Drainage Impact Assessment - August 2014

Airthrey Green, Stirling

Airthrey Green DIA

Ecus Ltd

Report to:

Graham�s Family Dairy Airthrey Kerse Farm Henderson Street Bridge of Allan

Report Title: Airthrey Green Drainage Impact Assessment

Revision: FINAL

Issue Date: August 2014

Report Ref: SC/4772//DIA

Prepared by: Ecus Ltd.

Brook Holt 3 Blackburn Road

Sheffield S61 2DW

0114 2669292

The report and the site assessments carried out by ECUS on behalf of the client in accordance with the agreed terms of contract and/or written agreement form the agreed Services. The Services were performed by ECUS with the skill and care ordinarily exercised by a reasonable Environmental Consultant at the time the Services were performed. Further, and in particular, the Services were performed by ECUS taking into account the limits of the scope of works required by the client, the time scale involved and the resources, including financial and manpower resources, agreed between ECUS and the client.

Other than that expressly contained in the paragraph above, ECUS provides no other representation or warranty whether express or implied, in relation to the services.

This report is produced exclusively for the purposes of the client. ECUS is not aware of any interest of or reliance by any party other than the client in or on the services. Unless expressly provided in writing, ECUS does not authorise, consent or condone any party other than the client relying upon the services provided. Any reliance on the services or any part of the services by any party other than the client is made wholly at that party�s own and sole risk and ECUS disclaims any liability to such parties.

This report is based on site conditions, regulatory or other legal provisions, technology or economic conditions at the time of the Service provision. These conditions can change with time and reliance on the findings of the Services under changing conditions should be reviewed.

ECUS accepts no responsibility for the accuracy of third party data used in this report.

Airthrey Green DIA

Contents

1. INTRODUCTION ............................................................................................... 1

2. SITE DRAINAGE ............................................................................................... 2

2.1 SITE HYDROLOGY ........................................................................................ 2

2.2 EXISTING SURFACE WATER DRAINAGE: FIELD DRAINS AND DITCH NETWORK ..... 2

3. SOIL CLASSIFICATION & HYDROGEOLOGY ................................................. 3

4. GREENFIELD RUN-OFF ESTIMATION ............................................................ 4

5. WATER QUALITY DESIGN CRITERIA ............................................................. 8

5.2 HAZARD CLASSIFICATION ............................................................................. 8

6. RUNOFF DESTINATION ................................................................................... 9

6.1 GROUNDWATER (VIA INFILTRATION) .............................................................. 9

6.2 OUTFALLS FOR SUDS .................................................................................. 9

6.3 CATCHMENT 1 (1001) RUN-OFF DESTINATION - ECR SUDS ........................... 9

6.4 CATCHMENT 2 RUN-OFF DESTINATION - WETLAND BASIN ............................... 9

6.5 CATCHMENT 3 - RUN-OFF DESTINATION � CAUSEWAYHEAD BURN .................. 9

7. DRAINAGE CONCEPT ..................................................................................... 1

8. FEASIBILITY ASSESSMENT ............................................................................ 3

8.1 OVERVIEW .................................................................................................. 3

8.2 FLOW MONITORING ...................................................................................... 3

8.3 HYDRAULIC CHARACTERISTICS OF C-SWS .................................................... 4

8.4 DRAINAGE MODEL DESCRIPTION ................................................................... 5

8.5 SUDS OUTFALL CAPACITY AND FLOW CONVEYANCE .................................... 13

FORGLEN BURN .................................................................................................... 13

WESTERN OUTFALL............................................................................................... 14

CAUSEWAYHEAD BURN AND SURFACE WATER SEWER (C-SWS) ............................. 15

8.6 C-SWS CAPACITY ..................................................................................... 16

8.7 ECR SUDS ............................................................................................... 21

8.8 TIDAL INFLUENCE ON THE DRAINAGE SYSTEM .............................................. 25

8.9 WESTERN OUTFALL ................................................................................... 28

8.10 CONCLUSIONS ........................................................................................... 32

9. MAINTENANCE RESPONSIBILITY ................................................................ 33

10. SITE-SPECIFIC DRAINAGE DESIGN CONSIDERATIONS ........................ 33

11. SUDS CONSTRUCTION ............................................................................. 34

12. RECOMMENDATIONS................................................................................ 35

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1. Introduction 1.1.1 This report provides an outline Drainage Impact Assessment (DIA) for surface water

drainage and initial guidance on implementation of SuDS for the Airthrey Green development site in line with current best practice.

1.1.2 The objective is to provide early guidance and assessment of site drainage characteristics, SuDS layout and components considered to be suitable within the site design. This is aimed at facilitating Planning discussions with the relevant authorities.

1.1.3 The DIA should be read in consultation with the Flood Risk Assessment and provides supplemental information regarding flooding for the drainage network. Information used in the preparation of this report is provided in Table 1, below.

Table 1: Sources of Information and Search Results

Source Results

Reports by consultants

Containing records of flood modelling, groundwater investigations and drainage strategy and information for the site.

· WWT Consulting (2012a) Preliminary drainage concept and feasibility study

· WWT Consulting (2012b) Progress report August 2012 � Airthrey Green FRA

· WWT Consulting (2012c) Airthrey Green SuDS and Fluvial-Pluvial Modelling

· WWT Consulting (2012d) Addendum to Airthrey Green SuDS and Fluvial-Pluvial Modelling

· WWT Consulting (2013a) Revised drainage concept

· WWT Consulting (2013b)) Addendum to revised drainage concept

· Hydrologic (2012) Airthrey Kerse Flood Risk Assessment 1D-2D Fluvial Hydraulic Modelling Technical Note

· Hydrologic (2012) Airthrey Kerse Flood Risk Assessment 2D Pluvial Hydraulic Modelling Technical Note

· Rigare (2012) Scoping-Level Hydrogeological Assessment;Airthrey Kerse, Stirling

· Rigare (2012) Technical memorandum Interpretation of borehole & short-term groundwater level data from site investigation boreholes, Airthrey Kerse

· Insight Geo. (2012). Factual ground investigation report. Airthrey hydrogeological assessment, Bridge of Allan. Consultant report P12057 for WWT Consulting, July 2012.

· CCTV survey of the network and depth of pipe invert from manhole lid where conducted in January 2014 by UIS Ltd.

· Flow monitoring of the C-SWS at two locations with rainfall monitoring during the same period by EMS Ltd. Concurrent rainfall and flow monitoring of the pipe network for a month (January-February 2014) was conducted by EMS Ltd. This provided evidence of winter base flows to the network and evidence for capacity. It also provides a data set for calibration of network response

· Topographic survey of manhole and street levels on the Easter Cornton Road and C-SWS.

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2. SITE DRAINAGE

2.1 Site Hydrology

2.1.1 The current site hydrology, watercourses and flood mechanisms are discussed in detail in the Flood Risk Assessment for the site and are not discussed further here. Site drainage descriptions are limited to the existing field drains and ditch network. Locations of the main hydrological features are shown on Figure 7.1.1 within this report.

