Impact on downstream environment of sediment flushing at the Dashidaira Dam in Kurobe River, Japan

Sediment flushing is vital to secure storage capacity in dam reservoirs. The Kurobe River is one of the steepest rivers in Japan, with a basin of 682 km2, 85 km in length, a gradient of 1/5 to 1/100, and extremely high precipitation. The Dashidaira Dam, located in the middle of the river, fulfills a role to control river flow downstream when the power plant, which is related to the Kurobe Dam, located in upmost stream (total storage capacity: approximately 200 million m3), increases or decreases power generation according to power demand. Therefore, securing the storage capacity of the Dashidaira Dam (total storage capacity: approximately 9 million m3) is extremely important to make the most of the upstream power plants effectively. This paper reports the long-term impact of flushing at the Dashidaira Dam on the downstream environment. Although the first flushing at the Dashidaira Dam in December 1991 caused environmental problems, flushing has been resumed since 1995 in cooperation with relevant parties, including academic experts and local government officials, etc., while studying and surveying the impact on the downstream environment. Since 2001, sand removal has also been conducted in cooperation with the Unazuki Dam (completed in 2001). Consequently, since the resumption of the flushing, it has been evaluated based on the survey results that although the environment would be impacted temporarily, a significant impact should not have occurred on the downstream environment of the river and river mouth. Additionally, the reservoir sedimentation rate in the Dashidaira Dam has become constant, although it had increased by more than 50 % in approximately 10 years since its construction completion (1985). In addition, the flushing caused the expansion of the beach toward offshore by approximately 50 m, compared to the shoreline in 1998.