Failure process of a lateral slope deposit and its effect on debris flood formation

Lateral slope deposit failure represents a major source of material for debris flow/flood formation in headwater regions. From the perspective of inducing factors, such failures are the result of combined action of hydrodynamic and gravitational forces. This work investigated the failure process of a lateral slope soil deposit using indoor flume experiments. The key factors considered were the fine particle content and bulk density of the soil deposit, inflow discharge, and channel slope. Overall, the failure process can be divided into three stages: slope toe erosion stage, frequent failure stage, and gradual stabilization stage. Results revealed that greater bulk density and more fine particle content decreased entrainment and that slope angle has a positive effect on entrainment of the soil body. Debris floods, which occurred only when the fine particle content of the soil deposit was < 30% and only when the channel slope was > 7°, formed more easily when the bulk density of the deposit was < 1.5 or > 1.8 g/cm³. Following analysis of the factors influencing soil body entrainment, an experimental-based critical runoff condition was proposed for debris flood formation. The findings of this study enhance understanding of the mechanism of debris flood formation, which could help improve debris flood forecasting in headwater regions of mountainous catchments.