2.2 Existing surface water drainage: field drains and ditch network

Forglen Boundary

2.2.1 Initial investigation of the field drain network from historical maps shows how this area was originally drained to a ditch or pipe which discharged along the western boundary of the then orchard, (now gardens of properties) to the Forglen Burn. Surface water accumulates at this point and it is apparent that the historical conveyance to the Forglen is impeded or is no longer connected. Many of the current drainage issues relate to changes in the site hydrology when the western boundary of the site was developed and field drainage was impeded to the Forglen Burn. This may account for the reported water logging on this boundary as flows collect in this area quite rapidly.

Western boundary with Railway

2.2.2 Other historical drains were bisected by the railway and culverts were installed at the Western Outfall location on the site (described in detail in subsequent sections). From the centre of the site, from east to west is the old ditch route which flowed to the River Forth via the stone culvert.

2.2.3 It is unclear if drainage passes under the railway at the pedestrian crossing as no records or obvious outlets exist to the perimeter ditch on Cornton Road. The construction of the railway has changed a significant flow pathway and exacerbates issues on the Easter Cornton Road. The historical maps show a ditch which at one stage was connected to the field ditch and outfall at HMP Cornton Vale. There was an open channel which originally had its upper reach within Airthrey Kerse flowing past Cornton Farm and running approximately along the current security fence boundary which had a confluence with the River Forth. No plans are available of the exact culvert routing but this has been replaced by twin 450mm pipes which outfall to the remnant channel and out to the River Forth.

Field Drainage

2.2.4 The remnant of much of the historical drainage system has been identified in Lidar terrain data following the field boundaries. Currently a herringbone sub-surface clay pipe drainage network provides drainage from many of the fields with routing to the central depression of the site and discharging to the western outfall. On the north-eastern boundary a field drains to the Forglen Burn along the boundary with the Primary School. A field on the Easter Cornton Road boundary drains to the Causewayhead Surface Water Sewer (C-SWS).

2.2.5 A depression from the clay extraction of the old brick works is the lowest point close to the site. The basin does not retain surface water all year round, or even seasonally which would be expected for a depression of this nature on heavy clay. There are small areas of permanent wetland marsh on the western boundary but the depression appears to drain relatively quickly. This indicates that this area has a higher soil infiltration rate, (possibly due to clay extraction and spoil/waste backfill activities) than the surrounding land. It is also possible that this area has greater sub-surface hydrological connectivity with the Allan Water and Forth water courses. No development is proposed in this area and the depression will be retained in the park plan.

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3. Soil classification & hydrogeology

3.1.1 For the purposes of estimating run-off, the soil classification for Airthrey Kerse is Soil Type 3 and 4 (based on FSR). This is consistent with borehole records from the site (Insight Geo 2012, 2014) that identify a silty to very silty and sometimes sandy clay in the lithology underlying the topsoil. Records from prior studies (Rigare 2012a&b) identify groundwater levels near to the surface.

3.1.2 Further investigation of seasonal groundwater levels has been undertaken to gain a fuller understanding of groundwater functioning. This has revealed variation in water table levels consistent with perched and impeded drainage but a general hydraulic gradient exists from the surrounding land into the existing field drainage network in the centre of the site and out to the Western outfall. If this is a groundwater pathway or just a feature of the outfall routing remains to be determined but general gradients to the River Forth appear to operate and may represent a significant unquantified pathway for slow infiltration processes on the site. Sub-soil porosity tests have not been conducted as it is anticipated that the conditions are generally unfavourable for infiltration mechanisms such as soak-away systems, although some limited infiltration will occur as the soil is not a pure clay and conditions may be patchy across the site.

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4. Greenfield run-off estimation

4.1.1 The IH24 method has been used for this estimate, as the total development area is less than 100 hectares.

4.1.2 Estimates for Greenfield run-off are provided in Table 2 from the site allocation areas (or sub-catchments), rather than from the entire site. They are included in this form to allow for differences in soil type and for further discussion on permissible discharges from the proposal SuDS scheme, to receiving water bodies and surface water drainage routes. The estimated run-off volumes for each catchment are provided in Table 2. It should be noted that the volumes provided in Table 3 are indicative and provide no additional volumes to support flood relief goals, especially on the Easter Cornton Road (ECR). The required volume to mitigate this surface water flooding issue is examined in Section 8 and incorporated into the preliminary design for the ECR SuDS.

4.1.3 It is therefore proposed that a peak discharge totalling 238 l/sec is permitted from the development area at the 0.5% Annual Exceedance Probability (AEP) with some allowance for additional flows during fluvial driven or extreme events with respect of the ECR and Wetland Basin features, which have dual flood protection facility. It is reasonable to allow some additional discharge given the level of flood protection the proposal will convey to the networks and Easter Cornton Road, railway line and Cornton. This is further discussed in Section 8.

4.1.4 The following discharge rates are proposed and examined in further detail in subsequent sections:

1) Flows limited to Qbar for catchment 3001 [Qbar 9 l/sec] and 3002, [Qbar 7 l/sec] direct to the Causewayhead Burn.

2) Flow limited discharge to the Western Outfall of [Qbar 168 l/sec at the 0.5%AEP].

3) ECR SuDS flow limited discharge at Qbar [54 l/sec]

4) Discharge to the Forglen Burn [120 l/sec initiating at the 1:5 year 20%AEP] or in response to fluvial flood flows to the Wetland Basin from the Forglen Burn

5) Tertiary flow pathway to the C-SWS from the Wetland Basin with flow limited discharge to the Western Outfall [168 l/sec at the 0.5%AEP].

4.1.5 Examination of the C-SWS presented in Section 8 provides evidence that flows equal to the proposed Western Outfall rate are feasible via this route as the additional network capacity provided by the flood alleviation and SuDS scheme buffers flows to the C-SWS providing significant surface water flood alleviation and reduces over-capacity issues of the baseline scenario. It is certainly viable to route flows from the Wetland Basin to the C-SWS.

4.1.6 It would be beneficial to the ecological integrity of the proposed habitat elements within the Wetland Basin to have multiple flow pathways out of the site. It is therefore proposed that a sweetening flow of up to 20 l/sec is allowed by this route, equal to baseflow flow conditions for the upper C-SWS. The ability to use this route for emergency discharge should the Western Outfall route fail or require maintenance provides additional protection to the Forglen Burn outfall.

4.1.7 This discharge proposal balances best-practice run-off rates with moderate additional outfall rates under more extreme conditions, reflecting the dual function of flood alleviation and SuDS required for the site.

4.1.8 The maximum discharge from the development catchments will be the mean annual maximum flood flow (Qbar), presented in Table 2, below. It is demonstrated that the current strategy will significantly reduce run-off from the site and maximum flood flow estimates are provided in Table 3, below.

4.1.9 Surface runoff that enters the site from adjacent areas will need to be managed on or

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safely conveyed across the site for all return period events. The Wetland Basin and Causewayhead Flood Alleviation Wetland have been designed to intercept fluvial and pluvial flooding.

4.1.10 Estimates for required attenuation storage for the SuDS elements are provided in Table 4, below. It should be noted that the SuDS elements will have additional flood alleviation functionality and that the estimates provided relate to the attenuation requirements of the development only. Further analysis of the sizing of the attenuation features are provided in Section 8.

4.1.11 The attenuation volumes will enable discharge of the stored volumes at Qbar for the development catchments, discharging at flow rates that are similar to the predicted greenfield rate, improving downstream flood risk during higher return periods.

