2011). This leads to major land use changes in and around urban areas, which has a complex set of economic, social as well as environmental impact on urban space. It is diffi-cult to predict the complete consequences of this growth, as it is a process influenced by both the type as well as the speed of growth. Simultaneously, climate change leads to increased torrential rains, flood risk, heat waves, drought, and sea level rise (Maksimović et al. 2015). A large part of its effects will be concentrated to urban areas (IPCC 2014), which will increase the demand on stormwater management and drinking water provision (Wong & Brown

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2008; Bates et al. 2008; Loftus et al. 2011). Due to a history of relatively sparse residential con-struction in Sweden, compared to other countries, housing shortages in attractive urban are-as and political ambitions to reduce emission of greenhouse gases and spare agricultural land drive the development of a denser urban space rather than urban sprawl, which put an even higher pressure on urban space management.

Due to these challenges it is argued new approaches to urban stormwater management is urgently needed (SOU 2014: 212). Stormwater management must be able to maintain its functionality during sudden local extreme events and at the same time provide benefits for people and nature in denser urban contexts (Ashley et al. 2011). This necessitates a holistic planning that minimize environmental impact, utilize land in a more multifunctional way taking social as well as well-being aspects into consideration and simultaneously is efficient with economic resources (Stahre 2004; Cettner et al. 2013). The ambition of holistic planning has been met with the idea that technical urban stormwater solutions should be combined with solutions using nature’s diversity as inspiration. The urban green infrastructure should in this perspective be seen as a part of the urban water management structure including fea-tures such as green roofs, rain gardens and rainwater ponds (Voskamp et al. 2015; Maksi-mović et al. 2015:3). The dynamic between water and vegetation can create an environment that can buffer and balance extreme weather events such as torrential rain and flooding (Dreiseitl 2015). It is also argued that such solutions can mimic natural systems and make them more resilience to climate change as well as cost-efficient and could therefore have the potential to increase and accelerate the desired transformation of urban water management (Mguni et al. 2015).

Blue-green solutions are frequently promoted as umbrella terms for such sustainable multi-functional measures able to reduce negative effects of urbanisation and adapt to a changing climate such as heat- and water regulation, air and water purification, increased biodiversity and recreation (National Housing Board 2010a; Cettner et al. 2014; Voskamp et al. 2015; Lerer et al. 2015; European Commission 2015). Other terms that are used are nature based solu-tions (NBS), sustainable urban drainage systems (SUDS), water sensitive urban design (WSUD), low impact development (LID), and sponge city (see Fletcher et al. 2014; Jia et al. 2016). These concepts have slightly different flavours from more green to more blue and usually have a stronger affiliation to one scientific discipline than another. Implemented in the urban context their aim is typically stormwater detention, infiltration and purification in a decentralised manner.

In Sweden, blue-green solutions are, in the area of stormwater management, known since the 1970s, primarily under the name lokalt omhändertagande av dagvatten (LOD, local treatment of stormwater) in the last decade there is an emerging trend where many municipalities sup-port open stormwater management and blue-green solutions to create a more robust and multifunctional management that is able to handle both social challenges as well as effects of climate change (Ashley et al. 2011; Cettner et al. 2014). However, the dominating solution to stormwater management is still pipe-bound solutions (Cettner et al. 2012), which is the cur-rent norm amoung both municipalities, developers and contractors (Ashley et al. 2011).

The dominating regime to stormwater management in Sweden is pipe-bound solutions (Cettner et al. 2012). Municipalities, are key players in the stormwater management as they are legally responsible for stormwater management, this responsibility is frequently delegat-ed to public water services. There are three main laws regulating the management of storm-water; the Planning and Building Act (PBL), the Environmental Code, and the Act on Public Water Services (SFS 2010: 900, SFS 1998: 808, SFS 2006: 412). PBL provides conditions for how to use the land. The Environmental Code and the Act on public water services govern the

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handling of stormwater at the municipal level (National Housing Board 2010b). A munici-pality can direct a certain stormwater management approach on private land as a part of the permission process, but have little influence on already built areas.

In addition, there is EU legislation implemented in Sweden, which may influence the regime of stormwater management: the EU Water Framework Directive (WFD) (Directive 2000/60/EC) and the Floods Directive (FD) (Directive 2007/60/EC). The WFD aims to en-sure access to water of good quality in river basins and the FD handles flood risk manage-ment. The two EU directives has introduced a new way of organising water management. Water issues have gained an increasingly larger place in the physical planning (National Housing Board 2010a). However, within the current stormwater management regime, it is not allowed for water utility companies to spend money collected for urban drainage on so-lutions that cannot directly be motivated to be a part of a structure for urban drainage in ac-cordance with municipal responsibility (Svenskt Vatten 2016). This means that reconstruc-tion of a park to handle stormwater during extreme precipitaton events can be difficult to motivate legally, as stormwater control during extreme events are not a legal responsibility for the municipalities in Sweden. Nevertheless there is an emerging trend where many mu-nicipalities support open stormwater management to create a more robust and multi-functional stormwater management system to handle both urban social challenges as well as effects of climate change (Ashley et al. 2011). In the two recent decades, numerous pilot pro-jects and experiments with different types of blue-green solutions have been implemented, both in Malmö and Helsingborg, the case municipalities in this article . These projects have been financed by external money in the form of various EU projects and are seen as an im-portant step to develop and gain practical knowledge in relation to alternatives measures for stormwater management suitable for local conditions (Farrely and Brown 2011). A question that can be asked is: Why are these solutions not being implemented at a faster rate and not spread beyond the externally financed projects? Several studies have noted that the uptake of blue-green solutions is progressing slowly (van de Meene et al. 2011; Brown et al. 2009c; ref). Cettner et al. (2012) describe this slow transformation of stormwater management in Sweden as caused by the “pipe bound culture”.

The aim of this paper is twofold. The first aim is to identify barriers and drivers hindering or encouraging the transition from a pipe-bound stormwater management to a widespread im-plementation of blue-green solutions to handle urban stormwater, and to analyse these bar-riers and drivers from a multi-level perspective. The second aim is to discuss potential ways to overcome the barriers and to encourage transition to a more sustainable and multifunc-tional urban stormwater management system.

2. Transition theory- an analytical tool to assess socio-technical change

In this paper, transition theory is used as an analytical tool to explore barriers and drivers that may exist in relation to an altered management of stormwater in Sweden. Transition theory sees socio-technical systems, in this case the stormwater management system, as com-posed of three constellations: regime, niche and niche-regime. Surrounding these is the land-scape that forms the pre-requisites for the system (de Haan & Rotmans 2011). The three con-stellations and their surrounding landscape is shown in Figure 1. While Rotmans et al. (2001) relate the levels to bureaucratic levels (macro, meso, and micro level) or nested hierarchies (2011), we perceive the levels as including organisational, technical as well as social systems as suggested by Geels (2011). In the following section we briefly explain the features of the different levels and thereafter we discuss how transition theory perceives change in a sys-tem.

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Fig 1. Multi-level perspective of socio-technical system with three constellations: regime, niche and niche-regime. The landscape forms the pre-requisite for the system (Ashley et al (2011) and de Haan & Rotmans (2011).

The regime dominates the system and is the way that societal needs are met (de Haan & Rot-mans 2011). A regime includes institutions, technologies, practical applications and social relationships (Geels 2002, Smith et al. 2005). The regime is the most powerful constellation of the system (de Haan & Rotmans, 2011). In Sweden, the current regime of urban drainage management consists of a pipe-bound system with a centralised management. Several actors are involved in developing and maintaining it(Cettner et al. 2012). A new regime is more than just an evolutionary transformation of a previous regime it has to be significantly dif-ferent. For instance, to change from combined to separate sewers is an evolutionary trans-formation, while the change from pre-sewered cities to sewered cities can be considered as a transition to a new regime. (Ashley et al. 2011).

A niche is a system that only meets certain, specific societal needs (de Haan & Rotmans 2011). The niche level is mainly referred to where emerging innovations are developed that are fundamentally different from solutions at the regime level (Geels 2011).

A niche-regimes is a regime that is not currently dominating, while it has significant power to compete with the regime for the functioning of the system (de Haan & Rotmans 2011). Thus, a niche-regime is somewhere between niches and the regime in strength.

The landscape level represents the wider context (legal systems, demography, economy and the natural environment) that in a long term perspective is able to influence and affect the development of the practices at regime, niche-regime and niche levels (Geels 2002; Geels 2011; Koppenjan et al. 2012).

A central idea of the transition theory is that change emerges from niche through alternative solutions to niche-regime to displaced or replaced regime but a regime can also be influ-enced by actions and ideas emerging from landscape level (Koppenjan et al. 2012, Geels 2011 Smith et al. 2005). The challenge of the regime emerge from the dispersal of new social norms concerning how to solve a problem, know-how and motivation that alternative solutions bring with them when developed and implemented at the niche level (Koppenjan et al. 2004; Van de Bruges et al. 2005; de Haan & Rotmans 2011; Ashley et al. 2011 Farrelly & Brown 2011).

Drivers for this type of socio-technical transition has been divided into three categories stress, pressure and tensions (de Haan & Rotmans 2011). Pressure comes from alternative technolo-gies that become viable competitors to the regime. Stress emerges when the regime is inade-

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quate or internally inconsistant in meeting the needs of the system. Tension is when the sys-tem compromises in its relation to its natural or social environment. The tensions can either be structural, related to physical aspects of the regime, legal or cultural, related to cognitive or discursive aspects of the regime (de Haan & Rotmans 2011). Structural tensions are more frequently related the landscape level where as the cultural tensions can emerge from both niche and landscape level (Ashley et al. 2011).

Due to entrenched technological path dependency and cognitive lock-in, actors may be una-ble or hindered to implement new solutions (Brown & Farrelly 2009a; Werbeloff et al. 2011). Such implementation barriers, have different origins and be technological, legal organisation, financial social, educational or related to political will (Holtz et al. 2008; Brown & Farrelly 2009a; Brown & Farrelly 2009b; van de Meene et al. 2011; Cettner et al. 2013; Cettner et al. 2014; Winz et al. 2014; Mguni et al. 2015). As a consequence a transformation of a regime must consist of both physical, administrative and structural changes in the regime (Bettini et al. 2015) and change usually has to occurs at multiple levels for a regime to alter (Geels 2002). Barrier and drivers can originate from one level in the multi-level perspective, but affect the outcome at another level.

3. Method

The empirical material to this study was collected through interviews with 20 municipal and water utility company bureaucrats in two municipalities and two water utility companies in the south west of Sweden. The two municipalities (Malmö and Helsingborg) focused on in this study, use two different water utility companies, VA-syd (Malmö) and NSVA (Helsing-borg), to provide stormwater management and water treatment services to a total of 730,000 persons (Figure 2). Malmö (330,000 inhabitants) and Helsingborg (140,000 inhabitants), are the two largest in the region and have a long experience of working with blue-green solu-tions.

Fig 2. Municipalities covered by the water utility companies in the study, NSVA (dark grey) and VA Syd (light grey).

Interviewees were chosen by contacting a key actors in each of the municipalities, and there-after new interviewees were chosen by snowball sampling aiming to cover an as broad range as possible of the municipal expertise related to the management of stormwater and blue-green solutions (Flowerdew et.al. 2005). The interviewed persons work at different depart-ments, as environmental administration and planning office, and have a varied academic

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background. A majority though have a background in natural and technical science (Table 1). In total, 20 persons were interviewed. Interviews were recorded, transcribed and written down in text format.

