112. Sediment Dredging: Unplugging Ikawa Dam's Bottom Outlets

When sediment threatened to clog the bottom outlets at the 62 MW Ikawa Dam project in Japan, Chubu Electric Power co. undertook a massive, first-of-its-kind dredging operation. Utility engineers overcame challenges such as mapping the area, precisely controlling the dredger, protecting downstream water quality, and disposing of the sediment. During the 40 years after the 1957 completion of the 62 MW Ikawa Dam hydro-electric project in central Japan, the effective capacity of the reservoir decreased by about 20 percent (22 MCM), owing to sediment inflow. Most of the sediment accumulated at the base of the dam. Engineers at Chubu Electric Power Co., the dam's owner and operator, monitored the gradual rise of the sediment pile. For many years, the sediment did not constitute an operational problem. However, by February 1997, the sediment level had risen high enough to cover the structure's sediment discharge outlet and water supply outlet, each screened at the inlet and containing two hydraulic high-pressure control gates. Both outlets are located about 20 m below the water intake for the powerhouse. It became apparent that the sediment soon would hamper the operation of the two outlets, and, therefore, alter the effectiveness of the project. Chubu Electric's alternatives for removing the sediment were limited. Ikawa Dam, a hollow gravity concrete dam, provides hydroelectric power for central Japan within a scheme of other hydropower projects located upstream and downstream. Moreover, plants downstream use water from Ikawa Dam for multiple purposes – storage, irrigation, recreation, and power. As a result, utility engineers did not have the option of significantly lowering the water level in the reservoir to remove the sediment in a land-based approach. The utility's only viable alternative involved dredging the sediment from the base of the dam using equipment floating on the surface of the reservoir. Chubu Electric considers the sediment dredging project at Ikawa Dam – involving extensive dredging and sediment disposal directly upstream of a large dam – to be the first of its kind in Japan, and that few similar operations have been done anywhere in the world.