The formation processes and development characteristics of sandbars due to outburst flood triggered by landslide dam overtopping failure

Abstract

Abstract. Sandbars are an essential form of riverbed morphology which could be affected by landslide dams. However, few studies have focused on the formation processes and development characteristics of sandbars triggered by outburst flood. In such a way, eight group dam failure experiments with 4 to 7 times of dam length movable bed is carried out to study the temporal and spatial distributions of 25 sandbars along the riverbeds, the sandbars geometric characteristics, and the influence of outburst flow hydraulic characteristics on developments of sandbars. The results show that sandbars are formed after peak discharge of outburst flow. The number of sandbars is 0.4 to 1.0 times the ratio of river bed length to dam length. Besides, sandbars have the characteristic of lengthening towards upstream during the failure process. Sandbars' upstream edges have a more extensive development than sandbars downstream edges. The length of a sandbar along the channel changes faster than the sandbar's width and height. The sandbars' length and width are about 10 to 80 and 1 to 7 times of average height, respectively, and the average heights of sandbars are about 1 to 3.5 times the maximum particle size. Sandbars' lengths make a more significant impact on sandbars' volumes than widths and heights. It found that the Froude number has a significant influence on the sediment carrying capacity. And the sediment concentrations in volumes of the outburst flow at the upstream edges of all sandbars are greater than those at the downstream edges of sandbars. Meanwhile, the sediment carrying capacities of the outburst flow at the upstream edges of sandbars are smaller than those at the sandbars' downstream edges. And the differences between the sediment concentrations and the sediment carrying capacities determine the sedimentation or entrainment. The results can reference the research on the river channel's geomorphological characteristics affected by the outburst flood.