Suspended Particle Settling Velocities in the Three Gorges Reservoir

Improved understandings of fine sediment settling velocities in The Three Gorges Reservoir are still important, due to the serious siltation since its impoundment. A method to measure the sediment settling velocity is proposed using ADV based on the sediment diffusion equation. Field measurements are conducted at the Zhongxian and Fengjie reaches. The sampled suspended sediments show that suspended particle diameters are mostly between 4 similar to 10 mu m and the concentrations are mostly between 0.1 similar to 2 kg/m(3). The backscatter acoustic intensities against sediment concentrations are calibrated according to field sampling results, and a formula for calculating the instantaneous sediment concentrations is given. The fluctuations of flow velocities and sediment concentrations are then obtained, hence the settling velocities according to the given method. The settling velocities of fine suspended sediment in the Three Gorges Reservoir are found quite different over the vertical profiles, being small near the water surface and the bed, reaching the maximum values at the depth between the surface and bed. With the increase of sediment concentrations, the settling velocities decrease, varying in a range of 0.1 similar to 10 mm/s with some negative values.