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Hokkaido Island
Honshu Island
Kyushu Island
Shikoku Island
TOKYO (Tama River)
Okinawa
River Restoration in Japan: Lessons from the Tama River
Keigo Nakamura, Ph.DNILIM, MLIT
TSUKUBA (PWRI)
ContentsContents
� Japanese nature and rivers� River restoration in Japan� River restoration in the Tama River
� Japan’s total area: 377,815 km 2 (Ger.357,000km 2)� Total Population: 127,740,000 (2006)� Mountains (Forest) account for about 71% of
Japan’s total land area.� The islands of Japan lie in the temperate zone and
northeast end of the monsoon area . � The annual average rainfall is 1,700 mm/year .� Earthquakes and Volcanoes
General Characteristics of JapanGeneral Characteristics of Japan
Joganji River
Abe River Shinano River
Seine River
Kitakami River Mekong River
Distance from river mouth (km)
1,200km1,000800600400200
Roire River
Colorado River
Rhine River
Joganji River
Abe River
Chikugo River
Yoshino River
Tone River
00
Elevation (m)
1,000
800
600
400
200
Longitudinally steepHigh Sediment yield
Topography
Geography of RiversGeography of Rivers
75% of Japanese Assets on the 75% of Japanese Assets on the FloodplainFloodplain
FP
FP
FPArea Population Assets
10
50 75
Anthropogenic Impacts on Anthropogenic Impacts on Rivers are enormous Rivers are enormous (Yoshimura. et al)(Yoshimura. et al)
� 23.5 % of river banks are artificial
� 2,675 dams (>15 m) have been constructed
Anthropogenic Impacts
http://www.hakkenden.net/hokuriku/a-toyama/anabatoyama.htm
Not only safe, but also natureNot only safe, but also nature
1
10
100
1000
10000
1945 1955 1965 1975 1985 1995Nos
. of
vict
imes
and
res
tora
tion
wor
ks
Nos. of victims due to flood, sediment,and volcanic disasterNos. of Nature-oriented river works
� Number of victims due to flood has decreased in the last 50 years.
� River restoration increases since 1990
Victims of flood
Restoration
History of River Environment History of River Environment � ~1960s economic growth and pollution� 1970s countermeasure for Pollution
� :Basic Law for Environmental Pollution Control was enacted after Minamata disease, Yokkaichi asthma, etc.
� 1980s Dawn for river environment� 1990
�Nature-oriented river works
River Restoration is rapidly River Restoration is rapidly increasing since 1990.increasing since 1990.
0
10000
20000
30000
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001A
ccum
ulat
ive
num
ber
of r
iver
res
tora
tion
proj
ects
Over 28,000 projects
(~37,000 in the US)
Agenda for river restorationAgenda for river restoration
� Balance flood control and conservation� Loss of wetland in watershed (60%)� Stable flow regime, loss of dynamics� Loss of gravel river bed (sediment yield)� Loss of Ecological network (e.g. fishway,
reconnection with paddy fields)
Early restoration work in urban Early restoration work in urban area (the Itachi River, 1982)area (the Itachi River, 1982)
Photos: Shinichi Yoshimura
Restoration of Backwater,Restoration of Backwater,Restoration of partial floodplain Restoration of partial floodplain functionfunction
Photo: MLIT, Japan
Design with Nature: Design with Nature: Restoration in active riverRestoration in active river
Restoration using channel narrowing process
Restoration of network between Restoration of network between paddy field and streampaddy field and stream
Paddy field plays important role for aquatic ecosystem in Japan.
Before After
Restoration of flow regime by the Restoration of flow regime by the operation of dam reservoirsoperation of dam reservoirs
River Ecology Research GroupRiver Ecology Research Group� Interdisciplinary association of ecologists,
civil engineers, and river managers founded by MLIT (river authority) in 1995
� Study at six rivers: Tama, Chikuma, Kizu, Kita, Shibetsu, and Iwaki rivers
The Society of Ecology and Civil The Society of Ecology and Civil engineeringengineering� Academic society established in 1997 to
support applied and practical studies� Journal “Ecology and Civil Engineering”
Keynote in 2006
Outline of the Tama River Outline of the Tama River
� Length: 136km� Basin area: 1,249 km2
� Source: Mt. Kasatori, 1,941 m� Population: 4,400,000� Visitors: 22,000,000 per year� Over 200 NGO groups
Tama River is a top runner for Tama River is a top runner for River Environment in JapanRiver Environment in Japan
� NGO for the Tama River environment was founded in 1970.
� River env. division was established in the Tama River authority in 1975.
� Tama Riv. Env. Management Plan was published in 1980.
River Ecology Research started River Ecology Research started at the at the Nagata areaNagata area in 1995.in 1995.
� Slope: 1/330� Sediment: 35 mm� Q=2m3/s� MHQ=620 m3/s� gravel-bed river on
the alluvial fan
Photo: Keihin river office
Nagata area
Research Group stimulated the Research Group stimulated the restoration project.restoration project.� Research group started meeting with citizens
since 1997.� People realized the necessity of restoring
gravel-bed river though the meeting.� Gravel-bed river restoration also fitted the needs
of river authorities for flood control.� Discussion has started for the restoration in
1999.
