Upstream Sediment‐Control Dams: Five Decades of Experience in the Rapidly Eroding Dahan River Basin, Taiwan

Abstract

Abstract Sedimentation is emerging as a key issue in sustainable reservoir management. One approach to controlling reservoir sedimentation is to trap sediment in hydraulic structures upstream of the reservoir. In the 1,163‐km 2 catchment of the D ahan R iver ( T aiwan) over 120 “ sabo ” dams were built to reduce sediment yield to S hihmen Reservoir. Built in 1963 for water supply, S hihmen has lost over 40% of its 290‐Mm 3 storage capacity to sedimentation. Most of these upstream structures were small, but three had capacities >9 Mm 3 . Field measurements and historical data from the Water Resources Agency show most smaller dams had filled with sediment by 1976. The three largest were full or nearly so by 2007, when one ( B arlin Dam) failed, releasing a pulse of 7.5 Mm 3 , most of its 10.4 Mm 3 stored sediment downstream. The Central Range of T aiwan is rapidly eroding (denudation rates 3‐6 mm/yr), so geologically high loads make sediment problems manifest sooner. Even in other environments, however, eventually small dams built upstream of large reservoirs are likely to fill themselves, creating multiple small sediment‐filled reservoirs, some located in sites inaccessible to mechanical removal. Our analysis suggests sabo dams do not offer a long‐term basis for controlling reservoir sedimentation in such a high‐sediment yield environment. Sustainable solutions must somehow pass sediment downstream, as would be accomplished by a sediment bypass around S hihmen Reservoir, as now being studied.