Environmental changes downstream of dams after sediment sluicing in the Mimikawa River system

Triggered by the extensive flood damage in the Mimikawa River system due to Typhoon Nabi in 2005, Miyazaki Prefecture, the river administrator, played a leading role in establishing the Integrated Sediment Flow Management Plan. As part of this scheme, Kyushu Electric Power Co., Inc., KEPCO, created a plan to mitigate the flood risk on the upstream of three dams, the Yamasubaru, Saigou, and Oouchibaru Dams, by sediment sluicing. Under this plan, lowering the water level of the dam reservoirs in case of flooding due to typhoons allows the reservoir to be close to the state of original river level before dam construction. Through this operation, the sediment into the dam reservoirs from the upstream is delivered directly to the downstream, thereby mitigating flooding risk in the upstream and improving the level of river safety. Whereas this operation benefits river safety, it is very likely that it dramatically alters the river environment, including riverbed material and habitat. For this reason, KEPCO has been implementing various investigations and analyzing the effects of sediment sluicing. Sediment sluicing operations have been conducted at the Saigou and Oouchibaru Dams since 2017, then since 2021, starting the operation at the Yamasubaru Dam, all three dams’ operations have been conducted. This paper focuses on the environmental changes downstream of those dams and describes the results and assessments of sediment sluicing that has been conducted so far. Below is the summary of the results: 1) gravel-sized materials are observed downstream of the dams, which has not been the case before the operation of the dam sluicing, 2) at several areas, diversity of riverbed materials and habitat has improved, and 3) at several areas, correlations among riverbed materials, habitat, and natural lives, such as fish and benthos, are recognized.