Table 1. Interviewees and their professional background.

Nr Municipality Department Proffessional title

1 Helsingborg Green Environmental strategist

2 Helsingborg Green Head of department, environmental strategy

3 Helsingborg Planning Water planner

4 Helsingborg Planning Landscape architect

5 Helsingborg Planning Physical planner (zooning planning)

6 Helsingborg Planning Plan co-ordinator (zooning planning)

7 Helsingborg Planning Landscape engineer

8 Helsingborg Planning Head of department, building permit

9 Malmö Planning Community planner

10 Malmö Planning Landscape architect

11 Malmö Green Project leader (leads projects and develop-ment)

12 Malmö Green Project leader (leads projects and respond to referrals)

13 Malmö Planning Building permits, architect

14 Malmö Planning Head of department, planning

15 Malmö Planning Physical planner

16 Malmö Planning Environmental strategist

17 Malmö Planning Environmental administrator

18 NSVA Blue Civil engineer

19 NSVA Blue Civil engineer

20 VA SYD Blue Head of department , new construction and exportation

The purpose of the interviews was to collect information about barriers and drivers related to the implementation of alternative solutions to urban stormwater management. The inter-views were semi-structured (Flowerdew et al. 2005) and based on an interview guide with three themes: 1) barriers respective 2) drivers related to implementation of blue-green solu-tions and 3) what would be needed to overcome barriers and start a transition towards a more holistic stormwater management.

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The material was analysed by the hermeneutic process – a circular analysis process where new insights are integrated from the understanding and interpretation of the studied materi-al through a constant exchange between the parts and the whole (Kvale & Brinkmann 2009).

Based on this anlysis, topics were identified in relation to the themes in the interview guide. Based on the themes and the topics a data table was compiled to categorize the informants’ quotes and expressions. This approach to categorizes the empirical material gives a clear structure and stability of how and which actor that express the drives and barriers. From this categorization, patterns could be distinguished among the answers from the interviewees.

Analysing and categorising of the empirical material was made in two steps. One part was to count how many of the interviewees mention a specific driver or barrier. This is reported in table 2 and 3 in the result section of the article. After this categorization, the next step was to understand how the barriers and drivers are framed in the context of transition theory and the multi-level perspective. The identified barriers and drivers were therefore first catego-rized by their origin in the multi-level perspective, for example is climate change a landscape factor..

4. Results and analysis

In this section, the result from the interviews and an analysis of identified barriers and driv-ers for the implementation of alternative stormwater structured is made.

4.1. Drivers for blue-green solutions

From the interviews, three types of drivers were identified: ecosystem services, climate change, and economy. In table 2, the identified drivers are presented in relation to how many of the informants that mentioned each driver.

Table 2. Drivers for increased implementation of blue-green solutions identified among the inform-ants. The categories is a categorisation where blue is bureaucrats working mainly with water, green is bureaucrats working mainly with ecology and vegetation and planning is bureaucrats working mainly with urban planning.

Drivers Number of informant that mention it

Green Blue Planning

Ecosystem ser-vices

20/20 4/4 3/3 13/13

Climate change 17/20 4/4 2/3 10/13

Economy 6/20 0/4 1/3 5/13

Politics of urban densification

7/20 2/4 0/3 5/13

Ecosystem services

All informants (20) mentioned ecosystem services such as recreation, biodiversity, delay and clean water as driving factors to work with blue-green solutions. Green and open solution provides values and qualities to the urban space (Informant 11, 2016). Environmental protec-tion of for instance watercourses is a driver for blue-green solutions (Widarsson 2016, Ashley

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et al. 2011), but also cultural ecosystem services, like aesthetics, are mentioned both by the interviewees and in the literature (Widarsson 2016). The interviewees mention different ser-vices as the most important for example;

The need for recreation areas are the primary drivers behind the implementation. (In-formant 10 2016)

The important thing is delay and treatment of stormwater. (Informant 7, 2016)

Another incentive could be to create more biodiversity and the desire to create a differ-ent environment, not just a lawn but something else. That can be an important driver. (Informant 20, 2016)

Water gives a beautiful aspect to a city, combining park and water facilities (Informant 18, 2016).

At the same time some of the respondents mentioned, that if there is a choice between biodi-versity and construction of houses, nature comes second. The following quote shows this argument in an illustrative manner.

There are few who experience a visible damage when dragonflies decline in a river. (In-formant 1 2016)

Climate change

Almost all respondents (18) mentioned climate change as a driver to implement blue-green solutions. The interviews show that there are awareness and knowledge about that the cli-mate is changing and that the actors need to adapt urban areas to be able to handle the in-creased precipitation.

Climate change is almost a positive thing and the fear of flooding. It has opened up for discussion with an entirely different width than earlier. (Informant 1 2016)

Climate change pushes society to adapt the existing infrastructure, to make it more flexible and robust (Ashley et al. 2011). Awareness among officials can stimulate a process of change and facilitate increased cooperation between administrations in a municipality, but also in-spire other actors to act, as developers or consultants. Climate change as a driver can lead to a greater consensus and dialogue between administrations, and between different disciplines in a city. To succeed with an implementation of blue-green solutions involvement of differ-ent actors, with different backgrounds, is needed for a successful change (Van de Bruges et al. 2005; van de Meene & Brown 2009; Mguni et al. 2015). Climate change is a driving factor for local authorities to work with multifunctional stormwater solutions.

Economy

Many of the respondents (14) mentioned that the financial aspect is a driver to implement blue-green solutions. There is an awareness of the economic impacts of downpours that may affect the municipality. One respondent argued that;

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There is a fear of the economic impact of downpours, such as basement flooding, dam-ages, compensation for poor planning (Informant 1 2016).

Several respondents mentioned that it is expensive to increase the number and dimensions of existing sewage pipes in the existing built environment. It was also argued that many pipes are getting old and will need to be replaced. From that perspective, blue-green solu-tions can be more cost effective (Ashley et al. 2011) and municipalities claim that they use the solutions to reduce costs (Widarsson 2007).

It is not economically viable to lay pipes in the existing built environment (Informant 10, 19 2016).

If one is to follow up on the grid with larger pipes it will be expensive (Informant 3 2016).

There is a difficulty working with nature, that it doesn’t have a defined cost, and natural values are therefore complex to include in city planning. Utility companies in England have to some extent started to see commercial advantages of blue-green solutions (Ashley et al. 2011) and more research focus on the cost-benefit of ecosystem services (Read et al. 2016, Ossa-Moreno et al. 2017). Interestingly, no bureaucrats with ‘green’ background mentioned economy as a driver (see table 2).

Politics of urban densification

Urban densification was mention both as a potential driver and a barrier in the interviews. There was 7 informants that mention urban densification as a potential driver to implement blue-green solutions. Most of them is working with planning (5). See the quote below.

We know that we make more and more impervious surfaces, and then we have more water to take care of. It is the main driving force. (Informant 7 2016)

I believe that the awareness grow continously, especially in the work with urban densi-fication. We need to address these questions. (Informant 6 2016)

This shows that there is a awareness about how a lager densification affects the urban cli-mate, with more water runoff, in a city. Urban densification is two folded which can create a awareness among actors, and obstruct green and blue structures in the urban matrix.

Existing drainage system often lack capacity when new areas are developed or densified (Widarsson 2007) as runoff are higher from denser urban areas (Sjöman & Gill 2014). This puts a stress on the regime, as it cannot any longer meet the main needs. In Västerås, such a stress within the current regime led to cooperation over organisational borders and a new surface water strategy was written with the goal to eliminate the combined system, reduce extraenous water to the waste water treatment plant and to reduce pollution of Lake Mälaren (Widarsson 2007).

4.2. Barriers for blue-green solutions

Eight types of barriers to the include new stormwater solutions were identified in the inter-views. In table 3, these barriers are presented in relation to how many of the informants that mentioned each barrier.

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Table 3. Barriers against increased implementation of blue-green solutions identified among the in-formants. The categories is a categorisation where blue is bureaucrats working mainly with water, green is bureaucrats working mainly with ecology and vegetation and planning is bureaucrats working mainly with urban planning.

Barriers Number of inform-ants mentioning it

Green Blue Planning

Economy 18 4/4 3/3 11/13

Roles and responsibilities 17 4/4 2/3 10/13

Lack of knowledge 17 4/4 2/3 11/13

Legislation 14 4/4 3/3 9/13

Municipal organisation 13 4/4 1/3 8/13

Urban densification and housing shortage

10 2/3 1/3 7/13

Political interests 9 3/4 1/3 5/13

Time and workload 9 1/4 2/3 5/13

Economy

The interviews conceived economy both as a barrier and as a driver, depending on which economical aspect the informants referred to. While 14 informants mentioned economy as a driver, almost all (18) mentioned economy as a barrier against implementation of blue-green solutions. Several informants mentioned the uncertainty of the costs for maintenance of the solutions. Economic reasons counteract an increased implementation of blue-green solutions (Cettner et al. 2014) and some developers perceive non-piped solutions as more costly (Ash-ley et al. 2011).

A majority of the respondents argued that economy is an important factor that define the outcome of the activities of the planning process. The different municipal department has their own budgets. Several informants argued that there is a difference between planning for a solution and a different story to implement them.

Many times these solutions exist at the planning stage. Space is allocated in the first sketches that the landscape engineers develop. The more time that passes, the more space the developer wants to exploit. The land is precious today and these solutions are excluded. (Informant 18, 2016)

Furthermore, according to the interviewees there is an uncertainty about which actor will finance solutions, with creating a resistance to implement blue-green solutions. The business model of water utilies can also inhibit implementation, like in Wales (Ashley et al. 2011).

There is a difficulty working with nature, that it doesn’t have a defined cost, and natural values are therefore complex to include in city planning, but it do act as a driver that puts a tension on existing stormwater regime.

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Roles and responsibilities

In the studied municipalities, there are several players with different skills, knowledge and training involved in the stormwater management chain, from the strategic and overall plan-ning to detail planning for building permits and to private individuals. In the interviews the lack of clarity of roles and responsibilities emerged as an issue.

It may also be that there is an unclear division of responsibilities or that we are not enough involved in each other's projects. (Informant 3 2016)

Van de Bruges et al. 2005 have argued that water management in an urban environment is a "persistent" problem. Mguni et al. (2015) and van Herk et al (2011b) describes this as a "wick-ed problem". Hordijk et al. (2014) suggest that the reason is that different stakeholders value water differently.

The interviews revealed a mixed picture regarding who has the overall responsibility for the implementation and management of blue-green solutions. Several interviews mentioned that the Department of Housing and Urban Development has the overall responsibility for estab-lish blue-green solutions, while the Water utility companies have an important role. But sev-eral informants also argued that there should be a shared responsibility to implement blue-green solutions involving not only the municipality but also consultants, property owners and developers. But it was argued that these actors do not realize their role in the big picture which increase the unclarity of responsibilities.

The responsibility for the stormwater lies with everyone actually, right down to the homeowner, but we are far from there. (Informant 16, 2016)

Lack of knowledge

In Sweden there is a widespread awareness about blue-green solutions (Ashley et al. 2011). Still, a majority of the respondents (14) mentioned that there is a lack of knowledge among officials regarding blue-green solutions. The comments shows that lack of knowledge creates an inertia to understand the importance of blue-green solutions in the urban landscape. As a consequence actors slow down or even block a change.