4.1.12 The 0.5%AEPCC (including an allowance for Climate Change) design standard has been incorporated into SuDS attenuation volumes to ensure that run-off from the SuDS does not contribute any flood risk up to this design event. Where possible additional exceedance storage will be provided with clear pathways should storms exceed this design standard.

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Table 2 Estimated Greenfield run-off rates for Airthrey Kerse development catchments - IH24 method, SOIL Type 3 or 4 as indicated in brackets. Qbar for SOIL Type 3 is 4.142 l/sec/ha and Qbar for SOIL Type 4 is 6.901 l/sec/ha. Soil Type 3 predominates in the Wetland Basin area and eastern portion of the site. N.B WB 2005 is the Wetland Basin area and associated swales and conveyance features on the development perimeters and is included within the positively drained calculations.

Catchment (SOIL type)

Area (m

2)

Qbar (l/sec)

1 in 1 year (l/sec)

1 in 30 years (l/sec)

1 in 100 years (l/sec)

1 in 200 years (l/sec)

1 in 200 years CC

(l/sec)

ECR SuDS 1000 (4) 77,670 53.60 45.56 104.52 140.97 160.81 209.05

WB 2001 (3) 59,210 24.46 20.79 47.71 64.34 73.39 95.41

WB 2002 (4) 51,159 35.31 30.01 68.85 92.85 105.92 137.69

WB 2003 (4) 50,020 34.52 29.34 67.31 90.79 103.56 134.63

WB 2004 (4) 59,720 41.21 35.03 80.37 108.39 123.64 160.73

WB 2005 (3) 80,130 33.11 28.14 64.56 87.07 99.32 129.12

Total WB 300,239 168.61 143.32 328.79 443.45 505.84 657.59

NE SuDS 3001 (3) 21,580 8.92 7.58 17.39 23.45 26.75 34.77

SE SuDS 3002 (3) 16,230 6.71 5.70 13.08 17.64 20.12 26.15

Development Total 415,719 237.84 202.16 463.78 625.51 713.51 927.56

Table 3 Run-off volumes for catchments for the 6 hour storm for the 3.33%AEP, 0.5%AEP and 0.5%AEPCC design standards. Footnotes are provided to demonstrate estimation methods. N.B WB 2005 is the Wetland Basin area and associated swales and conveyance features on the development perimeters and is included within the positively drained calculations.

4.1.14 Discharge from the SuDS system will be controlled by vortex control structures ensuring that the maximum permissible discharge is maintained across all storm durations.

1 Draindown volume calculated as 0.5*Qbar for 6 hours simulating the development of operating head during the storm and Qbar*18 hours. Note: this is a preliminary estimate and detailed design is required taking into consideration the flow characteristics of the proposed flow control.

2 Storage volume = Area (m

2)*PIMP*RD (m)*0.8 (Run-off coefficient) [PIMP � Percentage Impermeable

area, RD � Rainfall depth for storm duration. Run-off coefficient from impermeable areas is set to 0.8 in line with CIRIA 2007 guidance.

3 WB2005 is the Wetland Basin area which has been treated as impermeable (PIMP 100%) with run-off co-efficient of 1, i.e. 100% run-off to the feature.

Catchment (SOIL type) Area (m2) 3.33%AEP

6 hr storm (45.35mm)

0.5%AEP 6 hr storm (68.05mm)

0.5%AEPCC 6 hr storm (88.46mm)

Draindown1

volume in 24 hours at Qbar

Run-off volumes2 (m

3)

ECR SuDS 1000 (4) 77,670 1,832 2,748 3,573 4,052

WB 2001 (3) 59,210 1,181 1,773 2,305 1,850

WB 2002 (4) 51,159 1,021 1,532 1,991 2,669

WB 2003 (4) 50,020 998 1,498 1,947 2,610

WB 2004 (4) 59,720 1,192 1,788 2,325 3,116

WB 2005 (3)3 80,130 3,634 2,999 7,089 2,503

Total WB 300,239 8,026 9,590 15,656 12,747

NE SuDS 3001 (3) 21,580 431 646 840 674

SE SuDS 3002 (3) 16,230 324 486 632 507

Development Total 415,719 10,612 13,470 20,701 17,980

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Table 4 Required storage volumes for SuDS elements attenuating the 6 hour 0.5%AEPCC design standard. The WB attenuation volumes are shown in italics as these attenuation volumes are considered separately with the flood alleviation storage volume. The figures are provided to demonstrate the relative contribution of the Wetland Basin catchment on the total storage volume of 80,500m

3. In addition this hydraulic modelling has investigated the impacts of other

storm durations in excess of this design standard in line with prior studies of the site. Note: Storage volumes are preliminary estimates and detailed network design is required to finalise the required storage volume for the feature based on outfall flow control characteristics, climatic conditions and any upstream site control or interception features.

Catchment (SOIL type) Area (m

2) 0.5%AEPCC 6hr storm

(88.46mm) Design standard

Storage volumes (m3)

ECR SuDS 1000 (4) 77,670 2,994

WB 2001 (3) 59,210 2,041

WB 2002 (4) 51,159 1,610

WB 2003 (4) 50,020 1,574

WB 2004 (4) 59,720 1,879

WB 2005 (3) 80,130 6,731

Total WB catchment 300,239 13,835

NE SuDS 3001 (3) 21,580 431

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5. Water quality design criteria

5.1.1 Current best practice takes a risk-based approach to managing discharges of surface runoff to the receiving environment. The following section provides guidance on the extent of water quality management likely to be appropriate for the site.

5.2 Hazard Classification

5.2.1 Runoff from clean roof surfaces (ie not metal roofs, roofs close to polluted atmospheric discharges, or roofs close to populations of flocking birds) is classified as Low in terms of hazard status.

5.2.2 Runoff from roads, parking and other areas of residential, commercial and industrial sites (that are not contaminated with waste, high levels of hydrocarbons, or other chemicals) is classified as Medium in terms of hazard status.

5.2.3 The following treatment requirements are identified for disposal to surface water systems:

· Roof runoff does not generally require treatment prior to discharge to the Wetland Basin or SuDS features (Ciria 2007). It is therefore proposed that roof drainage will be directed by piped gravity drainage to the SuDS features following attenuation within individual plots using gravel trench systems and orifice plates to the disconnecting manholes. Ideally water harvesting will be incorporated into the development ethos to maximise the sustainability of the development.

· Runoff from other parts of this site such as roads, parking and other areas will require at least 2 treatment stages prior to discharge.

5.2.4 The developer will be responsible for incorporating 2 treatment stages into their allocation parcels, prior to discharge to the main SuDS attenuation feature servicing their allocation. This will ensure that the SuDS are not polluted and will maximise biodiversity and amenity value.

5.2.5 SuDS treatment stages will include swales, filter strips and bio-remediation features such as small Reedbeds.

5.2.6 Major roads will incorporate treatment features into their design. Interception storage and treatment stages can be incorporated into features thus minimising landtake.

