Amplification effect of cascading breach discharge of landslide dams

Abstract

Affected by earthquakes and heavy rainfall, multiple landslide dams often cluster closely together along river reaches or gullies. Compared with a single landslide dam, the burst flood produced by the cascading failure of multiple landslide dams can be enhanced, seriously threatening life and property downstream. Here, we conduct a series of experiments in a 42 m flume to investigate the failure mechanisms of single and paired dams with fine-grained, well-graded, and coarse-grained debris; analyze the effects of dam geometry and initial water level of a downstream dam on the cascading breach; and quantitatively evaluate the amplification effect of cascading breach discharge. Single dams fail by overtopping along with seepage instability for a fine-grained dam, headcutting for a well-graded dam, and overtopping for a coarse-grained dam, respectively. The type of failure which occurs for a single dam is influenced by the shear strength of the dam material and seepage. However, the downstream dams in cascading tests fail by overtopping irrespective of dam material due to the large outburst floods from the upstream dams. A general flat slope angle is maintained during breaching for the fine-grained and coarse-grained dams, while a step-pool structure is developed for the well-graded dams because the finer grains are easier to wash away than coarse grains. The peak breach discharge for a downstream dam is 1.4–1.9 times the value for an upstream dam in the experimental runs, indicating the amplification effect of breach discharge. The amplification effect has a negative linear correlation with the time interval between the peak breach discharges of the two dams because the overlap of breach processes of upstream and downstream dams is gradually reduced as the time interval increases.