There is need to work wider and to include the ecological expertise in a different way than we did before. Even more intersectoral and more ecological knowledge, either through the recruitment of new people or through educating those already working (Informant 11, 2016)

This has also been confirmed in other studies where it is forwarded that an increased knowledge among stakeholders can encourage the transformation and integration of differ-ent skills and approaches as well as open new ways of thinking as well as drive the uptake of new technologies in the urban context (McCormick et al. 2013).

The lack of knowledge does not only concern implementation of new solutions but also how solutions should be maintained. In several interviews lack of knowledge also concern main-tainance as even if knowledge of construction costs are known the maintainance costs are not. There is a lack of knowledge regarding cost-efficiency and lack of experience from estab-lished solutions (Widarsson 2007, Ashley et al. 2011).

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Research on organisational learning and climate adaptation shows that it is a challenge to spread learning outcomes (Storbjörk 2010). So far, much of the work in Sweden has been done by enthusiastic individuals that drives workings groups within the organisation (Widarsson 2007, Storbjörk 2010, Cettner).

It is also argued that the lack of knowledge also concerns other actors, like private property owners.

There is a very difficult actor and that is the property owner who has a rather big re-sponsibility too. All villa carpets that are hard paved. There is no awareness, or at least very few are aware. (Informant 12, 2016)

Private actors affect land use in urban areas through investments and the urban infrastruc-ture is largely influenced by their economic and social interests. Lack of knowledge and fo-cus on private exploiters desire to maximize profits is a factor that can both block and allow for a change (Olazabal & Pascual 2012).

Another factor that potentially can drive and push an for a increased implementation of blue-green solutions is forwarded in one of the interviews which is;

It feels like there is a new generation of officials on their way, so it feels like a lot has happened and blue-green solutions is a part of the work. If you think climate proof buildings. There is an increased awareness. It is clear that some senior officials contin-ue as they have always done and then it is difficult to change later. (Informant 18, 2016)

An increased knowledge of officials, developers and the public are probably important in order to achieve a holistic stormwater management.

Legislation

The legal structure related to the management of stormwater is complex and to a certain ex-tent unclear, which make it difficult to apply the different legislation that affect stromwater, which increase uncertainty for involved stakeholders; both municipalities and private actors (Persson et al. 2012; Dir 2015: 115). Legislation is crucial for how municipalities will be able to implement blue-green solutions in the future. It was highlighted in interviews, that the current legislation leads to a complex planning process of implementing blue-green solu-tions. Municipalities cannot demand special technical requirements for different landowners. This means that municipalities cannot require more than stated in the PBL or Building Regu-lations BBR (Building and Planning 2016).

In the legislation it’s possible to indicate a request but it is not possible to demand technical requirements. Land allocation is given through an exploitation agreement. The environmental Code and PBL are in coalition with each other. There is a conflict between environment and permission to build. The motivation is there, but you are re-strained. (Informant 17 2016)

The lack of special, technical requirements can therefore operate as a blocking barrier for experimentation and innovation to be established in stormwater regime. The municipality cannot make stronger demands on developers, which could have been a stimulating act for a

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change to occur. According to the interviews, the problem is that the detailed planning is mainly focused on exploitation. The focus on development slows down the transition, since the focus is to exploit and not to evaluate the potential positive effects of blue-green solu-tions.

Several of the informants mentioned that it is a problem that blue-green solutions do not have a quantitative value in the planning process. There are clear standards about the num-ber of parking spaces, the size of a turning zone or the number of schools, but there are no clear standards related to the size and number of blue-green space. This means that after demands for parking lots and schools are met, there is a risk that there is no physical space left for implementing blue-green solutions (Informant 15, 10 2016). Another respondent ar-gues in a similar manner;

If you let all functions add on top of each other, the distance between houses and people become even bigger. Then we do not talk about the dense city, but the sparse, and then transport need are increased. We need to dare to prioritize. It is not possible to add functions on top of each other. Let’s say that you must have six meters roadway, four-meter trees, and then you add on three-meter bike road and two-meter pavement. If you add all these measures on top of each other, the street canyons become very broad. (In-formant 14 2016)

It has been recognised at a national level that uncertainty about roles and responsibilities among actors is affected by unclear legislation which resulted in a government investigation in Sweden (SOU 2017:42). Brown & Keath (2008) have argued that drivers at the landscape level are required to stimulate a change in the regime, despite the innovations and new tech-nical solutions at niche level which is confirmed by the interviews in this study. Also Widarsson (2007) argues that top-down requirements from authorities are needed. In Swe-den, regulations in the Act of public water services inhibit implementation of blue-green so-lutions in practice (Ashley et al. 2011).

Despite implementation of the UK Flood and Water Management Act 2010, which was de-veloped mainly in response to the 2007 flooding, piped solutions are still the default solution (Ashley et al. 2011).

Municipal organisation

In several of the interviews it has been argued that there is a lack of structure that can pro-mote the cooperation needed to implement blue-green solutions.

The structure, how the organization is structured. It is this pipe thinking. You sit on your budget, your own resources, interests, responsibilities... Everyone talks about communication and collaboration. Everyone talks about it and then we use all the old tools and it's not working. (Informant 15, 2016)

Holtz et al. (2008) argues that innovation needs to be integrated into existing cultural net-works and structures. Based on the interviews involved authorities have their roles. There are various processes as well as projects "owned" by the Town Planning and the Environ-ment Department. But these innovative experiments and pilot projects have had difficulties to establish themselves as a mainstream solution.

The interviews also showed that there is a willingness to cooperate, but there are barriers linked to how the municipality is organised.

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The structure of the municipality leads to a "narrow-thinking", because each depart-ment has their own budget, interest and responsibilities when it comes to community planning (Informant 16, 2016).

Several studies have found that widespread implementation of climate adaptation measures and new solutions are highly dependent on single, engaged persons within the municipal organisation (Storbjörk 2010, Widarsson 2007, Cettner). This make the knowledge base vul-nerable and the work lacks continuity (Storbjörk 2010).

Urban densification and housing shortage

Several informants (7) mentioned densification and housing shortage as a barrier. Some in-formants said that there is a ‘housing shortage panic’ in the municipalities (Informant 9, 12, 15, 17, 2016) and that there is therefore a risk that officials don’t have time to take blue-green solutions into consideration (Informant 12, 2016). There is a general shortage of housing in Sweden that put pressure on the use of surface especially in urban areas and decrease the possibility to introduce blue-green solutions. Van de Meene et al. (2015) draws attention to the problem of a dense city, when a large part of a city is built up with impervious surfaces, there are limitations when it comes implement blue-green solution in the city environment. Other places, other development land use and roads are prioritised over blue-green solutions (Ashley et al. 2011).

It seems however as if the there is an awareness of the potential conflict between dense and blue-green solutions;

It was great that we recognized that there was a conflict between dense and green (Informant 15, 2016)

Political interest

Several informants mentioned the that the urgent housing need is a priority for the politi-cians. Other places urban flooding is so high on the agenda that the multiple benefits of blue-green solutions are forgotten (Ashley et al. 2011). If the need of blue-green solutions could be clarified, it could become a way to make it easier for officials to prioritize the implementation of more blue-green solutions. There is a tension between interests, values and prioritise (Storbjörk 2010).

Visions are hard to say no to. It is clear that we want multi-functional solutions and purify water, of course we want to save biodiversity. It's not hard. But it is when the individual decisions come in, not to make the connection between real estates and envi-ronmental goals. There is a will, but then it is the same politicians who approve urban densification and the constructions. (Informant 1 2016)

The lack of housing and need for densification influence the political agenda of stormwater management. This can have negative effects on an increased implementation of blue-green solutions. Politicians can obstruct and partially block an increased implementation of blue-green solutions as the politicians have the last word when it comes to what can be built and not.

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Time and workload

Several of the informants (9) mentioned time and workload as a barrier that hinders the im-plementation of blue-green solutions which is an important barrier and block implementa-tion of new solutions.

All municipal administrations are committed to an open stormwater solution, then how hard the issue is driven depends on how many other demands there are. (Inform-ant 10 2016)

In those cases where more has not been done to integrate multifunctionality, is due to lack of time in the projects and that the workload is high for many (Informant 7 2016)

There is little resources to work proactively with alternative solutions (Ashley et al. 2011), especially in many small municipalities (Widarsson 2007). Widarsson argues that the water utility often is weak in comparison with the municipal office for the built environment.

5. Discussion

5.1. Drivers

In this study, four drivers of change were identified: ecosystem services, climate change, economy, and politics of urban densification. The two first are external to the stormwater regime and has their origin in two different fields of international and national politics that has gained momentum under the last years. The debates about climate change and ecosys-tem services seem to have created awareness among involved actors and climate change work as a driver both from a general political level and at an individual level. Where the latter driver, climate change, is related to the concrete torrential rains and downpours that hit Malmö in 2014, resulting in major economic consequences for many actors, municipali-ties, private individuals, property owners and insurance companies. Previous research has shown that it often takes a crisis for a transition to take place (Loorbach 2007: 18; Brown & Keath 2008). The effects, both social and economic, of the downpurs act directly on the re-gime and created an internal stress and fear of more extreme events, which could open up for a shift towards an increased implementation of blue-green solutions in a city environ-ment.

The national politics of densification of urban areas also emerged as a driver for the imple-mentation of blue-green solutions, in particular in combination with the economic driver, where the inclusion of blue-green solutions is seen as more feasible than re-piping a whole city. Something which also has been identified in other studies (ref).

In the literature, other drivers are mentioned that were not identified in this study, this con-cerns for example technical problems with the piped system, stressing the regime such as reoccuring problems with flooding often initiate use of blue-green solutions (Widarsson 2007, Ashley et al. 2011, Sörensen & Emilsson under review). Another factor driving for change raisd by the literature is that in some cities it is critical to reduce the amount of extra-neous water to waste water treatment plants (Widarsson 2007).

5.2. Barriers

The barriers that were identified include economic factors, roles and responsibilities, lack of knowledge, legislation, municipal organisation, urban densification and housing shortage,

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political interest, and time and workload. Several of these barriers are closely related, like economy and political interest, and lack of knowledge and municipal organisation.

The legal barriers mainly concerns the water management. Regulation of water is a complex matter, mainly due to the liquid character of the substance, which influence the possibility to divide the responsibility of it, in a meat manner. This has been recognised and increasingly dealt with at a watershed scale level in rural areas, but in cities the watershed management idea has not been implemented (ref). One reason is the additional legal structures regulating urban stormwater management for which municipalities are responsible, a responsibility often delegated to water service organisations in Sweden. The responsibility of private land owners in connection to downpours is not clearly defined (SOU 2017:42). According to the interviews, one barrier is the lack of possibilities to pose special technical requirements on urban developers and property owners.

The barrier related to unclear roles and responsibilities is intimately linked to the legal barri-ers but also to the physical problems of managing water in the compartmentalised organisa-tion of the municipality, where different disciplines to a large extent are separated, and in an organisation that lack cooperative structures. According to Cettner et al. (2013), the urban planner has a key role in creating links between the planning process and the actors involved in stormwater management. However, according to Ashley et al. (2011) other actors such as constructors, possesses a great power, as they often get municipalities to opt for traditional stormwater management by the desire to maximally exploit the land.

Another barrier that appeared as important was the political pressure for urban densification as there would be little space for blue-green solutions in a dense urban matrix. In such a con-text, the problem is not only a potential lack of capacity in the stormwater pipes, but also the potential effects of the lack of green space for the urban dwellers (Tratalos et al. 2007). In the interviews it was clear that is a conflict between different aspects of building a city. There is a lot of different interest that need to be taken inte account. The lack of housing in Sweden has a large political focus, which make it difficult for official to argue for different values as blue-green solutions. In Scania, 28 of 33 municipalities has a housing shortage, and in on national level only one municipality has an excess of housing (Länsstyrelsen 2017).

