Abstract: Since its completion in 2006, the Zhikong Reservoir on the upper Lhasa River has trapped incoming sediment and released clean water, causing a relatively fast coarsening of the gravel riverbed in the downstream channel; but the degree and scope of the coarsening and the equilibrium status of gravel bed are not understood yet. In April 2018, we conducted a field survey of the riverbed around the reservoir focusing on five sites and manually measuring the median particle size of gravels and pebbles on the riverbed surface, and adopted unmanned aerial vehicles (UAV) taking photographs at a height of 30 m. Through image processing and generation of the high-resolution terrain data, we calculated spatial variations in the gravel particle size and relative roughness of the bar surface. Results show that the UAV aerial survey method can extract the particle size of gravel bed surface in good accuracy, and the mean error in median particle size calculations is about 18.7% in comparison with the manual measurements. Owing to flow scour and sorting, the coarsening in the river section of 1 km long below the dam has reached its equilibrium; and the particle size decreases along the course in the 10 km long section below the dam, and gradually recovers to the normal up to a distance of 22 km below the dam.

Key words: riverbed particle size, gravel bar, dam effect, relative roughness, UAV aerial survey