Maintaining Sediment Flows through Hydropower Dams in the Mekong River Basin

Abstract

The planning, design, and construction of hydropower dams are occurring at a rapid pace in the Mekong/Lancang River Basin in Southeast Asia. For years the river and its tributaries transported an average of 160 million metric tons of sediment per year to the South China Sea. Reservoirs are expected to trap significant fractions of this sediment, rendering much of it unavailable to transport nutrients and maintain the river’s geomorphic structure and ecological habitats. This paper introduces and applies a method for identifying and evaluating alternative dam siting, design, and operating (SDO) policy options that could help maintain more natural sediment regimes and for evaluating the effect of these sediment-management strategies on hydropower production. This paper focuses on the planned Lower Sesan 2 (LSS2) Dam in Cambodia. This dam would prevent a significant source of sediment from reaching critical Mekong ecosystems, such as Cambodia’s Tonle Sap Lake and the Mekong Delta in Vietnam. The concept of replacing LSS2 as planned with several smaller alternative dams with SDO modifications to enable reservoir sediment flushing is evaluated. These modifications could increase sediment discharge from this site by as much as three to four times compared to current plans but reduce short-term annual energy production by a third or more. Results demonstrate that most of the loss in energy production associated with SDO modifications (at least 94% in the cases considered) results from reduced reservoir size as opposed to modified reservoir operations. It is also found that, from an operational standpoint, sediment flushing is likely best conducted during the transition period between the dry and wet seasons.