Japan Incorporates Climate
Change Effects into River andDam PlanningYamba Dam was filled up by the rain due to Typhoon Hagibis
in 2019 and saved the Tone River Basin, including part of theTokyo Metropolitan Area, from possible levee failures.
Concerted pre-release from dams on the Nakagawa River
was conducted on September 8, 2020. Nagayasuguchi Dam(this picture) and three other dams joined this operation.

Sakamachi, Hiroshima, just after the 2018
July West Japan Rainfall Disaster. This shows
the vulnerability of a river without dams
against debris from the upper reaches.

Photo by Nakagawa River Office, MLIT
Source:Geospatial Information Authority of Japan (GSI)

Japan Commission on Large Dams

Kyushu has suffered from a spate of severe floods. Damage due to the floods differed substantiallydepending on the availability of flood control dams. For example, in the Chikugo River Basin, tributaries
without major dams, such as the Akatani River in this picture, were severely damaged, while the Sada
River with Terauchi Dam suffered little damage in the 2017 Northern Kyushu Heavy Rain Disaster. In 2020,a flood in the Kuma River Basin, Kumamoto, Kyushu, caused 50 fatalities and 6,110 houses wereinundated. Kawabegawa Dam, which would have been under operation by the time of this flood if theoriginal plan had gone ahead, was reauthorized to be built after this disaster.

The Ministry of Land, Infrastructure, Transport and Tourism (MLIT) manages 109 trunk
rivers in Japan. In 2019, MLIT formalized an increase in the amount of rainfall by 10 to15% in flood control planning to cope with the effects of climate change in the future. In
2020, the Government also established a general rule to change, as far as it is effective,
the operation rules of reservoirs to allow for pre-release from dams before the arrival ofheavy rains by utilizing weather forecasts. In 2021, “Basin-wide Concerted Measures forFlood Risk Reduction Acts,” which consists of amendments of nine affiliated acts, wasenacted to tackle the increased rainfall due to climate change.

Japan Incorporates Climate Change
Effects into Dam and River Planning
MLIT employed two climate change models. One is a 4 ℃ global
warming model using the d4PDF (5 km downscaling) dataset
comprising simulated weather data assuming weather in around 2090,
along with the IPCC’s RCP8.5 scenario. The other is a 2 ℃ global
warming model using the d2PDF (5 km downscaling) dataset. It is thesame as d4PDF except that the assumed weather is in around 2040.
With each model, MLIT conducted simulation of rainfall for a duration
ranging from 360 to 5,400 years. MLIT also conducted a simulation
with current conditions (1951 to 2010) to obtain a baseline, which is
not the same as the IPCC’s. Considering uncertainty in the future,
MLIT set the 2 ℃ warming case to be the main case used for dam andriver planning while using the 4 ℃ warming case as a reference case forunmodifiable structures with long-range service duration, for example.
Table 1 shows the projected rate of increase of rainfall. Table 2 shows
how much the flowrate and frequency of severe floods will increase.
The significant increase in the flowrates makes it almost impossible toabsorb the changes within current dam and river planning. Therefore,
alternative methods are to be explored.

Table 1. Projected Increase Rate of Rainfall by Regions
2℃
4℃ Increase
CaseRegion
Increase
Short Range
Hokkaido
1.151.41.5
North Western Kyushu 1.1
1.4
1.5
Rest of Japan
1.1
1.2
1.3
1. “Short-range” corresponds to rainfall within a range of three to nine hours.
2. This table is not applicable to rainfall within a range of three hours.
3. This table is applicable to a catchment area of no less than 100 km²
although it can be applicable to a smaller catchment area by taking into
consideration the possibility of a higher rate of increase of rainfall.
4. This table is applicable to floods with no more than a 200-year return
period.

Table 2. Projected Increase of Flowrate andFrequency of Severe Floods
Climate Change Scenario Rainfall Flowrate Frequency
2℃ Increase
approx.1.1 approx. 1.2 approx. 2approx. 1.3 approx. 1.4 approx. 44℃ Increase
1. The rate of increase of flowrate is the average rate of changes inpeak flowrates of design floods, which correspond to floods with a100- to 200-year return period, at Class A rivers in Japan.
2. The rate of increase of frequency is the average rate of changes in
the probability of design flood level rainfall at Class A rivers in Japan.

Climate change seems to bealreadyaffecting
Japanhydrologically.
Improving
concerted pre-release from dams
with more sophisticated use ofweather forecasting, revision ofdesign discharge, and effective
sediment control for sustainable
maintenance of dams are someof the imminent challenges weneed to work hard to resolve.