Lack of knowledge and know-how emerged as another barrier to why multi-functional blue-green solutions has not gained momentum as a general way of thinking. Several of the exist-ing nature-based solutions have been implemented through pilot projects funded by external project money, and knowledge about and processes for implementation has not been main-streamed into relevant part of the municipality. Due to the lack of knowledge of how to con-struct and maintain these solutions, as well as lack of funding in the everyday stormwater management, public officials and project leaders choose the easy and known way out, pipe-based solutions. Other interviewees argued that it is rather due to lack of time and an al-ready heavy workload and yet other argued that the problem is lack of willingness among public officials. What is lacking is a culture where public officials are permitted to test and fail as a part of a learning process, otherwise it will be difficult to increase the implementa-tion of blue-green solutions. Some interviewees also related the lack of implemented blue-green solutions to the lack of political interest.

5.3. Suggestions

Time is short when aiming to manage adverse effects of both urbanization and climate change. Blue-green solutions may handle some of these challenges through an increase or improvement of the green spaces for the benefit of both humans and wildlife (ref). Under this time pressure and in the midst of a densification boom of Swedish urban areas, there is

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an increased need to identify the factors causing the lack of transition and find counter-measures to speed it up (Liu & Jensen 2017). Based on the identified set of driving forces and barriers, this study is not surprisingly indicating that societal change is a complex process. There are multiple instances, connections, links, and levels that affect the outcome of a barri-er or momentum for change. A sustainability transition has to includes the improvement of several aspects of inter-institutional and public-private collaboration.

In a transition context it is often pointed out that innovation of conventional ways of design-ing and managing urban spaces and structures, and practical experiences of this approach through testing and improvement are essential for inducing change. It is argued that through such a development, new social norms are developed concerning how to solve a problem (Koppenjan et al. 2004, Van de Bruges et al. 2005, de Haan & Rotmans 2011, Ashley et al. 2011, Farrelly & Brown 2011). One reason for the strong attention given to the role of niche innovation is, according to Cettner et al. (2014), that a bottom-up transition is more visual to media, and that it leads to an increased public awareness and hence political ears. According to the approach, interpretation of change potential, a strong focus has been put on promoting different types of technical innovation at niche level. Based on the findings in this article, this view of change, provides a rather simplified view on change and the interaction or possible interaction between societal actors and legal structure. In the transition literature, there are also voices raising the importance of factors at the landscape level (Koppenjan et al. 2012, Geels 2011 Smith et al. 2005, Mguni et al. 2015). Brown & Keath (2008). Yet others have ar-gued that to achieve a transition, changes must occur and be integrated between levels (Geels and Schot 2007). We argue for the latter understanding of the potentials of change, where change is promoted by a combination of alternation at different scales of the system. Transition also requires a mutually-reinforcing shift in each of the cognitive, normative, and regulative pillars of a regime. A specific barrier experienced by a city often reflects the lag in the transition of one of the two other regime pillars, even though conditions in the other pil-lars may be ready. The existing implementation of blue-green solutions (normative) does not seem to be transformed into a mainstream solution until regulations are implemented (regu-lative pillar) and municipal organization (normative) are updated to support the application. So what can this study teach us about the potential ways to overcome the barriers, strength-en the drivers and to encourage transition to a more sustainable and multifunctional urban stormwater management system in a southern Swedish perspective?

Legal

One way could be reduce the legal barrier linked to the removal of the possibility to define technical requirements. Early 2018, the Swedish government present a national climate change strategy. The strategy is giving different measure to adapt the society to climate change. One measure concerns changes in the legislation, i.e. the PBL, and either reintroduce the possibilities to pose specific technical requirement concerning the water management capacity at each plot. Another possibility that actually is on the decision table is to demand private actors to submit site improvement permit when changing the infiltration capacity of their land (Government officials of Sweden 2018). The political debate anticipating the re-moval of the technical requirement mainly targeted construction technical requirement and issues concerning water management rather concerned issues relating to the management of public goods. If such changes are implemented it may provide a very important basis for change. According to Lui & Jensen (2017), green technology and innovations, such as blue-green solutions, cannot become mainstream until relegations exist to support their inclusion, which many of the informants in this study mentioned as a problematic area. Other research has shown that demands from authorities and legislation that puts a pressure on the current

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regime are important for transition towards widespread implementation of alternative solu-tions (Widarsson 2007, Ashley et al. 2011).

A more radical way to induce change through legal means could be to demand that each parcel of land to handle a certain percentage of the rain water landing on it, for example by implementation in a detail plan. It is clear that to use such an approach would be easier when building on new land, and in the already build space it is much more difficult, and with more conflicts of aims (include something about sponge city). In line with these sugges-tions, Widarsson (2007) suggests using a model from Franklin, US, in Sweden. Iin Franklin authorities demand a permit to outlet stormwater. The permit must be renewed every five years and can only be given if there is a stormwater programme with clear objectives, includ-ing routines for maintenance.

Economy

One way to strengthen the economic driver, might be to differentiate the water fee payed to the municipalities/water utility companies so that property owners with less impervious surfaces pay less (Widarsson 2007). Similar incentives have been tested for decades, for in-stance you can get 2500 SEK per disconnected downpipe in Malmö (VA Syd 2018), but such a possibility needs to be developed also in relation to what such a water fee actually can in-clude. Hence a redefinition of what a water fee could include as services should be devel-oped.

Money to increase the number of upstream wetlands and ponds could be introduced and in 2018 the Swedish government decided to distribute xx miljon to such wetlands (ref).

Knowledge and learning processes

In the interviews a need to learn and test solutions at the niche level is raised. Pahl-Wolst argued already 2008 that to test new solutions the municipalities can create a solution-oriented document base so a project manager can more easily find solutions which can lead to increased adaptive capacity at the regime level. From this study it is clear that if such test-ing is just performed by using external project money such as in Augustenborg (Stahre 2008), a stronger structure for organisational learning and knowledge transfer to the ordinary planning and development processes is needed. However, as noted by Geels (2011), if suffi-cient time is given to gain experience, develop knowledge and innovation, and update regu-lations, blue-green solutions could mature as an alternative approach to pipe bound solu-tions to urban stormwater management. In accordance to this reasoning, Mguni et al. (2015) argues that niche innovations have begun to mature and it has a much broader use than be-fore, which is also the case in Sweden.

It is clear that there is a need to increase the knowledge about blue-green solutions, but this is rather related to knowledge exchange and sharing between sectors, than lack of under-standing of what a blue-green solutions are. In this context, the perceived change in new generation employees more holistic understanding of the importance of blue-green solutions also could positively influence the future implementation. A complementary solution could be to develop a catalogue of solutions (including general ideas of their benefits, implementa-tion and maintainance cost) that different actors can use. Such a list could also include in-formation about other benefits of blue-green solutions than water management, and in that sense strengthening the knowledge of the multiple benefits of blue-green solutions.

Widarsson (2007) suggests that house owners should be informed about flood risk when they buy the house. Such information is for example available in Denmark (Danish EPA, 2018). In one of the interviews, it was mentioned that an important part for a change to occur

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would be to integrate property owners and the general public, in order to blur the bounda-ries between the blocks of private owned and public owned land. It could lead to a more efficient establishment of blue-green solutions in the urban environment. VA SYD has also recently started to work more directly with private property owners, through the project “Together we make space for water” (VA SYD, 2018).

Organisation of spatial planning

One solution to overcome the sectoral divide and increase the speed of organisational learn-ing could be to develop an organisational structure in the municipality that ensure the cross sectoral learning process in relation to both the implementation and management of blue-green solutions. Municipalities could for example employ a person in the planning system that ties together the tasks, facilitates dialogues between involved actors and ensures a joint understanding of the problem at stake. Also important would be to increase the interaction between green space development and stormwater management. In Wales, the surface water management strategies is linked to the green space strategy, which is probably a fruitful way to make the planning more holistic (Ashley et al. 2011). Widarsson (2007) suggest that groups are formed with representatives from all involved departments to increase cooperation with-in the municipal organisation. Another aspect in this is that the planning process include various municipal officials, which require trans-disciplinary of trans-professional approaches and skills (van Herk et al. 2011a). Storbjörk (2010) investigated barriers for organisational learning in two Swedish municipalities. In one of the municipality the city councils waits for national regulation of climate change adaptation, in this case flooding risk reduction, before they act. The other municipality engage neighbouring municipalities, arrange conferences, etc. While this is a more proactive approach, much of the work is done by a single servant and therefore vulnerable. It is therefore also important that every city has a surface water management strategy with clear goals (Widarsson 2007), like the climate adaptation plans in Denmark (Københavns Kommune 2012) or the surface water management strategy from Welsh Water (Ashley et al. 2011).

Another measure that supports the organisational capacity could be to increase dialogues and to strengthen capacity building among public and private landowners about the poten-tial benefits of introducing blue-green solutions. A capacity building that could increase the understanding and develop the benefits (for example recreation lower, temperature, noise reduction, health benefits) of blue-green solutions the cities in a similar ways as integrated water management does in rural areas (ref). In Copenhagen, public dialogues have been ini-tiated to locally verify and legitimise stormwater solutions for flood control. This type of initiative can create a consensus among actors in the regime and find common approaches to be able to mainstream the implementation of blue-green solutions at a broader scale than just particular project. It is also important that public and private landowners, officials, consult-ants and politicians find common approaches to achieve an integrated flood-proof storm-water management.

Lui and Jensen (2017) argues that a transition is two folded, innovative and experience is creating a deeper knowledge among actors, but more structural changes is also needed such as change inregulations so new solutions could mature. There is a need to combine o top-down, legal change, and bottom-up-down strategies, niche projects that promote learning processes, to accelerate a transformation process from pipe-based solutions to nature-based solutions. This is also clear from this study there need for top-down and bottom up changes or a transition to accrue. Niche project is more difficult to establish itself as a part of the planning culture in the municipality. Many of the innovative pilot-projects in Malmo or in Helsingborg, indicates that municipal actors are able operate at niche level, as they support,

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invest in and seek funding to start up pilot projects. However, there seems to have been a lack of link between this niche level project and the main planning culture as the ideas and knowledge have not been mainstreamed into the municipalities more daily development. Clear responsibilities among regime institutions, favorable conditions provided for develop-ing accountability and collective responsibility, and improved incentives as well as clear pro-curement rules, and municipal decision-making processes for public-private collaboration, may help overcome this (Cettner, Ashley, Hedström, & Viklander, 2014; Cettner, Ashley, Viklander, &Nilsson, 2013; Kennedy, 2011; Lund & Vaaben, 2014).

List of ideas to include or not include in the discussion:

• Clear responsibilities among regime institutions, favorable conditions provided for developing accountability and collective responsibility, and improved incentives as well as clear procurement rules, and municipal decision-making processes for public-private collaboration, may help overcome this (Cettner, Ashley, Hedström, & Vi-klander, 2014; Cettner, Ashley, Viklander, &Nilsson, 2013; Kennedy, 2011; Lund & Vaaben, 2014).

