Optimized Reservoir Operation for Flood Control under Extreme Events Based on Rainfall-Runoff-Inundation Analysis in the Upper Katsura River Basin, Japan

Reservoirs play a significant role in flood management in the river basins by storing flood water coming to the reservoir from the upstream as flood control operation so that the river water level decreases to the safe level with no inundation in the downstream during flood events. However, a reservoir can no longer store as much water as required to prevent from inundation in the downstream in extreme flood events beyond the design level where flood water to be stored in the reservoirs exceeds its storage capacity for flood control. Once water is stored to the full volume of the reservoir before the flood event ends, it can no longer control the flood and has to release the same amount of water as its inflow even around the inflow peak of that event, which can cause a great damage due to large-scale inundation in the downstream. Therefore, flood control operation under extreme flood events has to be carefully designed so that the reservoir can use its finite storage capacity for flood control in the most effective manner.

In order to develop a science-based method to design the optimal flood control operation of reservoirs during extreme flood events that may increase under the changing climate, a method is proposed for optimization of reservoir’s flood control operation based on inundation analysis in the downstream. At first, rainfall scenarios are developed for extreme flood events. Various patterns of rainfall scenarios with different scale and spatio-temporal distribution are generated considering stochastic characteristics and classified into representative scenarios that cause different patterns of runoff. River discharge, inundation area and depth in the target river basin are then estimated for the rainfall scenarios using RRI Model, a rainfall-runoff-inundation model developed by Sayama et al. (2010). Reservoir operation for flood control is optimized to minimize the expected flood damage due to inundation, which can be estimated from the inundation analysis of the considered extreme flood scenarios.

The proposed method was applied to the Hiyoshi Reservoir in the Upper Katsura River in Japan, and optimal flood control strategy of the reservoir was respectively identified for each scenario of extreme flood event. The results demonstrated that operation of the reservoir for flood control identified by the proposed model could successfully mitigate the flood damage under the extreme flood conditions allowing the minimal area of inundation by controlling flood water in the reservoir around the peak time of river discharge.