140. Reservoir Sedimentation Management at the Dashidaira and Asahi Dams

This paper introduces two cases of sedimentation management in reservoirs; one is for sediment flushing at the Dashidaira Dam, the other for sediment bypassing at the Asahi Dam. Sediment flushing at Dashidaira has been performed since 1991 in order to prevent the rise of the upstream riverbed, the erosion of the downstream river channel and seashore area and, of course, to ensure the proper storage capacity. A wide range of environmental monitoring has been undertaken to evaluate impacts downstream, the results of which have been used for improving the method of sediment flushing, and are periodically announced through an evaluation committee comprised of experts and related parties. Any particular environmental problems have not been found since severe environmental monitoring was introduced. A sediment by pass system at Asahi, which is comprised of a 13.5 m high diversion weir and a 2350 m long bypass tunnel with the maximum discharge of 140m³/s, was installed to mitigate both prolonged turbidity and sedimentation problems. The bypass system has showed a good performance for the above problems.