●Research Trends and Results

Evaluation of dam flood control using rainfall prediction
Water Management and Dam Division, River Department
TORII Kenichi, HeadINOMATA Hironori, Researcher(Key words) rainfall prediction, dam flood control operation

2. Evaluation by anticipating the inundation
damageFig.1 shows a comparison of the inundation
damages for several floods with different scales. Eachflood was controlled by the two types of flood control
operation. The one operation (hereinafter operation A)
is the conventional regular flood control operation andthe other (hereinafter operation B) is the operation
which sets the outflow discharge smaller than that ofoperation A by 100 m3/s. Those experimental floods
were obtained by multiplying several scale factors to acertain hyetgraph. In a case where the scale factor issmall, the inundation damage with operation B awhich stores large volume of flood water is smaller
than that of operation A. On the contrary, theinundation damage by operation B becomes larger
than that of operation A in the case where the scale
factor is large because operation B causes the flood
control capacity of the dam to be full earlier thanoperation A. As a whole, operation B is superior tooperation A on the condition that the scale factor issmaller than 1.7 and operation A is conversely
superior to operation B on the condition that the scalefactor is larger than 1.7. This relationship appears
when a rainfall prediction is referred to. In particular,
if the scale factor is replaced with the error of therainfall prediction,, operation B is superior on thecondition that the actual rainfall is smaller than 1.7
times the rainfall prediction and vice versa. This

relation is one of the reasons that rainfall predictions
cannot be practically applied to dam flood control
operation. The expectation of inundation damage is
proposed in this research to comprehensively evaluate
dam flood control effect while considering the error ofrainfall prediction. It is expressed as the integral of themultiplication of the probability on the error of rainfall
prediction and inundation damage (Eq.1).
350
Inundation damage

300(100 million yen)
浸水被害額（億円）

1. Introduction
Dam flood control operation using rainfall
predictions has been suggested as one way to improve
the flood control functions of existing dams. It can
optimally and efficiently use the flood control
capacities of dams. However, the result of flood
control operation based on rainfall predictions may beeven worse than that by conventional regular flood
control operation which does not use rainfall
prediction when the actual rainfall greatly exceeds theprediction. Therefore, rainfall prediction has not been
quantitatively utilized in practical dam flood control
operation . This report aims to describe the method
which evaluates merits and demerits of dam flood
control operation based on rainfall prediction byconsidering rainfall prediction errors.

Operation A is superior.
操作Bが有利 操作Aが有利

Operation B is superior.

250

OperationA: Regulation operation
操作A: 規則操作

200Operation
B: Regulation operation
操作B:
規則操作‐100m3/s
3-100m /s

150100500
0.00.51.01.5
Scalefactor
降雨倍率

2.02.5

Fig.1 Scale factor change on inundation damageE   Pr   C r drEq.1

Here, r is the error rate of rainfall prediction definedby actual rainfall/rainfall prediction, P(r) is theprobability density function on r and C(r) is thefunction of the inundation damage on r.
The expected inundation damage is estimated byintroducing the probability of error rate on rainfall
prediction to the result of Fig.1 and those values are
1.26 and 4.6 billion yen for operation A and B
respectively. This means that operation B is superior
to operation A as the expectation of inundation
damage. However, it is essential to be aware that there
is little possibility that flood control by operation B
might result in higher inundation damage than that by
operation A.
3. Conclusion
The practical application of the proposed method
will be studied by analyzing the rational method ofsetting the probability density function P(r) based on
accurate evaluation of a rainfall prediction and theimpact of P(r) and C(r) on the estimated expectationof inundation damage.