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6. Runoff destination

6.1 Groundwater (via Infiltration)

6.1.1 Infiltration (as a major process) is unlikely to be appropriate for managing runoff from this site. Studies have confirmed the following constraints for infiltration for source control as the sub-surface geology is primarily impermeable and the use of infiltration for surface water disposal is not suitable as a primary mechanism. The water table varies across the site and is considered perched at between 9.3 � 7.2 m AOD. Groundwater flows into SuDS features are unlikely to contribute significant volumes as horizontal transmission pathways from surrounding land into lower SuDS areas are unlikely to form in the clay rich soil.

6.2 Outfalls for SuDS

6.2.1 There are three main outfall routes established for the development:

· NEW OUTFALL - Forglen Burn (FGLN) � option of higher level discharge or exceedance routing with flap arrangement to prevent backflow and inhibit discharge when high flows occur in the receiving watercourse. Outlet from wetland basin at 8.5 m AOD with invert above or near Forglen Burn base flow level at 8.3 m AOD

· EXISTING OUTFALL - Western Outfall (WO) � main routing to the River Forth. It has been proposed that there is sufficient capacity to allow for routing via this method with preliminary maximum discharge of Qbar [168 l/sec at the 0.5%AEP].

· EXISTING OUTFALL - Causewayhead Burn and C-SWS. This route will provide the Primary route for the ECR boundary area with a preliminary maximum discharge of Qbar [53.6 l/sec at the 0.5%AEP].

6.2.2 The hydraulic profiles for the three proposed routes are shown in figure 6.2.1.

6.3 Catchment 1 Run-off destination - ECR SuDS

6.3.1 It has been determined that surface water runoff from the southern portion of the site cannot practicably be discharged to the Wetland Basin and that the area bordering Easter Cornton Road (Catchment 1001) has to discharge to the C-SWS.

6.3.2 The project surface sewer reference is: 675mm Easter Cornton Road; and the asset owner is: Stirling Council.

6.4 Catchment 2 Run-off destination - Wetland Basin

6.4.1 Catchment 2 [2001-2004] will drain via source control and site specific SuDS management train to a central Wetland Basin [Catchment 2005]. This wetland basin will drain to A) the Western Outfall at a peak discharge rate of 168 l/sec and B) Forglen Burn through a throttled link with a peak flow of 120 l/sec. This allows for more rapid draindown of the system and C) the C-SWS via a flow control structure (as a secondary route at up 168 l/sec to allow redundancy in outfall arrangements.

6.5 Catchment 3 - Run-off destination � Causewayhead Burn

6.5.1 Catchment 3 is divided into 2 sub-catchments (3001-3002); serviced by the NE SuDS and SE SuDS. Surface water discharge from NE SuDS will be to the Causewayhead Burn Flood alleviation Wetland at a maximum discharge rate of 9 l/sec (Qbar). Discharge from the SE SuDS will be to the Causewayhead Burn at a maximum discharge rate of 7 l/sec (Qbar)

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7. DRAINAGE CONCEPT

7.1.1 The drainage strategy for the site is presented in Figure 7.1.1 below with descriptions of the SuDS components.

7.1.2 The drainage system has two functions:

· Flood alleviation

· Surface water management and disposal

7.1.3 The key SuDS features, which allows for flood attenuation, storage and controlled release to surface water, are the central Wetland Basin and Causewayhead Flood Alleviation Wetland. The latter is controlled by the existing pipe arrangement. The Wetland Basin feature extends within the development allocations forming for the most part open green space. Although extensive in the park area, for most of the time this are will be a gentle sloping bowl in the landscape with a wetlands habitat mosaic at the base level. During extreme storms the Wetland Basin will fill and the parkland will become open water to a maximum depth of 1m.

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8. Feasibility Assessment

8.1 Overview

8.1.1 The following section provides an assessment of the proposed drainage strategy to determine the feasibility of options and determine existing network capacity.

8.2 Flow monitoring

8.2.1 Flow monitoring was conducted between 16/01/14 � 12/02/14 at two locations in the C-SWS, Lothian Crescent and Broomhill Drive. Rainfall was recorded at Airthrey Dairy using a Tipping Bucket Rain gauge (TBR), see Figure 8.2.1.

8.2.2 Flow monitoring of the upstream section (Lothian Crescent) reveals that there is a winter Dry Weather Flow (DWF) of approximately 20 l/sec. This is a combined flow from the Causewayhead Burn and field drainage of the site. The flow monitoring was conducted in winter (when it is anticipated that due to increased soil saturation and generally higher rainfall) responses to rainfall should be more rapid and thus are considered �worst-case� from a seasonal perspective. In addition, groundwater levels are likely to be fully re-charged within the catchment and spring-fed sources contribute to this base flow.

8.2.3 Peak flow for Lothian Crescent (pipe run S2-S4, 675mm) reached 100l/sec for short durations during rainfall events. Flow monitoring of the downstream section reveals that the base flow is far more variable at this point with up to a four-fold increase in DWF occurs as the network progresses, with a peak base flow of 80l/sec at Broomhill Drive (pipe run S19-S20, 1050mm).The influence of the River Forth is demonstrated in Figure 8.2.2 where flow is inhibited by initiation of the flap valve.

Figure 8.2.1 Flow and rainfall data for the C-SWS for 16/01/14-12/02/14. Note that there was an error in recording for a period from the 31st January-2nd February due to operational errors in data logging.

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Figure 8.2.2. Tidal locking of the outfall recorded in the monitoring data for Broomhill Crescent Nodes S19-S20

8.3 Hydraulic characteristics of C-SWS

8.3.1 A hydraulic profile for the drainage network is shown in Figure 8.3.1 below. Pipe diameter increases from 675mm (Chainage 0-669m) to 975mm (Chainage 669-813m) to 1050mm (Chainage 813-1280m).

8.3.2 Pipe roughness was assigned at the upper end of recommended roughness for concrete pipes at Manning�s 0.015 and any evident reduction in capacity from CCTV survey was incorporated into the model.

Figure 8.3.1 C-SWS long section from the Causewayhead Burn outlet to the River Forth

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8.4 Drainage model description

8.4.1 An assessment of the capacity of the drainage network is presented in the following section.

8.4.2 A combination of a) permeability mapping has been used to determine urban catchment contributions in the Causewayhead area b) flow monitoring of the upstream and downstream sections of the network combined with rainfall monitoring; and c) storm and flow modelling has been used to assess the capacity of the network using SWMM v5.1.

8.4.3 The percentage impermeable areas for each distinct catchment in Causewayhead (see Figure 8.4.1) were determined providing the basis for assessing run-off entering the network from roads, pavements, roof and other hard surfaces identifiable from the available mapping and aerial photography.

Figure 8.4.1 Summary of catchments and impermeability for sub-catchments of the C-SWS

8.4.4 Scottish Water maps of the network and information provided by CCTV survey and

topographic survey provide the invert levels of the pipes and associated manholes for the main drainage run. This information was used to build a hydraulic model of the C-SWS (see Figure 8.4.2, below).

8.4.5 It should be noted that a section of adverse gradient was determined by survey between S4 and S6. This was confirmed in part by the CCTV survey which indicated a very slow flowing pipe which crested and then fell to manhole S6 but didn�t appear quite as adverse as the topographic survey results indicated. This adverse gradient has been retained within the model but shows little effect in altering conveyance characteristics.

