Non-uniform Breach Evolution of Landslide Dams during Overtopping Failure

Abstract

<p>When natural dams formed by landslides and other mass flow events are breached by the impounded water, soil materials on the surface of the dam are eroded and entrained, creating erosional features that compromise its stability. The failure of landslide dams due to breaching often results in floods and debris flow downstream. Dam breach models often assume that surface erosion is uniform (surface gradient is constant) along the breach channel, despite observations from dam breach experiments that dam surface evolution is more complex. A thorough understanding of dam surface evolution during breaching is essential for assessing flood or debris flow risk, and hence for hazard mitigation. In this study, we use experiments and numerical modelling to investigate the mechanisms that influence the non-uniform morphological evolution of landslide dam breaches. Analog landslide dam models made from poorly sorted, unconsolidated soils are subjected to various inflow discharges. It is found that although the erosion along the bed and sidewalls both affect the evolution of the breach discharge, the bed erosion is what ultimately determines how the dam profiles change during the dam breach. Breaching flow scours the breach channel resulting in erosion rates, described as a function of the difference between the flow shear stress and apparent erosion resistance, that vary at different points along the dam surface. It is found that the soil erodibility remains constant while the apparent erosion resistance increases linearly along the dam surface. The complex dam failure process is numerically modeled using depth-averaged equations which assume that the evolution of the dam profiles results from the coupled effects of erosion, entrainment, and channel bed collapse. Good agreement between the observed and modeled dam profiles demonstrates that the gradual saturation of the breach flow with entrained sediment is responsible for the linear variation of the apparent erosion resistance, which in turn contributes to the formation of the surface non-linear scouring. Our work attempts to understand the mechanisms that control the breach morphology evolution of landslide dams during breaching. A new perspective on the dam erosion process and its relationship with the sediment concentration and the apparent erosion resistance is presented here, making it possible to provide more accurate physically-based outburst flood and debris flow predictions.</p>