Managing Sediment in the Mekong River Basin: Tradeoffs between Hydropower and the Environment

Abstract

The Mekong/Lancang River Basin is undergoing a period of rapid hydropower development. Construction of over 20 dams is currently underway just in the Lower Mekong Basin, with at least an additional 42 dams planned for construction in the next 20 years. The resulting alteration of the basin’s natural flow and sediment regimes, geomorphologic makeup, and fish migration patterns could significantly impact the exceptional biodiversity and food production characteristics of the basin’s ecosystems. This study is devoted to assessing the improvement in sediment passage (compared to current and expected conditions) that could be achieved at various locations in the basin by implementing reservoir sediment management practices such as sediment bypasses, drawdown flushing, seasonal drawdown routing, and sluicing. Specific sediment management options are discussed for selected reservoirs in the Sre Pok, Se San and Se Kong sub-basins (called the “3S” basins). These sub-basins are particularly important with respect to biodiversity and ecological productivity, and serve as an important source of flow, sediment and nutrients to the mainstream Mekong River. Simulations are performed with the Sediment Simulation Screening (3S) Model. Using a daily time-step, this model simulates the mass-balance of flow and sediment to predict in relative terms the spatial and temporal accumulation and depletion of sediment in river reaches and in reservoirs under different sediment management policies. It also identifies the relative tradeoffs between hydropower output and reliability and sediment regime alteration for these sediment management options. INTRODUCTION In the recent past, wars have kept the Mekong Basin natural, and as such it has supplied the fish that support the livelihoods of more than 50 million people living in the basin. The production of fish results from the natural hydrological and sediment (and nutrient) regimes as the water flows for 4,750 km from the upper Mekong (called the Lancang) River basin of China through portions of Myanmar, Laos, Thailand and Cambodia on its way to the Vietnam Delta. Given the current absence of military conflict in the basin the river has become attractive to hydropower developers and those who will benefit economically from that development, at least in the short run. The Mekong River and its tributaries form one of the world's most diverse ecosystems, containing over 560 endemic fish species and over 300 globally threatened vertebrate species. The basin’s productivity and biodiversity are driven by an annual flood pulse, which carries most of the annual 1297 World Environmental and Water Resources Congress 2013: Showcasing the Future © ASCE 2013