8.4.6 An additional un-surveyed pipe run has been estimated within the model for a 150-375mm pipe (joining at Manhole S14) which drains the Easter Cornton Road to the pedestrian railway crossing area which is flooded on an annual basis. CCTV found flows

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from this point on the Easter Cornton Road via this route. The 225mm pipe running along the Easter Cornton Road is very badly choked with root mass and couldn�t be surveyed but it is believed that other pipes service the road drainage and join at S2-S4 and the 675mm Causewayhead Burn culvert. These possible outflows from the ECR have not been simulated in the model but would be limited to inflows from 150mm diameter pipes.

8.4.7 This simplification of the network also included intermodal connections between manholes with inflows from the majority of the drainage catchments simply fed into the nodes of the main C-SWS modelled run. Catchment characteristics were estimated and differences applied (where appropriate) to the Baseline and Future scenarios. The differences included changes in run-off destination of catchments and catchment characteristics of permeability. Other factors such as pipe roughness and soil infiltration characteristics were consistently replicated between the baseline and future scenarios.

8.4.8 In addition to the C-SWS pipe flow model, the drainage model was extended to include Airthrey Kerse and the Causewayhead Burn and Western Outfall to examine flow routing and behaviour of the catchment for the Baseline (see Figure 8.4.3) and proposed future scenario for the site (see Figure 8.4.4). Topographic survey data was used to simulate flow pathways from the upper Causewayhead Burn to the outfall, the Easter Cornton Road to railway flow route, and the Western outfall arrangement at HMP Cornton Prison.

Figure 8.4.2 C-SWS and Easter Cornton Road to the River Forth

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Figure 8.4.3 Baseline model of the site showing ECR flood route, Causewayhead Burn and Western Outfall

Figure 8.4.4 Future model of the site showing ECR flood route, Causewayhead Burn and Western Outfall

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8.4.9 A range of simulations have been conducted with the following hydrological scenarios

tested for the Baseline and Future scenarios:

SuDS and outfall capacity

· <99%AEP Calibration flow event from monitored rainfall and flow data (see Figure 8.4.5)

· 3.33% AEP (1:30 year storm) (see Figure 8.4.6)

+ Forglen Burn to CWHB flow derived from Fluvial-Pluvial Model (Peak 1.0 m3/sec, see Figure 8.4.8)

+ Forglen Burn to ECR flow derived from Fluvial-Pluvial Model (Peak 2.1 m3

/sec, see Figure 8.4.9)

· 0.5%CCAEP (1:200 plus climate change) (see figure 8.4.7)

+ Forglen Burn to CWHB flow derived from Fluvial-Pluvial Model (Peak 2.6m3/sec, see Figure 8.4.8)

+ Forglen Burn to ECR flow derived from Fluvial-Pluvial Model (Peak 5.8 m3 /sec, see Figure 8.4.9)

8.4.10 The influence of tidal/fluvial levels on the River Forth was investigated for the Western Outfall and the C-SWS:

· 3.33% AEP (1:30 year storm) - 10%AEP level in the River Forth (6.4m AOD)

· 3.33% AEP (1:30 year storm) - 0.5%AEP level in the River Forth (8.43 m AOD)

· 0.5%CCAEP - 10%AEP level in the River

· 0.5%CCAEP - 0.5%AEP level in the River

8.4.11 Additional scenarios were tested using data derived from prior modelling. CWHB flow derived from Fluvial-Pluvial Model (Peak 0.78 m3/sec, see Figure 8.4.10) entered into the CWHB at node (CWHD0089). This flow rate was derived from the FP-M model but is considered to be exaggerated as the model simulated the CWHB at bank-full and didn�t include for the ditch and pipe in the Upper CWHB catchment which provides significant attenuation of flows from over-topping of the Forglen Burn channel.

8.4.12 The catchments� responses were calibrated to observed flow monitoring data and calibration plots are provided in Figure 8.4.11 - 8.4.12, below. The plots demonstrate good fit with the upper model zone but slightly poorer fit in the lower reach but are considered to be acceptable for the purposes of this study.

8.4.13 The option to mitigate flooding on the Easter Cornton Road is explored with reference to the 0.5%AEPCC and the 3.33%AEP in line with the FP-M study for the site.

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Figure 8.4.5 Observed rainfall data from Tipping Bucket Rain Gauge on the 29th

of January 2014. Used for calibration of the model.

Figure 8.4.6 3.33% AEP Rainfall curve applied to simulate 11.5hr storm � storm duration consistent with FP-M model

Figure 8.4.7 0.5%AEPCC Rainfall curve applied to simulate 9.5hr storm-� storm duration consistent with FP-M model

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Figure 8.4.8. Flow introduced to the Causewayhead Upper Catchment (Node CWHD0882), simulating the overbank flow from the Forglen to the CWHB network. Flows are derived from the FP-M (Hydrologic 2012c) for the 3.33%AEP and 0.5%AEPCC.

Figure 8.4.9. Flow introduced directly to the Wetland Basin, simulating flow rates observed from the Forglen Burn to the central depression in the FP-M (Hydrologic 2012c) for the 3.33%AEP and 0.5%AEPCC.

Figure 8.4.10 Flow introduced directly to the Causewayhead Burn (Node CWHD0661), simulating flow rates observed in the FP-M (Hydrologic 2012c) with flow rate interpolated between 14 and 24 hours.

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Figures 8.4.11- Calibration runs for a simulated and observed rainfall event on the 26th of January 2014 showing flow, depth and velocity for link node (pipe-run) S2-S4 where flow monitoring was conducted at Lothian Road.

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Figure 8.4.12 - Calibration runs for a simulated and observed rainfall event on the 26th of January 2014 showing flow, depth and velocity for link node (pipe-run) S19-S20 where flow monitoring was conducted at Broomhill Crescent.

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8.5 SuDS Outfall capacity and flow conveyance

8.5.1 The proposed outfalls from the site are discussed in the following section with reference to hydraulic characteristics and flow conveyance.

Forglen Burn

8.5.2 The route of the new outfall to the Forglen is shown in Figure 7.7.1, above. Although it is not possible to fully drain the Wetland Basin via the Forglen Burn Outfall the route is effective at 8.5 m AOD and above. Through further modelling and detailed design work of the Wetland Basin it may be possible to elevate the base level and use this route to fully drain the system. Using a pipe (450mm) to a pre-outfall chamber with a flap valve provides protection for backflow and inhibits flow to the Forglen Burn when levels are elevated in the stream channel, thus preventing any flood risk downstream of the site on the Forglen Burn channel due to the discharge. Although the 300m pipe-run would be consequently of shallow gradient (1:1500), it would require no flow control device with a peak flow velocity of approximately 120 l/sec at maximum operating head, (see Figure 8.5.1). This outfall provides a mechanism to drain fluvial flood water back to the Forglen Burn in a controlled manner and only when levels have receded following spate flow, see Figure 8.5.2.

8.5.3 The outfall will initiate at approximately the 5-10%AEP storm event and act as a passive emergency outfall for the system should blockage occur elsewhere. This level is essentially at the upper level of the wetland base zone and provides a simple fail-safe to the system.

