Hydrological and Sediment Transport Simulation to Assess the Impact of Dam Construction in the Mekong River Main Channel

The downstream impact of dams is a complex problem in watershed management. In the upper Mekong River watershed and its main channel, dam construction projects were started in the 1950s to meet increasing demands for energy and food production. Dams called the Mekong Cascade were completed on the Mekong River in China, the Manwan Dam in 1996 and the Dachaoshan Dam in 2003. We evaluated the impact of the Manwan Dam and its related watershed development on seasonal water discharge and suspended sediment transportation using hydrological simulations of target years 1991 (before dam construction) and 2002 (after dam completion). Our study area was the main channel of the Mekong River in northern Thailand extending about 100 km downstream from the intersection of Myanmar, Thailand and Laos. We used the MIKE SHE and MIKE11 (Enterprise) models to calculate seasonal changes of water discharge and sediment transport at five points 15-35-km apart in this interval. Sediment load was calculated from a regression equation between sediment load and water discharge, using suspended sediment concentrations in monthly river water samples taken between November 2007 and November 2008. Finally we estimated annual sediment load along the study reach using from both of simulated annual hydrograph and the regression equation. Our simulations showed that after construction of the dam, there was a moderate decrease in peak discharge volume and during the rainy season in August and September and a corresponding increase in the subsequent months. Accordingly, sediment transportation budgets were increased in months after the rainy season. The suspended sediment transportation in Chiang Sean was increased from 21.13 to 27.90 (M ton/year) in our model condition.