• Bottom up transition: Brown and Keath (2008) argue that a key factor to implement sustainable transformations is to improve the link between actors at the regime and niche level. The actors at the niche level can respond to barriers in the regime such as lack of knowledge and awareness and could lead, for example, to lager financial sup-port and legitimacy at the regime level in the longer run (Brown & Keath 2008). But there is a problematic side to the bottom-up, niche project. Its more difficult for such a project to establish itself as a part of the planning culture in the municipality. Many of the innovative pilot-projects in Malmo or in Helsingborg, indicates that municipal actors are able operate at niche level, as they support, invest in and seek funding to start up pilot projects. However, there seems to have been a lack of link between this niche level project and the main planning culture as the ideas and knowledge have not been mainstreamed into the municipalities more daily development. urban envi-ronment (ref Cettner? Farrelly?).

• The barriers and uncertainties related to the roles and responsibilities make it difficult to change the regime. Links between different groups within the regime needs to be clarified in order to raise awareness. This barrier hinders the link between the level niche and regime level as it leads to imbalance between different players and that no one wants to take responsibility for increasing implementation. Learning and cogni-tive changes of the system could lead to legislative and regulatory changes within the regime (Geels 2006).

• In the end handling water need to take space in the urban matrix. More space effi-cient and multi functional solition could be introduced especially if planners learn more about the different qualities a blue-green solution could have. But there is a lim-it to how little space they can take in use. There is a huge focus on building houses due to a large house shortage, which complicates the implementation of blue-green solutions. In extension there are conflicts of aims for blue-green solutions possibility to outweigh the number of parking spaces or the turning radius of a turning zone, which there are establish standards for in the community planning.

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• Det finns nya tänkandet som blivit regiem for ex in Malmö nya detaljplaner får man in my a lösningar (Ashley 2011) men Ashley har ju fattat fel….

• The municipality has, due to the planning monopoly, a large potential to change the management of stormwater and creating a more holistic approach to policy devel-opment and planning through dialog.

• Every city must have a surface water management strategy with clear goals (Widarsson 2007), like the climate adaptation plans in Denmark (Københavns Kom-mune 2012) or the surface water management strategy from Welsh Water (Ashley et al. 2011).

• Efficiant routines for blue-green solutions are needed in the planning process (Widarsson 2007).

• Blue-green solution is left to a question of negotiation between byggherrre och kom-mun…

• Annika Kruuse :Vi har gjort oss kända över pilotprojekt här i stan med Bo01 och Au-gustenborg och på så vis har vi ju faktiskt gått från ord till handling rätt så snabbt men att få in det som ett arbetssätt och generellt tänkande där är vi inte än och det har gått betydligt trögare. Det beror på att stadsplanering är så bred och komplex fråga och många frågor som ska hanteras parallellt.

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A framework for strategic urban development using blue-green infrastructure and

nature-based solutions (NBS)

Johanna Sörensen1, Johanna Alkan Olsson2, Anna Persson2

1. Water Resources Engineering, Lund University

2. Centre for Environmental and Climate Research, Lund University

Abstract

A common strategy to lower the demand for intensive land use and energy consumption is the

densification of urban areas. While this might be a fruitful strategy for its main objectives, one

concern is the concomitant decrease in space for urban green areas and surface water.

Therefore, urban densification increases the need of strategies to implement, develop and

manage multifunctional blue-green infrastructure with nature-based solutions (NBS) in urban

areas. Smarter spatial planning strategies are needed, both for maintenance of existing NBS

and for implementation of NBS in new developments. Spatial planning should ideally be

based on well-evaluated context specific solutions. Such evaluation must in turn be based on

appropriate data material for each city. Today there are several types of difficulties related to

adequate provisioning of data to ensure appropriate implementation and maintenance of NBS.

This study focuses on exploring the gap between data availability and implementation of NBS

that persists in many city-planning authorities today. By identifying information barriers, this

study aims to develop a framework to serve as a basis for a discussion on strategic urban

development and help urban planners overcome these gaps and facilitate the inclusion of

context relevant and multifunctional high quality NBS in the larger blue-green infrastructure.

Selected interviews with urban planners and experts in the area of NBS and GIS were

performed to develop the framework based on the idea that well-structured and user-friendly

data, supporting city-planning strategies, are essential to facilitate the implementation of NBS.

The identified challenges arise from issues such as lack of data and patchy data sets, lack of

access to data and data rights issues, lack of knowledge both concerning other sectors and

scientific disciplines, data storage not adapted to users’ needs, organisation of data storage

and the difficulties concerning how to use the existing data strategically.

The conclusion that can be drawn is that there is need for an improvement of the organisation

of data management including better trans-disciplinary skills to understand and interpret

different types of data in the municipality.

What can be understood as well-structured and user-friendly data depends on the goals and

needs expressed in strategic plans, which mean that there is a need for a close interaction

between the collection of data and the development of strategic political goals expressed in

spatial planning documents.

KEY WORDS: nature-based solutions, blue-green infrastructure, spatial planning, strategic

planning, stormwater management, climate change adaptation, data, GIS

1. Introduction

A common strategy to lower the demand for intensive land use and energy consumption is the

densification of urban areas (Grimm et al. 2008). While this might be a fruitful strategy for its

main objectives, e.g. to decrease urban sprawl on agricultural land and increase the population

density for an efficient public transport, one concern is that space for green areas and surface

water will decrease (Haaland and van den Bosch 2015). E.g. the urban green space of

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Swedish cities and towns decreased by 0.6–1.5% between 2000 and 2005 (SCB 2005). Other

challenges may be a lack of freshwater at certain times and in certain places, pollution of

different kinds, lack of areas for recreation, increased noise, increased urban temperatures and

loss of biodiversity (Grimm et al. 2008, Stott et al. 2015). Therefore, densification increases

the need for strategies to preserve, build, develop and ideally simultaneously increase the

quantity (area) and quality of green and blue spaces (vegetation and surface water) in urban

areas in a multifunctional manner (Hansen 2014; Pauleit 20011), to ensure that these areas

provide the necessary functions to counteract a broad array of challenges caused by

urbanisation. This approach is also supported by goal 11 of the UN’s 17 Sustainable

Development Goals aims to “make cities inclusive, safe, resilient and sustainable” which

includes providing access to green and public spaces for all strata of society (UN 2015).

On top of an increased densification of urban space, climate change puts a pressure on the

urban environment. In the future, more intense rainfall events and thereby more flood events

are expected in several regions worldwide (IPCC 2014). In combination with current land use

changes due to urbanization and increased industrialization of agriculture and forestry,

problems with urban flooding will increase further (Semadeni-Davies et al. 2008; Voskamp

and Van der Ven 2015). At the same time, droughts are expected to become more frequent

(IPCC 2014). Again, both quantity and quality of blue green spaces in urban areas will be

important to counteract effects of climate change.

Many cities have ambitious strategic plans for green urban spaces (Turner 1995, Sandström

2002). When such spaces are designed to align with the natural water cycle, we hereafter call

them blue-green infrastructure. These blue green areas are seen as important for recreation,

urban biodiversity as well as climate adaptation and handling of cloud bursts. Recently the

European Commission has introduced the concept of nature-based solutions (NBS) and states

that NBS are “actions inspired by, supported by or copied from nature” (EC 2015). According

to the Commission’s definition these should both address specific demands or challenges and

seek to maximise other environmental, social and economic co-benefits, including habitat for

biodiversity creating a multifunctional green space. In this article, we have chosen to use the

concept blue-green infrastructure for the overall strategic planning, while we use NBS for

individual solutions (e.g. a green roof or a water-retention pond). We assume that municipal

authorities have an overarching goal to strategically coordinate the implementation of blue-

green infrastructure into the built environment and improvement of existing to provide good

environmental living conditions for inhabitants in a changing climate, while simultaneously

providing ecologically sustainable cities and habitat for biodiversity. Working towards such a

goal requires strategies that combine maintenance of existing blue-green space with strategic

placement and good quality of NBS, both in existing and new urban developments. The aim

should be to optimise the delivery of benefits over the whole city, rather than in any one

individual NBS. The type of solution that will be introduced depend on the main purpose of

the blue-green infrastructure, water retention, biological diversity or recreation. This is a

challenge for many reasons, one being that administrative borders often do not coincide with

the spatial or temporal scales necessary for adequate maintenance and governance of

ecosystems and ecosystem services (Borgström et al. 2006, Faehnle et al. 2015). In Sweden

for example, the National Board of Housing, Building and Planning (Boverket) has the

responsibility for guidelines regarding green infrastructure in urban areas. Municipalities have

a monopoly on spatial planning, and municipalities or separate water utility companies are

responsible for urban drainage, but neither can in detail govern what is built on private land.

Furthermore, the Swedish Environment Protection Agency through the County

Administrative Boards is responsible for the green infrastructure in general both rural and

urban areas. Thus, there is potentially a weak link at the regional scale between green and

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blue structures in, and between rural and urban areas. Water is often managed within

municipal borders, which do not follow hydrological catchments.

As there will be less available space when urban areas grow denser, we therefore argue that an

urban development introducing one or several NBS must be founded on well-evaluated

solutions and based on relevant (spatial or other) data for each city, to make optimal use of the

limited space. Today there are several types of difficulties related to gathering and accessing

adequate data for appropriate implementation of blue-green infrastructure as well as NBS.

These difficulties are related to both economic, social and environmental factors and span

from lack of information about the extent of the existing blue-green infrastructure, cost of

maintenance and citizens’ perceptions and preferred use of NBS, to difficulties of a more

technical character such as data base structure, the data format of available GIS information

and patchy data provision based on lack of data, including lack of coordination between

different data collecting authorities at different scales (Hansen and Pauleit 2014; Ahern et al.

2013).

The aim of this study is to address the information gaps in data availability for adequate

planning of blue-green infrastructure and NBS that persists in many municipalities today. We

do so by identifying information barriers between expressed data need, data collection and

use, and use this information to develop a framework to help strategic spatial planners

overcome information gaps to facilitate the inclusion of high quality and context relevant blue

green infrastructure and NBS in urban planning.

2. Definition of Green Infrastructure, Sustainable Urban Drainage Systems and

Nature-based Solutions

Several concepts have been launched by researchers and authorities at different administrative

levels to describe and aid planning and implementation of green and blue structures in both

urban and rural areas. While NBS is a relatively new concept, proposed by the European

Commission (EC, 2015), Sustainable Urban Drainage Systems (SUDS) and Green

Infrastructure (GI) have been used in urban planning since the 1990s. In this section, we will

define the differences between these concepts and how they relate to each other by focussing

on different scales and aspects of blue green structures.

Green Infrastructure (GI) or ‘green structure’ has been perceived as a way to upgrade green

space as a coherent planning entity and incorporate aspects of both ecological sustainability

(including biodiversity conservation) and the well-being of urban dwellers (Sandström 2002,

Tzoulas et al. 2007, Ahern 2013, Hansen and Pauleit 2014). The concept of Green

Infrastructure is used as an urban planning tool emphasizing the quality as well as quantity of

urban and peri-urban green spaces (Turner, 1996; Rudlin and Falk, 1999), their

multifunctional role (Sandström, 2002), and the importance of interconnections between

habitats (van der Ryn and Cowan, 1996). Green infrastructure should be a coherent planning

entity where individual urban green spaces are multifunctional systems (Sandström 2002). GI

builds on landscape ecological theory, with ecological or functional connectivity as an

important tool to manage e.g. meta-populations and biodiversity conservation in human

modified landscapes (Ahern 2013, Snall et al. 2016). In the urban landscape GI is currently

seen as a way to reduce the negative effects of urbanisation and to adapt to an anticipated

changing climate, including an increased need for surface water and stormwater management,

and to create multifunctional areas with both environmental and social functions (Hansen and

Pauleit 2014). In 2013 the GI concept was launched by the EC as a way to preserve and

enhance healthy green infrastructure to help stop the loss of biodiversity and enable

ecosystems to deliver their many services to people and nature. GI was described as a

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strategically planned and multifunctional network of natural and semi-natural habitats, as well

as other environmental features, in rural and urban areas (EC 2013). The Commission is

arguing that because GI is partly defined by its geographical extent it is a suitable tool to

bridge the urban-rural interface and to use in planning at several spatial scales (EC 2013).