Figure 8.5.1 Water level in the Western Basin (0.5% AEPCC) and flow rates for the proposed outfall pipe. The simulation assumes free-flowing conditions at the Forglen Burn

Figure 8.5.2 Flow rates for the proposed outfall pipe. The simulation shows the flow behaviour of the outfall (red line) during a 0.5%AEPCC event in response to the Forglen Burn levels (Pink line) and water levels in the Wetland Basin (Blue line). It demonstrates that flows can be effectively limited by a flap valve during high levels in the Forglen Burn

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Western Outfall

8.5.4 The Western outfall conveys sub-surface drainage for the majority of the fields in the current baseline (>18 ha). Currently a 275mm pipe coveys flows from the field drainage network by the central depression. The flow route is illustrated in Figure 8.5.3, below. The culvert is a 600mm wide by 750mm deep, rectangular stone culvert under the railway line at Point A, running for approximately 120 m to Point B. Just after the culvert reaches the other side of the railway there is a short section of 500mm pipe used as a replacement section of the damaged culvert. Dye tests and CCTV survey have revealed connection with Point B where flows exit into a ditch and at Point C, where two 450mm pipes convey flows from this ditch to piped culvert to Point D and into the River Forth. The section of 500mm diameter pipe has been used to repair a short section in the culvert just after the railway line and thus exerts a throttle to the larger culvert. The levels are summarised below:

· Inlet IL to 600 x 750mm stone culvert from development area, �A� = 7.51m

· Outlet IL of 600 x 750mm stone culvert from development area,�B� = 6.43m

· Twin 450mm pipe culvert inlet IL at Cornton Road, �C� = 6.06m

· Twin 450mm pipe culvert outlet IL at River Forth, �D� = 4.33m

· Grade of 600 x 750mm stone culvert = 1:113.75

· Grade of twin 450mm pipe culverts = 1:162.6

Figure 8.5.3. Western Outfall route

8.5.5 Using conservative Manning�s roughness of 0.02 and 0.015 for the stone culvert and 500mm pipe section respectively gives a current maximum conveyance estimated of 0.38 m3/sec (as shown in Figure 8.5.4) and that this flow rate is limited by the throttle behaviour of the 500mm pipe section. This demonstrates that the available conduit capacity is well in excess of requirements for discharge at sustainable discharge rates.

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Figure 8.5.4. Peak conveyance of the Western Outfall with current arrangement. Upper Plate

shows 500mm pipe section. Lower Plate shows flow in the pipe section using increasing flow rates to test maximum conveyance.

8.5.6 Using Manning�s roughness of 0.015 for the twin 450mm pipes yields a combined peak

flow rate of 0.46 m3/sec as shown in Figure 8.5.5 for a storm simulation for one of the pipes. As there is no tidal gate on this outfall to the River Forth this also represents the peak backflow via this route during high level events on the River Forth.

Figure 8.5.5. Peak conveyance for one of the 450mm pipes to the River Forth

Causewayhead Burn and Surface Water Sewer (C-SWS)

8.5.7 The Causewayhead Burn will receive flows directly from Drainage Catchments 3001 and 3002. Catchment 3001 will drain via the ECR Flood alleviation wetland and catchment 3002 will have its own dedicated SuDS system with outfall to the Causewayhead Burn.

8.5.8 In addition the C-SWS will receive flows from the ECR SuDS and the Duty/Emergency outfall from the Wetland Basin. For the ECR SuDS it is proposed to construct a

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replacement drainage run to the 675mm T-point at manhole S2, using a 500 mm pipe. This connection will service both as a secondary discharge route from the WB SuDS and the primary route for the ECR (limited to a maximum 54 l/sec at 0.5%AEPCC). The invert level for the culvert manhole on the site is at 6.160 m AOD and is therefore well below the level of the site boundary at this point. The flow behaviour of the C-SWS is discussed in further detail below.

8.6 C-SWS Capacity

8.6.1 Total pipe-full capacity is 0.45m3/sec for the 675mm culvert. The upstream CWHB S2 to S4 pipe run has a maximum flow capacity of 0.55m3/sec when surcharged. The Mean Annual Flood flow rate (Qbar) is 53.6 l/sec for the proposed ECR catchment and this is suggested as a peak permissible discharge rate from the ECR SuDS. This proposed peak flow is approximately 13% of pipe full capacity and 2.7 times greater than winter base flows of 20 l/sec (DWF). It is therefore concluded that the C-SWS can easily accommodate this proposed flow rate.

8.6.2 The outfall will discharge to the CSWS via a newly constructed 500mm surface water sewer, connecting with the existing 675mm diameter culvert at manhole S2.

8.6.3 Conduit capacity and flow is provided in Figure 8.6.1-8.6.2 below for the Lothian Crescent S2-S4 675mm pipe and the final run (S24-S25FLAP) before the Flap valve chamber to the River Forth outfall.

8.6.4 During the 3.33%AEP Baseline the S2-S4 node almost reaches capacity, whereas in the Future scenario there is capacity available due to the drainage proposal. The S2-S4 node reaches capacity in the 0.5%AEPCC event for both the Baseline and Future scenarios. The Causewayhead Burn outfall is the primary source of flow during this high capacity period between 5.5-8.5 hours. Flooding of the Easter Cornton Road is apparent in the Baseline but not in the Future scenario, (see Figure 8.6.3, below). This is due to the attenuation provided by the Causewayhead Flood Alleviation Wetland which limits flow rates to acceptable levels and prevents over-topping at the outlet to the C-SWS.

8.6.5 To examine the potential for the C-SWS to receive additional flows in the Future scenario, outfall from the Western Basin was also simulated to the C-SWS at a peak discharge of 168 l/sec at maximum operating head of 9.35 m AOD as per the Wetland Basin design. Therefore including the ECR flow rate of 54 l/sec, a total peak flow of 235 l/sec was routed to the C-SWS for capacity testing, see Figure 8.6.4.

8.6.6 The results demonstrate that the proposed drainage strategy increases network capacity due to attenuation within the SuDS features and that the network can accept the proposed discharges without causing flooding.

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8.7 ECR SuDS

8.7.1 The ECR SuDS concept was tested in the model to assess the feasibility of both providing drainage to the site and alleviating flooding issues on Easter Cornton Road, Figure 8.7.1 below demonstrates the flooding issue in the Baseline, where flows exit the site over the railway. The ECR is crudely represented in the model but is relatively consistent with the behaviour demonstrated in the detailed FP-M. Although flooding is not as severe during the 3.33%AEP event it still results in flooding across the railway line and similar inundation of the Easter Cornton Road.

Figure 8.7.1 Easter Cornton Road flooding during the 0.5%AEPCC A) Baseline and B) Future scenarios.

8.7.2 The assessment was carried out to examine storage volume requirements and depth of

the features. Figure 8.7.2 and 8.7.3 provide volume and depth analysis. Figure 8.7.4 provides evidence that the SuDS features can accommodate the full 0.5%AEPCC flow from the Causewayhead Burn should the outfall become blocked.

8.7.3 From this analysis it is concluded that a total attenuation volume of 5,000 m3 is required to mitigate flooding on the Easter Cornton Road and provide attenuation for the development at Qbar for the 0.5%AEP event. This volume provides additional protection for a Causewayhead Burn blockage scenario and has some additional freeboard even during this event.

