Ambuklao hydroelectric scheme ― sedimentation of the reservoir and rehabilitation program

摘要

From the beginning of its operation, the Ambuklao scheme, located in the Benguet Province in the north of Luzon Island in the Philippines, suffered a rapid depletion of the reservoir active storage due to the considerable quantity of sediments transported by the various tributaries that flow into the reservoir. The scheme was severely affected by the 1990 Baguio earthquake of exceptional magnitude that triggered the movement of sediments accumulated in the reservoir towards the dam and the intake tower. The intake was submerged by a few meters of sediment leading to the immediate shut down of the plant. Although the plant generation has been restored some time after the event, the difficulty experienced in dredging the intake area and the impossibility to close the intake prevented a proper rehabilitation of the waterways and turbines, which continued to be operated at partial capacity from 1995 until the complete shut down of the plant in September 1999. All possible solution to delay the filling of the reservoir by sediments including flushing and dredging of sediments have shown to be unworkable, environmentally unfriendly or economically unattractive. Sustainable solutions for the rehabilitation of the scheme will therefore have to accept that the filling of the reservoir by sediments is unavoidable, and that the operation mode of the scheme will evolve towards that of a run of river plant. The rehabilitation works, and particularly the new intake structure, have been designed accordingly.