Abstract: Most sediment deposition in the Three Gorges Reservoir is fine sediment with complex transport laws. The erosion and deposition calculated from traditional theories, in which sediment carrying capacity is regarded as a criterion, is inconsistent with actual measurement results. On the basis of field measurement results of Huanghuacheng reach, a typical reach of sediment deposition, flume experiment was conducted to further explore the transport of fine sediment. According to the experimental data, three different areas can be divided according to flow velocity and water depth: erosion area, sediment transport area with relative equilibrium, and deposition area. The discriminating standard of critical condition of erosion or deposition status was not determined by the comparison between sediment concentration and sediment carrying capacity, but by the flow rate.

Key words: Three Gorges Reservoir, backwater area, fine sediment, prototype observation, flume experiment, erosion and deposition