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8.8 Tidal influence on the drainage system

8.8.1 As the proposed development and drainage strategy improves the current situation, the Baseline has not been covered in the following assessment. Rather the assessment focuses on issues relating to the prolonged periods of high discharge from the site and the flood risk that this could pose to the downstream network.

8.8.2 Within this assessment flood levels have been simulated at a static boundary condition on the River Forth. This is clearly exaggerating the influence of the River Forth as the peak level would only occur for a relatively short period of time, and that the combined probability of simultaneous events on the site and in the River Forth (i.e. 0.5%AEPCC Site and 0.5%AEP River Forth) are unlikely to occur on the same day.

8.8.3 Irrespective of the concurrence of events, this highly precautionary measure is taken to illustrate the differences between the baseline and future response of the outfalls to the proposed discharge of prolonged high flows to the C-SWS and Western Outfall.

8.8.4 The fluvial levels on the River Forth were examined:

· 3.33% AEP �- 10%AEP level in the River Forth (6.44m AOD)

· 3.33% AEP � 0.5%AEP level in the River Forth (8.43 mAOD)

· 0.5%CCAEP - 10%AEP level in the River (6.44m AOD)

· 0.5%CCAEP - 0.5%AEP level in the River (8.43 mAOD)

C-SWS outfall to the River Forth

8.8.5 The behaviour of this outfall is provided in the following section. The C-SWS has a flap valve which is designed to prevent back-flow to the drainage network. The network becomes locked as the head increases from the River Forth with flow out of the valve dependant on higher water levels in the upstream network. The tidal influence on the C-SWS from the River Forth is shown in Figure 8.8.1 - 8.8.4 below for the scenarios listed above in Section 8.8.4.

8.8.6 The study demonstrates that at the 10%AEP level on the River Forth, the C-SWS will still convey flows once the drainage system becomes surcharged and this flow limits flooding at manholes. No manhole flooding is predicted during this event.

8.8.7 When the River Forth is at the 0.5%AEP level, the system is completely locked and surcharge from manholes below 8.46m AOD is predicted. Subsequently, manholes in the lower reaches of the C-SWS will flood. As the flows received to the C-SWS are attenuated on the site it is considered that the potential flood risk is lower than the Baseline scenario during these events.

8.8.8 Although the proposal will improve the capacity issues in the C-SWS compared with the Baseline, an outfall control chamber will be included in the outfall arrangement which limits flows to the C-SWS during surcharging of the network. The proposed mechanical solution is detailed in Figure 8.9.4 and discussed for the Western Outfall in more detail in Section 8.9.4. This provision will further reduce the risk of surface water sewer flooding due to outfall from the site, blockage or tidal and fluvial inhibition by the River Forth of the C-SWS.

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Figure 8.8.1 C-SWS � Scenario 3.33%AEP on site and static 10%AEP level (6.44 mAOD) on the River Forth. A) Flow in link S24-S25FLAP B) C-SWS hydraulic profile shown at approximate peak.

Figure 8.8.2 C-SWS � Scenario 0.5%AEPCC on site and static 10%AEP level (6.44 mAOD) on the River Forth. A) Flow in link S24-S25FLAP B) C-SWS hydraulic profile shown at approximate peak.

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Figure 8.8.3 CC-SWS � Scenario 3.33%AEP on site and static 0.5%AEP level (8.43 mAOD) on the River Forth. A) Flow in link S24-S25FLAP B) C-SWS hydraulic profile shown at approximate peak.

Figure 8.8.4 CC-SWS � Scenario 0.5%AEPCC on site and static 0.5%AEP level (8.43 mAOD) on the River Forth. A) Flow in link S24-S25FLAP B) C-SWS hydraulic profile shown at approximate

peak.

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8.9 Western Outfall

8.9.1 The behaviour of the Western Outfall and the culvert to the River Forth was examined in response to the 0.5%AEP and 10%AEP levels. The results of the analysis are provided in Figures 8.9.1 - 8.9.4 below.

8.9.2 It should be noted that the flood levels are exaggerated and primarily used to examine the impact of the Western Outfall flows from the site. Absolute flood levels would require accurate modelling of the River Forth rather than using a fixed stage level.

8.9.3 At the 10%AEP level flooding is predicted to 8.06 mAOD in the Cornton Road ditch, with the 450mm pipe conveying flows once sufficient head has developed to overcome the River Forth levels. The 3.33%AEP shows no level difference between the flow and no-flow scenario. At the 0.5%AEPCC the level in the ditch increases by 100mm due to flows out of the site. Therefore flow from the site has the potential to contribute to flooding on the Cornton Road during high water levels on the River Forth which restrict flow from the Cornton ditch.

8.9.4 Due to this risk of flooding the outfall poses to the Cornton Prison area, it is proposed that a floating-arm valve (see Figure 8.9.5) will be used to prevent flow from the site during high level events. The mechanical flow control device initiates when levels in the ditch rise, shutting-off the valve and preventing flow out of the site. The floating arm would be designed to initiate at 6.51 m AOD, (soffit level of 450mm pipes to the River Forth), thus allowing flows to discharge up to the pipe full level but inhibit flows when levels in the Cornton ditch exceed this level and there is a risk of flooding issues developing.

8.9.5 At the 0.5%AEP the ditch floods to 8.44m AOD and flooding is predicted up to the pedestrian crossing at Cornton, see Figure 8.9.6. Significantly, this flood level inhibits flow out of the site as the level is the same as the base storage level of the Wetland Basin.

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Figure 8.9.1 Western Outfall. Scenario 3.3%AEPCC site and static 10%AEP level (6.43 m AOD) on the River Forth. Red line - Flow for 450mm pipe to the River Forth. Blue line - Water levels in the ditch adjacent to Cornton Road (0m depth = 6.06m AOD). Maximum ditch depth is 8.16 m AOD. Pink line - Flow from western outfall to the ditch via the 600mm x 750mm stone culvert.

Figure 8.9.2 Western Outfall. Scenario 0.5%AEPCC site and static 10%AEP level (6.43 m AOD) on the River Forth. Red line - Flow for 450mm pipe to the River Forth. Blue line - Water levels in the ditch adjacent to Cornton Road (0m depth = 6.06m AOD). Maximum ditch depth is 8.16 m AOD. Pink line - Flow from western outfall to the ditch via the 600mm x 750mm stone culvert

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Figure 8.9.3 Western Outfall. Scenario 3.33%AEPCC site and static 0.5%AEP level (8.44 m AOD) on the River Forth. Red line - Water levels in the ditch adjacent to Cornton Road (0m depth = 6.06m AOD). Maximum ditch depth is 8.46 m AOD. Blue line - Flow for 450mm pipe to the River Forth � note negative flow rates indicating backflow. Pink line - Flow from western outfall to the ditch via the 600mm x 750mm stone culvert route which is inhibited during this scenario

Figure 8.9.4 Western Outfall. Scenario 0.5%AEPCC site and static 0.5%AEP level (8.44 m AOD) on the River Forth. Red line - Water levels in the ditch adjacent to Cornton Road (0m depth = 6.06m AOD). Maximum ditch depth is 8.46 m AOD. Blue line - Flow for 450mm pipe to the River Forth � note negative flow rates indicating backflow. Pink line - Flow from western outfall to the ditch via the 600mm x 750mm stone culvert route which is inhibited during this scenario.