SUDS are drainage systems constructed to handle stormwater in a decentralised manner,

typically through stormwater detention, infiltration and purification in e.g. open channels,

ponds, wetlands, and via green roofs (Stahre 2008, Cettner et al. 2014, Hoang and Fenner

2015). SUDS aim to copy the natural hydrological cycle, to prevent harmful effects of

urbanisation on streams, groundwater and marine environments.

To some extent SUDS are interchangeable with other water management related concepts,

such as Sponge City in China, Water Sensitive Urban Design (WSUD) in Australia, Best

Management Practice (BMP) and Low Impact Development (LID) in the US, and local

control and use of stormwater (LOD) in Sweden (Jia et al. 2016, Brown et al. 2009, Fletcher

et al. 2014). The term used in non-urban contexts is Sustainable Drainage Systems (SuDS).

More recent concepts, like the Sponge City and WSUD have incorporated the idea of

resilience. The city is viewed as a “sponge”, soaking up water during rainfall and releasing it

when needed (Jia et al. 2016). The idea of the above mentioned, concepts is to mimic

functions from nature that are valuable in urban areas, such as purification of water, flood

control, water storage, and temperature regulation.

NBS is one of the newest ‘green’ concepts and was launched as a way to adapt to climate

change by the International Union for Conservation of Nature (IUCN) in 2009 in its position

paper on the United Nations Framework conventions on climate change (UNFCCC) COP 15

(IUCN, 2009). In 2015, the European Commission (EC) launched the concept in relation to

the Horizon 2020 call. In recent years, NBS has been put forward within the European

Commission, as a measure to increase innovation and development of multifunctional

solutions inspired, supported by or copied from nature (European Commission, 2015). The

introduction of nature-based solutions is expected to support economic development and

deliver multifunctional benefits for the health of people, society, economy and the

environment. Key focus areas are sustainable urbanisation, restoring ecosystems, climate

change mitigation, improved risk management, and resilience (European Commission 2015).

The three concepts have a slightly different focus, mainly related to their uptake and use in

different disciplines and their related sectors. SUDS consist of techniques (e.g. swales, green

roofs, canals) and concepts (e.g. multifunctionality, decentralisation), while the GI concept is

a broader principle including the need to improve the quality of green urban space (Fletcher et

al. 2014) and when the European Commission launched it as a tool in 2013 it also

incorporated a strong rural component. Whereas GI has a strong link to large scale green

planning and water is a secondary feature, SUDS is a concept that has emerged from practice

and disciplines of urban storm water management with emphasise on the use of ‘green’

solutions for storm water management. It has been argued that for SUDS, other benefits are

considered added value, but not targeted per se (Stahre 2008, Voskamp and Van de Ven

2015). The GI concept on the other hand, does not explicitly include water. NBS on the other

hand include both blue and green features and solutions. NBS is focused on solving specific

challenges, but concept is also used when discussing larger scale green structures both outside

as well as inside urban areas (Keesstra, 2018). What the concepts have in common is that they

all require work beyond administrative 'silos' and engaging across disciplines and sectors, as

well as systemically involving stakeholders, including citizens (e.g. Hansen and Pauleit 2014;

Ahern et al. 2013).

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Our brief assessment of these concepts, shows that these green planning tools has a history

and they have different relations to; urban/rural, green structures/water, and large scale/small

scale, which make the recycling of ideas complex and to some extent even confusing. In this

study we focus on spatial urban planning and, we use ‘blue-green infrastructure’ when we talk

about large-scale structures in, while NBS or ‘blue-green solutions’ are used for one or

several connected elements of the blue-green infrastructure. Thus, blue-green infrastructure

defines the spatial extent and distribution of mainly green structures that are in line with the

hydrological cycle, while different NBS define the content and quality of separate elements.

There are two important reasons for this “redefinition”, the first is that different actors are

involved in planning large scale structures and smaller elements, the second concerns the

addition blue to green infra structure which highlights the importance of water in strategic

planning.

3. Method

The framework was developed in three steps:

Step one, brainstorming within the author group, with the aim to identify barriers influencing

the flow of information. The authors represent different disciplinary background and

approaches to blue green infrastructure and NBS (more blue to more green) and their

implementation. In step 1 some NBS elements (Table 2) were selected to illustrate a variety of

suitable placements (street, roof, park, garden, square, etc.), functions (hydrological,

biological, aesthetic, social, pedagogical, etc.), needs for maintenance, ownership and scale of

implementation. In step one a draft framework was developed.

Step two, interviews with selected municipal officials and practitioners working with

development and implementation of NBS from different parts of the implementation chain,

with the aim to complement the initial list of potential barriers through the experience from

practice. The main focus of the interviews was to identify what data or information was

missing to meet urban challenges and plan for an optimal distribution and placement of NBS.

The drafted framework was used as a basis for the questions. Six practitioners were

interviewed. Interviews were semi structured (Bryman 2015) taking the point of departure in

the experience of each interviewee.

Step three, analysis of findings using triangulation and development of the final framework.

In triangulation results from several methods are used to grasp a complex set of relations

(Mathison 1988). In this case we combined our three disciplinary backgrounds with a “reality

check” based on practical experiences of bureaucrats working at different levels in the NBS

implementation chain, and additional information from scientific articles.

Table 1. Interviewed practitioners

Interview Name Position

Educational

background

1 Annika Arvidsson Malmö city planning office,

planning department Landscape Architect

1 Jonna Nilsdotter Malmö city planning office,

strategy department Ecologist

2 Kristina Hall Planning engineer at water

utility company (VA Syd) Civil Engineering

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2 Pär Svensson Malmö city streets and parks

department ???

3 Peter Olsson

Researcher with GIS

expertise (CEC, Lund

University)

PhD in Applied

Biochemistry

3 Mats Elfström

Responsible for GIS in two

municipalities (Tomelilla and

Simrishamn)

???

Table 2. Elements selected for comparison and discussion

Element Geographical

extent

Typical

property

owner

Placement Main

function

Maintenance

need

Trees in

streetscape

Point City council,

private

Along

streets and

on parking

Aesthetic

Climatic

Yearly

Raingardens Point City council,

private

Along

streets

Hydrological

Aesthetic

Continuously

Extensive

green roofs

Point/area Private, city

council

Roof Aesthetic

Biological

Seldom

Daylighting

of urban

streams

Linear

element

City council Larger

dedicated

areas

Hydrological

Biological

Continuously

Permeable

pavement

Point/area City council,

private

Parking,

school yards

Hydrological Seldom

Retention

pond

Point City council Park Biological

Hydrological

Continuously

Park or

urban forest

Area City council Bigger,

dedicated

areas

Recreational

Biological

Continuously

4. Results

Results step 1: Brainstorming

A set of brainstorming sessions with the authors were conducted in autumn 2016 and spring

2017. During these sessions, different perspectives on data availability and data management

in the urban spatial planning of blue-green infrastructure and nature-based solutions were

discussed.

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Several gaps as well as question were identified already in the first brainstorming sessions,

such as lack of information about the extent of existing NBS, costs of maintenance, citizens’

perceptions and preferred use of NBS, difficulties of a more technical character such as data

base structure, the data format of available GIS information, and patchy data provisioning

based on lack of certain data, including mismatches in the coordination between data

collecting authorities working at different scales. Three main areas of problems were

identified: the multiple need of data, lack of GIS data, and problems related to data

management.

4.1.1. Multiple needs of data

Several kind of data is demanded, e.g. information that can aid actions to reduce noise, control

pollution, limit lack of green areas, reduce heat islands, reduce stormwater runoff and increase

infiltration and evapotranspiration. There is a need for data that can be developed into maps

that show the availability, lack of and need for, as well as provide information on what

functions and services the NBS blue-green infrastructure can deliver.

4.1.2. Lack of data and GIS data

Some data are missing due to difficulties related to data collection. It is for example difficult

to investigate underground infrastructure and to keep track of maintenance needs for less

visible systems. In some cases, it is costly to maintain often updated datasets, for instance on

the spatial distribution of species in different urban habitats.

Data collection is affected by ownership structures. For example, green roofs built by city

council or a water utility company are more likely to be registered in a database compared to

those constructed by private house owners.

Inventories of existing biological and ecological values are often made based on available

knowledge rather than actual need of information. The Swedish Species Information Centre

uses crowdsourcing as one of their data collection methods through the Swedish Species

Observation System. This will lead to a more reports on rare species close to where interested

biologists live, and a lack of information in other areas. A SIS standard for biodiversity

surveys exist since 2014, and is starting to be implemented at the municipal level to inform in

planning processes (SIS/TK 555 Naturvärdesinventering).

4.1.3. Problems related to data management

Several problems related to data management were identified, from technical issues such as

incompatibility between database structures, to problems related to human resources. The

latter includes low priority among assignments, scant knowledge about available data in other

departments and organisations, competition, prestige and difficulties to cooperate between

departments as well as leadership related issues.

Collection and processing of data to assess the development and function of NBS are divided

between different national, regional and local authorities. Data is collection in relation to the

legal responsibility of authorities, e.g. the responsibility to assess future rainfall and the

capacity of current stormwater system, to handle precipitation based on climate modelling, to

assess spatial distribution and quality (through different types of surveys) of green areas, or to

assess nutrient or pollution run-off to surface and ground water, etc. Because the purpose of

collecting and assessing data differ between authorities, the collected data seldom comprise a

comprehensive ground for planning blue-green infrastructure and its elements. Neither is it

comprehensive in spatial cover, content or quality.

8/20

4.1.4. Raised questions

Several questions were raised during one of the first brainstorm sessions which together with

the initially developed framework were used to guide the framing of questions during the

interviews:

• Which data exist at which scale?

• What kind of data is lacking?

• Is the data format convenient or not when to cooperating with other professions in the

planning process?

• Are authorities working at different spatial scales aware of the need to pool the data to

understand NBS at the local scale?

• What are the costs of maintenance of future and existing NBS? Maintenance is closely

linked to quality and functions of green spaces and SUDS.

• What are citizens’ perceptions and preferred use of NBS? Are their perceptions

obtained and used in the planning process?

4.1.5. Present situation

A list of available data and data in current use (Table 3) was collected during the

brainstorming and later supplemented by the authors. The list includes data on different

planning scales, provided by various authorities and organisations. From the list, the

following categories of data needed in the planning process of blue-green infrastructure

evolved.

• Cadastral

• Technical

• Geological

• Biological

• Environmental

• Risks

• Social

• Administrative

• Meteorological

Table 3. Datasets available for planning.

