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

PrecipitationPrecipitation

Rainy season Typhoons

Floods and debris flowFloods and debris flow

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

River Restorationin Japan

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)

Restoration (rehabilitation) Restoration (rehabilitation) projectsprojects

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

Lakeshore restoration

July 9, 2002

Restoration of locally extinct aquatic plants by soil seed bank

Academic activities…Academic activities…

Aqua Restoration Research CenterAqua Restoration Research Center

Three 800 m experimental rivers

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

Gravel-bed restoration in the Tama River

Location Location

10km

Tokyo

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.

Problems in the Tama River?Problems in the Tama River?

River incisionRiver incision

Gravel mining (stopped in 1967)Dams and weirs

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)

Studies on restoration projects

→ 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-

Lessons from the Tama River

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

Thank you for your attention !

Nishizawa Valley in Yamanashi Pref.