Rate and Distribution of Sedimentation in the Three Gorges Reservoir, Upper Yangtze River

Abstract

Sedimentation in the Three Gorges Reservoir (TGR) has been studied with a primary focus on the amount due to the decrease in sediment inflow caused by water and soil conservation projects and sediment retention of recently constructed upstream cascade reservoirs. In this study, the rate and distribution of sedimentation in the TGR were investigated. A total of 1.6 billion tons of sediment was trapped in the TGR from 23 to 215. The sediment retention rate increased from 6% during the initial operation stage between 23 and 25 to 82% during the normal operation stage between 26 and 215. Sediment finer than .8 and .16 mm contributed 44 and 61% of the total sedimentation, respectively, and the median size of the sedimentation was .1 mm. The spatial distribution of the sedimentation was discontinuous, and 8% of the sedimentation occurred in one-third of the backwater region, with primary deposition zones being at the wide reservoir reaches. The sedimentation was assessed by the sediment transport capacity, which was embedded in the 1-D sediment transport and bed deformation equations. Due to the impoundment of the TGR, the sediment transport capacities decreased significantly at the wide reservoir reaches where flocculation occurred, resulting in deposition. However, the sediment transport capacities were still greater than the sediment concentrations at the gorges, which will be the fixed stretches with no deposition. The surplus sediment transport capacity at the gorge stretches would not allow cumulative sedimentation in the future, leading to a spatially discontinuous equilibrium sedimentation along the TGR instead of the predicted spatially continuous distribution. A uniform bed slope is not expected under the new sedimentation equilibrium state, and the actual sedimentation will be considerably less than the initially predicted value, generating a sustainable storage volume with about 84.7% of the initial volume, which will facilitate the engineering functions of the TGR significantly.