Geographical

scale

Planning scale Authority/organisation Kind of information

Local Comprehensive

planning

The Swedish Species

Information Centre,

SLU

Artportalen

(Swedish Species

Observation System) --

citizen science, observations,

new findings

River basins Comprehensive

planning

Competent Authorities

of the Swedish Water

Districts, the County

Administrative Boards

and the Swedish

VISS (Water Information

System Sweden) – Status

classification, environmental

quality standards,

environmental monitoring,

protected areas

9/20

Agency for Marine and

Water Management

Properties Comprehensive

planning

Insurance companies Flood claim data

All of Sweden All Lantmäteriet – The

Swedish mapping,

cadastral and land

registration authority

Cadastral maps

All of Sweden All Lantmäteriet – The

Swedish mapping,

cadastral and land

registration authority

Orthophotos (also historic

orthophotos)

All of Sweden All Lantmäteriet – The

Swedish mapping,

cadastral and land

registration authority

Digital elevation model

(Nationella höjdmodellen)

- Livsmedelsverket –

National food agency

Quality standards for

drinking water

Regional Comprehensive

planning

Länsstyrelsen – County

administrative board

Pluvial flood risk assessment

(doubtful quality in urban

areas)

Local Comprehensive

planning, zoning

Länsstyrelsen – County

administrative board

Environmentally hazardous

business

City, regional Comprehensive

planning

Länsstyrelsen – County

administrative board

Targeted inventories

Local,

regional

Comprehensive

planning

Länsstyrelsen – County

administrative board

Ad hoc inventories

Local Comprehensive

planning

Länsstyrelsen – County

administrative board –

SGI coordinates

research and training

Polluted soil and

groundwater

National Comprehensive

planning

Metria, Lantmäteriet,

Länstyrelserna i

Östergötland och

Stockholm, etc.

National land cover map –

CadasterENV

River basins Comprehensive

planning, zoning,

building permit

MSB – Swedish civil

contingencies agency

Flood risk assessment along

watercourses (prioritise

rivers)

Municipal emergency

service

???

Local Comprehensive

planning, zoning

Municipalities Reports related to

environmental control

programme: air, water (EU

10/20

Water Directive), and local

enthusiasts/projects

City Comprehensive

planning, zoning,

building permit

Municipalities Municipal maps for planning

(plankartor)

Local Comprehensive

planning

Municipalities Ad hoc inventories of

species

City Comprehensive

planning

Municipalities Groundwater

City Comprehensive

planning

Municipalities – some

of them

Pluvial flood risk assessment

City Comprehensive

planning

SCB – Statistics

Sweden

Urban green areas, extent

City Comprehensive

planning

SCB – Statistics

Sweden

Public access to urban green

areas

River basins

– few

Comprehensive

planning

SGI – Swedish

geotechnical institute

Landslide risk assessment

River basins Comprehensive

planning

SGI – Swedish

geotechnical institute

Observed landslides and

other ground movements

(Swedish Landslide

Database)

Coastline,

river basins,

lakes

Not good enough

for detailed

studies

SGI – Swedish

geotechnical institute

Coastal vulnerability index

(erosion)

Local Building permit SGI – Swedish

geotechnical institute

Stability – hållfasthet

(myndighetsansvar?)

- Zoning, building

permit

SGI – Swedish

geotechnical institute

Costs calculation for

management of masses and

for foundation

- Zoning, building

permit

SGI – Swedish

geotechnical institute

GeoEkoKalkyl (GIS-baserat

verktyg) för att identifiera,

dokumentera och visualisera

betydelsen av

ekosystemtjänster (EST) och

byggkostnader vid

exploatering av mark

Coastline Comprehensive

planning, zoning,

building permit

SGU Sea level rise

Coastline Comprehensive

planning

SGU Ongoing erosion

Local Comprehensive

planning, zoning,

building permit

SGU Well and borehole archive

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Local Comprehensive

planning, zoning

SGU Rock and soil properties

Local Comprehensive

planning

SGU Exploitation of soil, gravel,

and stone

Regional Comprehensive

planning

SGU Groundwater

Regional SMHI Climate data (temperature,

wind, precipitation, etc.)

River basins SMHI Flow

Regional, city Comprehensive

planning, zoning,

building permit

SMHI Risk assessments (water

level, drought, floods, etc.)

National SMHI Climate models

City Comprehensive

planning, zoning

Stadsfastigheter Properties owned by

municipals

River basins Comprehensive

planning

Vattenmyndigheterna –

Competent Authorities

of the Swedish Water

Districts

Results step 2: Interviews with municipal officials and practitioners

From the interviews, six themes could be distinguished: lack of data, data access and rights,

lack of knowledge, data storage tools not adapted to users’ needs, strategic data use, and

organisation of data storage.

4.2.1. Lack of data

In two of the interviews, lack of GIS data related to ecosystem services such as

pollinators/pollination, tree shade, the marine environment, and mapping of biotopes

(Interview 1, 3) were mentioned. Another area mentioned was the lack of ‘green’ data for

private land (which related to lack of inventories of green cover and biodiversity). One of the

interviewees mentioned that there is a tree plan for public land in Malmö but no such plan for

private land, whereas the downpour plan (a plan to handle rainstorm and flood events)

concerns the whole city. As a consequence, it becomes difficult to assess the real lack of

green space (1).

In interview 1 it was argued that the Swedish Species Information Centre has uneven data and

to interpret available data correctly it is important to have the proper meta knowledge of the

structure and purpose of the data collection (1).

In relation to storm water information it was mentioned that maps do not follow the

development of the city. Deep holes at construction sites can accept a lot of water during

downpours, but what happens with the water when the site has been built on? Even if the data

quality is improving, the resolution is still low and data update processes are slow (2).

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4.2.2. Data access and rights

In interview 1 and 2, the lack of access to inundation data from insurance companies was

mentioned as a problem. However, in interview 2 it was mentioned that insurance companies

are working on facilitating access to these data and the possibility to combine insurance data

and data from their own flood claims to enable calculation of expected flooding costs in the

future (2). It is also argued that meta data related to different flood hazard modelling scenarios

as well the model outcome is not stored systematically which mean that other persons than the

one running the models have difficult to access data as well as the underlying scenario(2).

It was mentioned that municipal maps of the blue structures cannot easily be used for strategic

planning at the municipal level, because the Department for Strategic Development does not

have access to certain GIS layers from the Streets and Parks department within the same

municipality (1, 3). In interview 2 it was also argued that the strategic service providers (e.g.

energy suppliers, fire and rescue services) claim that they have control over their

infrastructure and environmental risks related to them, but the municipality doesn’t know on

what they base their assessments (2).

Another problem related to data access is that data is physically scattered. For example, data

about pipes and property owners covers the whole municipality but is spread between

different databases and formats, such as digital maps, but also as scanned documents as well

as paper (1). In relation to the downpour plan it was argued that one simulation result, namely

the flood hazard risk map for a so called hundred-year-rain, is available in ArcGIS, while

other simulation results only exist in the computers of the consultants that performed the

downpour modelling (2). It was also mentioned that an evaluation of green values such as

ecosystem services were made for the downpour plan but did not enter the final report (2).

One of the interviewees had experiences from data management in different municipalities

and argued that it is especially difficult in some municipalities, as they are afraid of making

mistakes which result in a very restrictive access (3).

4.2.3. Lack of knowledge

Many municipality staff members use either GIS or CAD based planning platforms, but very

rarely both (3). Planners most often use CAD and do not know GIS very well (2). Small

municipalities are generally not using any of these platforms, but if using GIS it enters at the

end of the planning process and the production of maps. This situation is mainly due to lack

of local GIS knowledge, as a lot of data is available to the municipalities through the Swedish

geodata collaboration (3). Using GIS is a matter of practice: if you do not work in a GIS

environment every day there is a threshold to open these type of programs (2). Even staff

members that use ArcGIS on regular basis are not aware of all possibilities related to the

software. In interview 3 it was argued that there is a need for basic GIS education, at least one

day for middle management positions, to make them aware of the importance of GIS

competence among the staff members.

The City of Malmö is working to improve the so called Malmö City Atlas, where the goal is

that employees should have their own GIS layer. In the future, the atlas will be linked to the

Swedish Species Information Centre to show reported species occurrences (interview 1).

4.2.4. Data storage tools not adapted to users’ needs

A problem identified is that ArcGIS costs 30,000-50,000 SEK (3,500-6,000 USD) for a

licence plus some 1000 SEK for maintenance per year. This system is not perceived to be

adapted to municipal needs. There is a sort of oligopoly of ArcGIS in Sweden where three

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companies deliver services and municipalities are bound to the structure they choose to buy

from the start (3).

Another problem related to tools is that persons working in the field of maintaining

stormwater systems lack access or the routine to add new data when they find that existing

data is wrong or missing (1,2).

In interview 2 several wishes for tool developments were expressed. It was for example

mentioned that it would be great with a system where different rainfall scenarios could be

tested in real time. Today it takes two weeks to run a model for a scenario and a consultant is

needed to make the simulations. In the detailed planning stage, it would be valuable to be able

to assess how much water an area undergoing planning receives and how much it drains to

other areas (2). It is however difficult to develop tools that are able handle all needs (3). An

initial step is to provide open access data (3). A dream would be to have lists of open data

including meta data, with different security levels depending on the type of data (3)

4.2.5. Organisation of data storage

In interview 2 it was mentioned that there are too many different plans and programs to relate

to when developing GI and NBS. The fact that these plans are often only available in different

PDF documents makes it difficult to handle and compile the information during the planning

process (2).

The GIS infrastructure is organised very differently in different municipalities. In interview 3

it was argued that in Malmö, there is one staff member responsible for GIS at each

department, but the departments do not work together. In another municipality, Helsingborg,

there is instead one GIS unit serving the other departments. Moreover, GIS competence is

often sorted under the City survey department situated under the planning unit. However, the

budget work is done far from this unit and data collection and data infrastructure may

therefore become of low priority. Information Technology issues are on the other hand often

organised under the leading functions of a municipality and therefore higher prioritised. As

data availability is of strategic importance, data storage infrastructure and collection strategies

should be organised from a more strategic position to increase its status and priority (3).

It was further argued that the only solution to the lack of GIS data is to sit together in the

municipality and focus on getting the existing data into a system. It is a one-time-only job, but

it is very important to do it in a proper way. Lack of progress is in several municipalities

probably due to a combination of lack of knowledge and economic resources (3).

To create a structure for data flow, it is important to handle the three pillars of information

handling: storage of data, analysis and visualisation, and data maintenance. These three pillars

can use different software but they should preferably be based on open source code (3).

Data follow different standards which has lately been harmonised in Sweden. Most

municipalities follow SWEREF as geographic coordinate system. Before the standardisation it

was more confusing and e.g. Malmö sometimes still uses a different coordinate system (3).

Several ongoing initiatives to improve data organisation to ensure that it is more available, for

example are SKL and Vinnova currently cooperating to make planning more digital. More

and more data become open, for example the data from the cadastral and land registration

authority is now open access for municipalities (3). Another example mentioned was the

current cooperation around “ekogeokalkyl”, led by the Swedish Geotechnical Institute, which

will include green space and ecosystem services in urban spaces. This tool will be open for

anyone that work in ArcGIS/ArcScene and (2).

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4.2.6 Strategic data use

An issue regarding strategic use of data that was brought up in two of the interviews is that

there is too much information. This leads to a risk to “get lost in the data”. Both in interview 1

and 2 it was argued that there is a need of more data, but not too much data. There is also a

need to know which data to focus on. In the neighbourhood Söderkulla in Malmö in total 14

models have been run to simulate the effects of downpours. A multitude of simulations make

the information difficult to handle (2).

Another issue brought forward is that politicians want to be good at everything and consider

everything. As a municipal staff member you have to be clear about what you have prioritised

and know that not everything can be prioritised (2).

In interview 1 it was argued that strategic data is available in the Malmö City Atlas, databases

about areas of national interest and the Swedish Species Information Centre. This data can be

used to prioritise what to protect and what action to take, and it is possible to be strategic if

you are prepared early on in the planning process. The interviewee gave an example of a park

and a dam in two newly developed areas where this information had been used (1).