1997
1974
Encroachment of invasive Encroachment of invasive vegetation (Robinia pseudoacacia )vegetation (Robinia pseudoacacia )
Copyright Prof. Oohori
Eusphingonotus japonicus Floodplain grasshopper
GravelGravel--bed species declinebed species decline
Phragmites Japonica
Robinia pseudo-acacia
Aster Kantoensis
Restoration measuresRestoration measures
� Channel widening (Aufweitungen)� Removal of Forest of Robinia
pseudoacacia (invasive)� Sediment augmentation (sediment from
upstream weir)
→ Channel widening is necessary for restoration to reduce the tractive force.
No recovery with the incised channel
Irreversible impact of gravel mining Irreversible impact of gravel mining at the incised reachat the incised reach
110
120
130
140
150
160
50 52 54 56 58 60 62
Ave
rage
bed
ele
vati
on
(m)
Longitudinal distance (km)
Time interval between lines: 96 years
48 years624 years
Hamura weir
Ozaku weir
Kabe districtNagatadistrict Narrow
section
Groundsill
Riverbed elevation in 1923
(Hattori et al. 2003)
Sediment fills only upstream of Nagata area
110
111
112
113
114
115
116
0 50 100 150
Ele
vatio
n (m
)
Distance From the Starting Point on Left Levee(m)
Bar
Water Level at Average Annual Maximum Discharge
BeforeExcavation
Transversal distance (m)
0.02
0.04
0.06
0.08
0.1
0.12
0.14
2 4 6 8 10
Before Excavation
Shi
elds
Num
ber
at th
e T
op o
f B
ar
Occurrence Probability (year)
Critical Value of Common Reed Destruction
Sediment supply is necessary to sustain gravel bed
Sediment supply to restore Sediment supply to restore the fluvial systemthe fluvial system
Return period (year)
110
111
112
113
114
115
116
0 50 100 150
BeforeExcavation
Ele
vatio
n (m
)
Distance From the Starting Point on Left Levee(m)
Just AfterExcavation
Excavated AreaBar
Water Level at Average Annual Maximum Discharge
Transversal distance (m)
0.02
0.04
0.06
0.08
0.1
0.12
0.14
2 4 6 8 10
Before Excavation
Just After Excavation
Channel Widening
Shi
elds
Num
ber
at th
e T
op o
f B
ar
Occurrence Probability (year)
Critical Value of Common Reed Destruction
Shi
elds
num
ber
at th
e to
p of
bar
110
111
112
113
114
115
116
0 50 100 150
Ele
vatio
n (m
)
Distance From the Starting Point on Left Levee(m)
Just AfterExcavation
Excavated AreaBar
Water Level at Average Annual Maximum Discharge
Excavation + Increase of Riverbed Slope
Riverbed Slope Changes1/350→1/250
Transversal distance (m)0.02
0.04
0.06
0.08
0.1
0.12
0.14
2 4 6 8 10
Before ExcavationJust After ExcavationExcavation + Incarease of Riverbed Slope
Shi
elds
Num
ber
at th
e T
op o
f B
ar
Occurrence Probability (year)
Riverbed Slope Changes1/350→1/250
Critical Value of Common Reed Destruction
Shi
elds
num
ber
at th
e to
p of
bar
Return period (year)
(Hattori et al. 2003)
Sediment augmentation volumeSediment augmentation volume
� Sediment volume was estimated 5,500 m3/year to maintain gravel bed by calculation and large scale experiment.
River wideningVegetation removal(~21,000 m2)
A工区
B工区
D工区E工区
C工区
Sediment supply1.7 km upstream
400m
Outline of the restorationOutline of the restoration
Enlarged gravel river bedEnlarged gravel river bed
2001 Sep 20 2002 Jul 5 2003 Aug 25
2004 Aug 4 2006 Jul 282005 Jul 22
Photos: Fukushima M, PWRI
Results: GravelResults: Gravel--bed restorationbed restoration� Increased the number of plover
(Charadrius placidus), floodplain grasshopper, gravel-bed plants (e.g. Aster Kantoensis), benthic fishes.
� Mitigation of river incision
DecisionDecision --making processesmaking processesInterdisciplinaryresearch group
Citizen
Tama RiverAuthority
Committee forrestoration
Tama RiverAuthority
Elicitation
Demand
Decision
Strategy
Implementations
meeting, seminar
consultation
1995-
1997-
1999-
2001-
Interdisciplinary research group Interdisciplinary research group including river authorities are of including river authorities are of great importance for river great importance for river restoration. restoration.
Researchers Practitioners
Scientist should explain their Scientist should explain their research results to local people.research results to local people.
� Scientific and accurate information gives great effect for decision making process.
� Importance of sharing information and enlightenment
Scientist should give quantitative Scientist should give quantitative estimation as much as possible.estimation as much as possible.
� Gravel augmentation and that volume was decided by large scale experiment.
� River widening was decided based on the calculation.
Scientist should publish the Scientist should publish the results to multiple readers.results to multiple readers.
Books for citizens Reports for specialists