Prof. Tetsuya SUMI

Ph.D. (Kyoto Univ.)
1985 Public Works Research Inst., MLIT
1998 Associate Prof., Kyoto Univ.
2009 Professor, Kyoto Univ.
2019 Vice-President Candidate at ICOLD
87th General Assembly at Ottawa
2020 Vice President, Japan Society of DamEngineers

Northern Hokkaido

Southern Hokkaido

Western Tohoku
San’inSetouchi
WesternChugoku
NorthWesternKyushu

Hokuriku

Eastern Tohoku

KantoChubu
Kinki
Southern Kii
Southern Shikoku
South-eastern Kyushu

OkinawaBasin-Wide Concerted Measures for Flood Risk Reduction
Japan’s response to intensified rainfall is “Basin-wide Concerted Measures for Flood Risk Reduction”(which MLIT refers to as “River Basin Disaster Resilience and Sustainability by All”). With this concept,
all stakeholders related to flood damage reduction are encouraged to work together to reduce flood
disaster risks.
Chisan - ForestThe Taskforce to Promote River Basin
Construction/Conservation
Sabo Works
Upgrade of Flood
Disaster Resilience and Sustainability byControl
Dams
Flood Water
Effective Use
All, which includes members from 16
Storage inof Non-Flood
Paddy Fields
Control Dams
ministries and agencies, was established at
Use ofAgricultural
the central government level in 2020. The
Reservoirs for
Urban Flood
Flood Control
Storage
Taskforce drew up the “Action Plan toMeasures against
Backwater Flooding Facilities
Promote River Basin Disaster Resilienceand Sustainability by All” in 2021. It
Relocation toPumping Station
Low Risk Areas
comprises various measures such as paddyRetarding Basins
field dams (floodwater storage in paddy
Dredging
High Risk
Flood Defense
fields), relocation to low-risk places,
Areafor Schools
Embankment &delineation of inundation areas by PMF
Reduction of Levee
Fragility(probable maximum flood), and flood
Drainage
Facilities
defense measures for key infrastructure as
Coastal Levees
well as traditional flood control measures
Image of Basin-wide Concerted Measures
such as dams, levees, and retarding basins.
For Flood Risk Reduction

Concerted Pre-release from Dams
A highlight of the basin-wide concerted measures for flood risk reduction is “concerted pre-release fromdams.” Under this scheme, various dam owners such as MLIT, MAFF, prefectures, power generationcompanies and irrigation organizations work together in each river basin to cope with heavy rains in thebasin. By lowering the water level of a dam reservoir further below the top of the conservation pool, ineffect, flood control capacity can be expanded or created. The Government has issued “Basic Policies forEnhancing Flood Control Function of Existing Dams.” Under this new policy, based on “flood controlagreements” among dam owners and river managers, the pre-release operation is conducted when anexpected amount of rainfall in the upper reaches of a dam reaches a threshold beyond which flooding islikely to cause damage downstream. MLIT calculates the expected rainfall with weather forecasts
supplied by the Japan Meteorological Agency (JMA). Larger estimates produced either by an 84-hour
range Global Spectral Model (GSM) or by a 39-hour range Meso-Scale Model (MSM) are used forcomparison with the threshold. MLIT has also formalized a compensation scheme in the event that prerelease operations happen to have adverse impacts on water supply and hydropower.

To see all

200 km
100 miDams Joining Concerted Pre-release Operation
Others
Source of the base map : Geospatial Information Authority of Japan (GSI)
For the details, see the notation at the map on the next page. The same notation applies to this map, too.

Dams Joining Concerted Pre-release Operations Based on Flood Control Agreements

Welcome for your
Virtual Visit toDams in Japan

LegendRockfill
Gravity
EarthOthers

Dam Maps
200 km
100 mi

JCOLDFootages

Publications

200 km
100 mi200 km
100 miTechnologies

Source of the base map : Geospatial Information Authority of Japan (GSI)（https://maps.gsi.go.jp/#5/37.932645/138.251953/&base=english&ls=english&disp=1&vs=c1g1j0h0k0l0u0t0z0r0s0m0f1）
Japan And Its Surroundings (Zoom Level 5-8), 1:1,000,000 INTERNATIONAL MAP (Zoom Level 9-11)
The bathymetric contours are derived from those contained within the GEBCO Digital Atlas, published by the BODC on behalf of IOC and IHO (2003) (https://www.gebco.net)
海上保安庁許可第292502号（水路業務法第25条に基づく類似刊行物）
Shoreline data is derived from: United States. National Imagery and Mapping Agency. "Vector Map Level 0 (VMAP0)." Bethesda, MD: Denver, CO: The Agency; USGS Information Services, 1997.
Location of the dams on the map is basically derived from MLIT (https://nlftp.mlit.go.jp/ksj/gml/datalist/KsjTmplt-W01.html). Modification and addition of data are conducted by JCOLD andCampo Salado. QR codes on this map are added by JCOLD. Web design and programs are powered by Campo Salado.