To mitigate flood damage due to a recent increase in the frequency and magnitude of heavy rainfall events, the Kamihayashi district in Niigata prefecture, Japan, has undertaken flood mitigation measures using paddy fields by installing runoff control devices in drainage boxes of paddy field plots. The purpose of this study is to evaluate the flood mitigation performance of the Paddy Field Dam project in terms of a decrease in discharge volume, drop in channel water level and reduction of inundation damage using combined hydrologic analyses and flood routing. The model constructed for runoff analysis is composed of three modules: a hilly/residential area module in which the overland flow is estimated using the kinematic wave method, a paddy field module in which runoff from paddy fields is calculated using water balance analysis, and a channel network module in which flood routing is performed using a one-dimensional unsteady flow model. The outputs of the first two modules are the input of the third module. The result of the simulation shows the main channel discharge decreased by 26% and the water level dropped by 0.17m in the case of the largest observed rainfall event. The simulated effect was larger for larger rainfall events. In terms of flood water volume, the runoff control devices have the effect of reducing the flood damage due to the 50-year return period rainfall event to almost that due to the 10-year return period rainfall event.
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