٨Research Trends and Results

Using rainfall predictions toimprove dam flood control
methods
MITSUISHI Shinya, HeadOZEKI Toshihisa, Researcher
Water Management and Dam Division, River Department

Flowrate
ᵹ㊂

Flowrateᵹ㊂Inflow
ᵹ౉㊂
quantity

Inflow
ᵹ౉㊂
quantity

Quantity
discharged䉎ᠲ૞ⷙೣ䈮䉋
under operating rules

᡼ᵹ㊂

Increaseof dam’s 䉍੐೨᡼ᵹ䈮䉋
freecapacityኈ㊂by䉻䊛ⓨ䈐
priordischarge䉕 Ⴧᄢ䈢䈣䈚 theᦠ䈐ᠲ૞䉕
Avoiding
normaloperatingmethodto࿁ㆱ䈚 䇮ᦨᄢ᡼ᵹ
reducemaximum㊂䉕 ૐᷫdischarge

Quantityᧄ⎇ⓥ䈮dischargedfor this䉋 䉎 ᡼ᵹ㊂
research
Flowrateto start floodᵩ᳓⺞▵㐿ᆎᵹ㊂
control

Quantity
discharged䉎ᠲ૞ⷙೣ䈮䉋
underoperating rules
᡼ᵹ㊂

Flow
rate to startᵩ᳓⺞▵floodcontrol㐿ᆎᵹ㊂Quantity discharged for thisᧄ⎇ⓥ䈮䉋 䉎 ᡼ᵹ㊂researchFigure 1. Comparison of Flood Control EffectivenessᤨೞTimeSuper large flood㪌㪇㪇㪇㪋㪌㪇㪇ታ❣ᵹၞMeasuredactualdrainage basin
average rainfall (mm)ᐔဋ㔎㊂㪲㫄㫄㪴
3 䊮䉲䊚䊠䊧䊷䉲Simulation
discharge (m䊢/s)᡼ᵹ㊂㪲㫄㪊㪆㫊㪴ታ❣䉻䊛quantity (m3/s)
Actualdam inflow
ᵹ౉㊂㪲㫄㪊㪆㫊㪴
Actualdam dischargeታ❣䉻䊛quantity(m3/s)
 ᡼ᵹ㊂㪲㫄㪊㪆㫊㪴㪋㪇㪇㪇㪊㪌㪇㪇㪊㪇 㪇㪇㪉㪌 㪇㪇㪉㪇㪇㪇㪈㪌㪇㪇㪈㪇㪇㪇ᤨೞTime

Medium-small floodDesign maximumdischarge⸘↹ᦨᄢ᡼ᵹ㊂㪈㪊㪇㪇㩷㪲㫄㪊㪆㫊㪴 ήኂ᡼ᵹ㊂Harmless discharge㪌㪇㪇㪇
Sept./10㪐㪆 㪈㪇㪋㪇㪍㪇㪏㪇㪈㪇㪇㪈㪉㪇㪈㪋㪇㪈㪍㪇㪏㪇㪇㩷㪲㫄㪊㪆㫊㪴

Sept./11㪐㪆㪈㪈㪇㪉㪇

Rainfall (mm/hr)
㔎㊂㪲㫄㫄㪆㪿㫉㪴
1. Introduction
It has been pointed out that under the effects ofglobal warming, it is highly probable that extreme
rainfall phenomena will continue to occur withincreasing frequency. On the other hand, the Ministryof Land, Infrastructure, Transport and Tourism has
been constructing fewer dams in Japan in recent years,
so it must enhance the functions of its existing stock ofdams in the future.
This research project was undertaken in order tominimize flood damage downstream from dams by
predicting rainfall with high spatial resolution suitableto manage dams in dam catchment areas wherelarge-scale flooding has occurred in the past, by
studying a more effective flood control method (Fig.
1) which maximizes the full capacity of dams, and by
verifying the applicability of this new method under
actual floods including super large floods. 
2. Rainfall prediction and rational dam operating
methods Rainfall was predicted by performing predictioncalculations in 2km meshes through down-scaling by ameteorological model developed and released in theUnited States, and based on predictions by theMeteorological Agency (20km meshes) at the time ofactual floods with reference to the occurrence of local
torrential rainfall and the dam’s catchment basin area.
The predicted inflow was obtained by calculating theinitial loss in the area upstream from the dam, theprimary runoff, and rainfall loss based on themaximum underground water holding capacity and its
reduction curve, then finding the effective rainfall byreducing the hourly rainfall by the rainfall loss, andthen obtaining the product of this value and thecatchment area.
The total inflow during the predicted period which
has been calculated and the dam’s free capacity at thetime of the prediction were compared to perform aflood control simulation which performs operating
judgments every hour.

Flowrate (m3㫊㪴/s)
ᵹ㊂㪲㫄㪊㪆

Research Trends and Results(Key words) Rainfall prediction, flood control, prior discharge 

Sept./12㪐㪆㪈㪉

Sept./13㪐㪆㪈㪊㪈㪏㪇㪉㪇㪇
Sept./14
㪐㪆㪈㪋Figure 2. Results of Simulation of the Flood of
September 2000 at the Yahagi Dam
3. Simulation results
Even when a super large flood has occurred, as
shown in Figure 2., prompt adjustment operations
sharply reduce the maximum discharge more thanwhen normal operation based on the operating rules isperformed, and it is possible to restrict inundation
damage by many floods. And damage to downstream
regions by many medium and small floods can beprevented by holding the discharge below theharmless discharge level. 
4. Conclusion
While flood control effectiveness can be enhanced
by using rainfall predictions, prediction errors may
endanger flood control or water use. In the future, theDivision will conduct research to accurately clarify
characteristics including errors in rainfall predictions
and reflect them in flood control operation judgments
in order to minimize these risks. [Sources]
1) S. Mitsuishi, T. Sumi, T. Ozeki: Dam Operations
Using Precipitation Forecast by Weather Research andForecasting Model, Journal of Japan Society of Dam
Engineers, 20(2), p94-104, 20102) S. Mitsuishi, T. Ozeki, T. Sumi: Applicability of aNew Flood Control Method utilizing Rainfall
Prediction by WRF, Journal of Japan Society ofHydrology and Water Resources, Vol. 24, No. 2,
p21-31, 2011HP㧦http://www.nilim.go.jp/lab/fdg/page/ronbunn_1.htm

٨Research Trends and Results