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Figure 8.9.5 Proposed outfall control mechanism for the Easter Cornton Road and Western

Outfall arrangement

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Figure 8.9.6 A) Western outfall to the and B) Cornton Road to the pedestrian railway line crossing illustrating the extent of flooding during the 0.5%AEP level on the River Forth

8.10 Conclusions

8.10.1 The following conclusions can be drawn from the feasibility assessment:

· The proposed outfalls can convey the proposed flows for the SuDS features

· Sufficient capacity exists in the C-SWS to receive flows limited to Qbar for all proposed catchments, including the duty/emergency discharge from the Wetland Basin

· The drainage strategy does not contribute to over-capacity issues in receiving outfalls and in fact reduces flows to the C-SWS through attenuation

· The strategy manages flood flows effectively on the Easter Cornton Road

· The C-SWS will become ineffective in events exceeding the 10%AEP on the River Forth. Manhole surcharge is likely but flooding and overspill from manholes is only predicted during extreme events in the lower section of the network. The development reduces flows to the C-SWS in comparison with the Baseline and therefore the risk of surcharge and overspill is lower in the Future scenario. To further reduce this risk it is proposed that an outfall control chamber will be installed in the upstream network to inhibit flows from the site when the C-SWS is surcharged.

· The proposed outfall discharge rates may increase risk downstream of the site during high level events on the River Forth, increasing flood levels by up to 100 mm during the 0.5%AEPCC event on site. The proposal includes a outfall control mechanism to inhibit

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flows during high water levels in the receiving ditch at the Western Outfall which negates this risk. In addition to this a similar arrangement is proposed at the Easter Cornton Road outfall to further reduce the risk of surface water sewer flooding during high level events on the River Forth.

9. Maintenance responsibility 9.1.1 Like all drainage systems, SuDS components should be inspected and maintained. This

ensures efficient operation and prevents failure. The proposed source, site and regional SuDS components are mainly on or near the surface and can be managed using landscape maintenance techniques.

9.1.2 The development allocation areas may also include below-ground SuDS such as permeable paving and potentially use of modular storage/treatment elements in roads, and car park areas to facilitate treatment. The manufacturer or designer will provide maintenance advice and should include routine and long-term actions that can be incorporated into a maintenance plan which would be developed for the site. The design process will consider the maintenance of the components (access, waste management etc.) including any corrective maintenance to repair defects or improve performance. A SuDS management plan for the maintenance of SuDS will be prepared upon detailed design of the full drainage network.

9.1.3 It is envisaged that private maintenance provision will be allowed to service SuDS in catchment 2 due to the scale of the Wetland Basin feature.

9.1.4 Catchment 1 and 3 SuDS (ECR and NE AND SE SuDS) will be designed and implemented to a standard that is acceptable for adoption by Scottish Water.

10. Site-specific drainage design considerations

10.1.1 The development regional SuDS feature is located within the floodplain. The design of the drainage is therefore likely to need to consider high ground water levels, downstream water level control during extreme events, and the possibility of inundation of the drainage infrastructure.

10.1.2 The design of SuDS with access to temporary or permanent water should consider public health and safety as well as issues associated with construction and operational management of the structures. Health and safety issues and risk mitigation features are presented in the CIRIA SuDS Manual.

10.1.3 Individual SuDS components should not be treated in isolation, but should be seen together as providing a suite of drainage features that are appropriate in different combinations for varying scales. It is always desirable to have a mix of SuDS components across the site as different components have different capacities for treatment of individual pollutants.

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11. SuDS Construction

11.1.1 SuDS are a combination of civil engineering structures and landscaping practice. Due to the limited experience of building SuDS in the water industry, there are a number of key issues which need to be particularly considered as their construction requires a change in approach to some standard construction practices.

· SuDS components should be constructed in line with either the manufacturer�s guidelines or best practice methods.

· The construction of SuDS usually only requires the use of fairly standard civil engineering construction and landscaping operations, such as excavation, filling, grading, top-soiling, seeding, planting etc. These operations are specified in various standard construction documents, such as the Civil Engineering Specification for the Water Industry (CESWI).

· During construction, any surfaces which are intended to enable infiltration must be protected from impaction. This includes protecting from heavy traffic or storage.

· Water contaminated with silt must not be allowed to enter a watercourse or drain as it can cause pollution. All parts of the drainage system must be protected from construction runoff to prevent silt clogging the system and causing pollution downstream. Measures to prevent this include soil stabilisation, early construction of sediment management basins, channelling run-off away from watercourses and surface water drains, and erosion prevention measures.

· After the end of the construction period and prior to handover to the site owner/operator:

- Subsoil that has been compacted during construction activities should be

broken up prior to the re-application of topsoil to garden areas and other areas of public open space to reinstate the natural infiltration performance of the ground;

- Any areas of the SuDS that have been compacted during construction but are intended to permit infiltration must be completely refurbished;

- Any blockages must be removed; - Any silt deposited during the construction must be completely removed; - Soils must be stabilised and protected from erosion whilst planting becomes

established.

11.1.2 Detailed guidance on the construction related issues for SuDS is available in the SuDS Manual and the associated Construction Site handbook (CIRIA, 2007).

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12. Recommendations

12.1.1 The following recommendations are provided:

· The design of the SuDS and flood alleviation system will require detailed design and further hydraulic modelling to determine the habitat design element of the Wetland Basin. This will take into consideration the groundwater, annual and seasonal patterns of rainfall, evaporation and infiltration.

· Further analysis of the flood behaviour of the site is recommended during later stages of planning to confirm the effectiveness of the proposed drainage strategy and ensure that the finalised development design does not pose a flood risk both on and off site.

· The drainage model requires linkage with the existing Fluvial and Pluvial model to demonstrate that flood flows and surface water will be effective in managing surface water and inform the design of the SuDS features.

· The Wetland Basin is currently proposed to have a storage volume of 75,000m3 according to the precautionary modelling exercise. From the preliminary modelling work conducted for this drainage strategy it would appear that this storage volume exceeds requirements.

Appendix D: Groundwater monitoring

Groundwater modelling on Airthrey Kerse � Technical note In addition to the studies conducted in 2012 (Rigare2012a&b), groundwater monitoring has been conducted since February 2014 and is on-going for the foreseeable future to inform design of the Wetland Basin. The locations of the monitoring locations are provided in Figure E1 with maximum and minimum levels for the recording period. The data collected from February � June 2014 is presented in Figure E2 below. The dip-wells show no surface water flooding during the recording period, in contrast with 2012 data which was a particularly wet during the summer months. Consequently, the soil has been less water logged and a lower water table is observed. The data reveals a potential north to south hydraulic gradient to the central basin. Groundwater behaviour is typical of a clay lithology with impeded drainage. An update is proposed in October 2014.

Figure E1 Location of dipwells and minimum and maximum levels (mAOD)

Figure E2 Groundwater levels for dipwells installed on Airthrey Kerse

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l fl

uc

tua

tio

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OD

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Groundwater Levels at installed dipwell locations

BH1 9.12 mAOD BH3 9.45mAOD BH4 9.18mAOD BH6 9.52mAOD BH2 9.36mAOD