Modeling reservoir sedimentation in the Agno watershed, Philippines

The exceptionally high sedimentation rate in the mountainous Agno River Watershed in the Philippines has affected dam operations on the Ambuklao and Binga reservoirs which were built in the late 1950s. In addition, sediment inflow scenarios have revealed that sedimentation will significantly reduce the total storage volume in the new San Roque facility which has been constructed downstream of those reservoirs. As such, watershed management plans will need to address conditions in the entire basin, not just the portion downstream of Binga Dam. Sediment will be deposited in the reservoir in the form of a delta front that will advance from the head of the reservoir towards the dam. Sedimentation in water reservoirs affects the utility to sustain power production, water supply and flood control objectives. It will likely be very difficult to reduce the sediment yield to any great degree by watershed restoration such as re-vegetation or tree planting. However, since sediment production from road-related slope failures appears to the main contributor to reservoir sedimentation, future developments in the basin related to road construction, mining activity and construction of new towns will need to adopt best management practices to avoid increased erosion or land disturbance. Empirical and analytic techniques were used in this study to assess sedimentation volumes and patterns, with particular emphasis on a GIS-based sediment yield model. The GIS model identified where sediment yield is greatest within the watershed, providing a means for developing sediment management and mitigation strategies that focus limited resources on key areas that give the highest rates of return. 25 refs., 3 tabs., 4 figs.