Climate change is increasing the frequency of extreme rainfall events and the snow cover to melt at high altitudes, which may exacerbate the threat posed by debris flows. Soil bed erosion, the process by which the bed material fails under loadings from a debris flow, is perhaps the most important momentum exchange process that governs the destructive potential of debris flows. Existing erosion theories adopt saturated soil mechanics to describe the failure of soil bed and place a strong emphasis on the basal friction induced shear stress as the driving mechanism. However, soil beds in nature are rarely saturated and field observations have hinted at the importance of collisional stresses as a driving mechanism. In this study, an unsaturated soil mechanics framework is used to characterize soil bed erosion by collisional flows. Experiments were conducted to model the erosion of unsaturated sandy beds with a wide range of initial matric suction values, which is a measure of capillary stresses, by gravel flows. Contrary to the existing literature, the rate of erosion does not increase linearly but demonstrates a parabola-like relationship with the bed water content because the shear strength of unsaturated soil is governed by capillary stresses. The importance of collisional stresses on soil bed erosion is demonstrated by a newly proposed dimensionless number. Findings indicate that existing erosion models largely underestimate channel bed erosion, especially for soil beds with low water content, and stress the importance of hydro-mechanical coupling to advance the current state of debris flow hazard delineation.

中文翻译：

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揭示毛细管和碰撞应力对泥石流引起的土壤床侵蚀的重要性

气候变化增加了极端降雨事件和积雪在高海拔地区融化的频率，这可能加剧泥石流带来的威胁。土壤床侵蚀，即床层材料在泥石流的载荷作用下失效的过程，可能是控制泥石流破坏性潜力的最重要的动量交换过程。现有的侵蚀理论采用饱和土力学来描述土床的破坏，并以基底摩擦引起的切应力为驱动机理。但是，自然界中的土壤床很少饱和，现场观察表明碰撞应力作为驱动机制的重要性。在这项研究中，采用非饱和土壤力学框架来表征碰撞流对土壤床层的侵蚀。进行了实验，以砾石流模拟了具有广泛范围的初始基质吸力值的非饱和砂层的侵蚀，这是毛细应力的量度。与现有文献相反，由于非饱和土的抗剪强度受毛细应力的控制，因此侵蚀速率并没有线性增加，但与床层含水量呈抛物线状关系。新提出的无量纲数证明了碰撞应力对土壤床侵蚀的重要性。研究结果表明，现有的侵蚀模型在很大程度上低估了河床的侵蚀，特别是对于含水量低的土壤床，并强调了水力-机械耦合对推进泥石流危险性描述的现状的重要性。