Proposed framework

Through the three steps we have identified issues relevant for a strategic use of data to aid the

development of NBS. It includes now eight main issues (Figure 1). Three boxes lie outside the

main flow in the figure: citizen’s needs, the fact that values change over time and scientific

evaluations and support. Appropriate data to develop new NBS, or maintain current ones, is

essential to ensure a strategic development to benefit both people and nature (biodiversity)

itself. Hence the step evaluation and data collection, is essential and can be seen as both the

starting and end point of the framework. It is here essential that the evaluation and data

collection step includes links to broader municipal goals to ensure a relevant collection of

data. It is also essential to keep track of time related changes in data needs. A structured and

continued evaluation of implemented structures is essential to assess their success. However,

if the collected data is not stored in a proper way, access to it may in reality be limited.

Moreover, as urban areas change quickly, data maintenance is essential to ensure its relevance

and quality. This step has to include both data maintenance in a more technical meaning as

well as information on how collected data has been collected.

As the main purpose of the data discussed here is to be used in further analysis, there is a need

for both tools and structures for proper use once data is collected. If different types of models

are used it may be important to save and store analyses of different scenarios in a structured

way. The collected data and performed analysis thereafter has to serve as a basis for planning

which need some type of visualisation of the data at a scale that is useful for spatial planning

but also form smaller project.

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Figure 1. Framework for improved information flows in city planning including Green infrastructure (GI),

Nature-based Solutions (NBS) and Sustainable Urban Drainage Systems (SUDS).

5. Discussion

In recent years a set of semi-scientific/semi-policy concepts have entered the field of spatial

planning and management and development of green space. In this paper we have discussed

GI, NBS and SUDS. These concepts focus on different issues, mainly related to their uptake

and use in different scientific disciplines and sectors. The GI concept is frequently related to

larger scale green structures whereas NBS mainly focuses on smaller and more separate blue

green elements. SUDS on the other hand has a stronger focus on urban storm water

management and is scale wise situated between the other two concepts. Depending on which

concept that is in focus in the particular planning situation, different sets of data will be

needed. All concepts incorporate ambitions which to be fulfilled require work beyond

administrative and disciplinary 'silos', including a more systematic involvement of relevant

stakeholders, including citizens. Based on this understanding this article has aimed to better

understand the information gaps and develop a framework that can support adequate planning

of blue-green infrastructure.

The interviews indicate that to ensure work beyond administrative (disciplinary) silos

especially five were brought forward. Three of the issues brought up were new and was used

to improve the drawn-up framework form the brainstorming set; data storage technology, data

maintenance and visualization of analysis and data. In the following sections, we will discuss

these issues in relation to relevant scientific literature and suggest how the framework could

be used in the future strategic work to implement long term sustainable NBS in urban areas.

One of the major obstacles related to a strategic development of NBS in urban areas turned

out to be lack of data related to the quality of green areas and the marine environment,

patchiness of existing data and the continuous change of the urban matrix. If aiming for the

development of multifunctional green infrastructure and nature-based solutions this is a

challenge that has to be tackled. The collection of people’s perception of green space nor data

related to human wellbeing was mentioned in the interviews neither the economic benefits of

a solution or the cost of development and maintenance. This may reflect the selected

interviewees but of relating to the identified list of data in step 1 this data does not exist on a

more national level. Similar results have been identified in an article, mapping research

priorities for green and public urban space in the UK (Bell 2007) where health and well-being

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were identified as areas with weak support and where research had been undertaken on

physical aspects, and on biodiversity. In relation to lack of data it was also mentioned that

there was an unsystematic use of data and a similar observation was made by Eliasson (2000)

in relation to the use of knowledge about climate in urban planning especially when

conflicting interest appears in the end of the planning process.

In the interviews, it was also argued that different actors produce and have access to different

sets of data. Lack of access was both related to security (sensitive societal activities such as

electricity and transport infrastructure and hospitals) and privacy issues (i.e. information

collected by insurance companies on events in specific households). The ethical aspects of

data access, is increasingly brought forward in the literature (see for example Yeh 2005;

Newman 2010; Schweitzer 2017) raising questions concerning access and not access between

municipal departments, municipality and private actors as well as between these actors and

citizens. One major concern is that the increased use of informatics may increase the gaps in

power and political voice between experts and non-experts (Viitanen, 2014; Grindrod 2016).

With the development of smart green cities where information systems are out-sourced ethical

assessments and consideration of consequences of data collecting systems will consequently

grow in important and it will probably therefore be essential for municipalities to consider

how to store model results as well as scenario structures in a transparent way as an

increasingly amount of especially water related data to support urban planning is produced by

models frequently run external consultancy firms. For public decision-makers especially in

small municipalities with only a few employees it may be difficult to keep up with the

development of a specific model and to interpret what the produced data actually mean and at

what scale it mean something. And at a more general level it may even be difficult to discern

what type of model or consultancy advice that is procured for, which can be illustrated by a

review a of decision support tools for informed decisions on urban water management made

by Lerer et al. made in 2015. The reviewed tools were categorized in three groups, i.e. "How

Much"-tools, "Where"-tools, and "Which"-tools. None of the reviewed tools addresses all

aspects of water and they are influenced by the local context of where they were developed.

Another challenge brought forward in the interviews was the lack of knowledge related to or

shortage of human resources knowledgeable in GIS. This lack is related to knowledge about

other sectors and disciplines problems and perspectives as well as how to use and organize

data storage and data management. The difficulty for different disciplines to understand and

the negative consequences related to practical and scientific development is not a new

discussion (Brogren 1998). When it comes to the use and organization of data storage and

data management, Yeh (2005) has concluded that the main constraints concerning the use of

GIS in urban planning are not technical, but linked to the availability of data, staffing and

need of organizational change, which confirm the findings in this paper. One example of the

lack of knowledge of perspectives and techniques used by different expert groups in the urban

planning process can be illustrated by the identified mismatch between non-GIS supported

data, GIS data and visualization of urban visions and designs. For architects, visualizations

are an important tool to covey his or her ideas but these ideas need to be connected to plans at

more general levels as well as data supporting the understanding of context specific

parameters relevant for a continued provision of high quality green space and vice-verse.

Consequently, some kind of translation structure is needed from data to visualizations and

from visualizations to data, to ensure communication across disciplines and between actors. In

this structure there is also a need to move between a larger more strategic scale and a more

concrete level where a specific NBS are implemented. The interviews showed that the GIS

skills is weak to support such a “translation”. Häggquist and Nilsson (2016) examined

officials’ use of geological information in Swedish municipalities and found that the perceived

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usefulness, educational efforts, work tasks and gender affected the rate of usage which may be

something to consider when aiming to increase digitalization.

Different municipalities have chosen to organize its data storage in different ways, a fact also

indicated by Hansen (2017). Some municipalities have a central unit for data storage and

management whereas others have a more decentralized organization. To ensure a more

holistic approach a more centralized system where data storage and management for strategic

urban development purposes is part of a centrally organized municipal service such as Malmö

StatsAtlas may be the way forward. But then they probably would need to be connected to for

example the IT unit of the municipality. When developing such systems, it is essential to

develop good and solid data categorization systems to ensure that data is found when needed

as well as maintained.

Moreover, it was argued by several of the interviewees that there is too much data or that the

politicians want to do everything which makes strategic use of the data difficult. This finding

can be interpreted in several ways the data handling systems are not good enough or the goal

of the NBS are not clear enough. The first issue has been discussed above, as for the second

issue it seems therefore essential to more clearly define or decide for what or who, a NBS is

developed which clearly indicate a need to include data related to green structures as an

integral part of general planning documents. Several municipalities have initiated the develop

of such assessment they should be used as inspiration for how to organize the work in

Swedish municipalities.

This article has focused on Swedish municipalities, which due to the Swedish municipal plan

monopoly, are relatively independent in their role in promoting urban development. The

Swedish decentralized organization of urban development may influence the result as each

municipality are free to organize their work in different ways creating different structures for

promoting the development of blue green solutions. Despite the possible different situation for

Swedish municipalities compared to municipalities, we believe that there are more similarities

than dissimilarities due to similar challenges in urban areas and the similarities related to

challenges and possibilities of the use of information systems.

The findings indicate that if a municipal ambition is to ensure a development of green

infrastructure and NBS in urban areas that are both cost efficient and contributes to the

general well-being of both nature and people it is essential to develop a data collection and

data management system that is a pan-municipal, bridging sectors and disciplines and is able

to involve citizens as a data source, especially at the super local scale thorough different types

of crowd sourcing structures. The digitalization of information is developing fast on Sweden

which opens the way for integrated decision platforms. Through for example the Geodata

Cooperation, Swedish municipalities are able to using a more similar set of GIS data. But as

noted in the interviews there is still a lack of knowledge of how to use it and there are several

datasets are missing. The constructing sector is increasingly using collaborative IT systems

such as BIM where expert areas are integrated into one IT structure (Dossick, 2011). Recent

research has showed their ability to work as digital boundary objects (Alin 2013) but the

potential of these structures is strongly interlinked with how a user perceive the organization

of the form and content of the use if such a tool (Poirier 2017) which ties back to the findings

in this article that organization seems to play a central role in the success of the integration of

new IT technology to support decision making. Spatial planning could be supported by

similar tools where experts on different aspects of planning as well as citizens could create a

collaborative space and learn from each other. In Hopkins et al. (2005) has developed an

initial version of a planning data model (PDM), will enable urban development decision

makers to conceive of planning as involving many actors whose many plans can provide

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useful and usable information. There are also other potential data sources that could be

explored as a part of the development of spatial planning and mainly the inclusion of citizens

such as mobile phone data (Reades et al. 2007)

The developed framework accompanied by the list of existing data set could serve as a basis

for an identification of gaps in data availability, acknowledging the need of different kinds of

data to ensure holistic, strategic planning of blue-green structures and features in urban areas.

The dilemmas that have been brought up can be used as basis to identify the more specific

challenges in municipalities to strengthen and develop a clearer structure for how the different

sectoral practices and their interrelated disciplines can collaborate.

We believe that such a gap analysis could provide local politicians with more useful

information concerning the value and cost of blue green solutions better as if including

knowledge about various urban values at early stages of the spatial planning process, it will be

easier to assess the impact of the direction of the urban development to defined environmental

and sustainable development goals and unnecessary high costs later in the planning process

could be avoided. This suggestion goes in line with the idea that multi-functionality should be

promoted through planning objectives or an agreed set of functions or standards that need to

be considered in green space planning (Ahern 2014; Haaland 2015) and where a sound

knowledge of the green space potential is needed to exploit its opportunities (Lennon 2015).

This could support a governance approach where data collection management and use to

sustainable cities as an infrastructure comparable to other urban infrastructures such as roads,

electricity net and sewage and storm water systems. In the long run this could facilitate a

systematic assessment of the urban space, improved links to defined guidelines and goals for

sustainable development, the provision of a variety of functions throughout the urban space, a

basis for assessments of synergies trade-offs between different functions and a basis for

collaboration between departments as well as with citizens as well as between municipal

authorities and science.

As a step for further research we suggest that the framework is used in a set of real contexts

which in addition to assessing the framework in itself and identifying more specific

challenges to provide input on how to organize the data collection, management and use, one

important area to further explore is the need of data at different scales, supporting the

understanding of how to enhance multifunctionality at all spatial levels and indicating how

this multifunctionality is distributed in space and time to help identify priorities in conserving

and enhancing certain functions and ecosystem services.

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