Experimental Investigation on Scouring v.s. Mass Failure of Unsaturated Soil Bed: Implications for Debris Flow Initiation and Erosion

Scouring and mass failure are two common mechanisms used to describe soil bed erosion, but their combined effects are often not considered. To better understand how these mechanisms compete and under what conditions they prevail, it is essential to consider infiltration and a more realistic unsaturated soil bed. This study investigates soil bed erosion by considering unsaturated soil mechanics, a wetting front, and both erosion mechanisms of scouring and mass failure. Physical experiments were conducted on model water runoff over an unsaturated sand bed to investigate the effects of soil water content and flow velocity on erosion. Experimental results show that current understanding of soil bed erosion can be enhanced by adopting unsaturated soil mechanics and considering the combined effects of scouring and mass failure. The scouring rate is found to be independent of the bed water content because it only affects the uppermost soil particles, which immediately become saturated once water flows over them. Mass failure, on the other hand, is initiated at the wetting front when the rate of infiltration exceeds that of scouring. The depth of mass failure can be described by the net infiltration depth, which is defined as the difference between the infiltration and scouring depths. The net infiltration depth is jointly governed by the soil water content and flow velocity. The crucial role of the coupled effects of the hydro‐mechanical behavior of unsaturated soil in the realistic modeling of soil bed erosion is demonstrated. Outcomes present advancement toward improved hazard assessments of debris flows. Plain Language Summary When water flows over unsaturated soil on a slope, the soil bed can be eroded by either scouring or mass failure. Scouring occurs when the surficial soil particles are transported by the flow, and mass failure occurs when the stress exerted by the flow exceeds the soil strength at a potential failure plane. It is important to study how these two mechanisms compete and the conditions that enable each mechanism to prevail. In turn, improved predictions of soil bed erosion can be obtained. Furthermore, erosion is governed by the infiltration of flow water, which changes the soil water content and the soil strength. In this study, soil bed erosion was investigated by considering different erosion mechanisms and flow water infiltration, and a unique set of physical experiments was carried out. The results show that the rate of scouring is not