Though widespread in disaster-prone areas, debris flow dams have unique characteristics in their formation and failure. An accurate description of their failure is the prerequisite to predict the dam-break floods, although little has been done in this research area. This paper describes the longitudinal evolution processes of a debris flow dam with 15 different grain-size distributions through flume tests, and put forward the formula of the downcutting rate for the whole process of overtopping failure. Results have shown that, according to the longitudinal evolution characteristics of the debris flow dam failure, the breach process can be divided into three stages: the formation of a retrogressive scarp, the erosion of the retrogressive scarp, and its decline stage. When the dam overtops and breaches, the downcutting rate for the whole evolution process can be expressed as E = k v sin(0.85 + θ)³, where E is the erosion rate, v and θ are the flow velocity and the longitudinal slope of the breach respectively, and k is the erosion rate coefficient. The larger the value of the erosion rate coefficient k is, the faster the erosion rate E is. The minimum correlation coefficient R² between the calculated and the measured erosion rates is 0.73. In addition, the relationship among erosion rate coefficient k, grain-size distribution parameters μ, and D_c is also proposed, and the correlation coefficient R² reaches 0.965. The results provide an important theoretical basis for the prediction of debris flow dam-break floods, which can facilitate evidence-based decision-making in disaster prevention and mitigation.

泥石流坝虽然普遍存在于灾害易发地区，但其形成和破坏具有独特的特征。准确描述它们的故障是预测溃坝洪水的先决条件，尽管在该研究领域几乎没有做过。本文通过水槽试验，描述了15种不同粒度分布的泥石流坝的纵向演化过程，并提出了满顶破坏全过程下切率计算公式。结果表明，根据泥石流溃坝的纵向演化特征，溃坝过程可分为退坡形成、退坡侵蚀和衰退阶段三个阶段。当大坝越过堤坝决堤时，E  =  kv sin(0.85  +  θ ) ³，其中E为侵蚀速率，v和θ分别为裂口的流速和纵向坡度，k为侵蚀速率系数。侵蚀速率系数k的值越大，侵蚀速率E越快。计算的和测量的侵蚀率之间的最小相关系数R ²是0.73。此外，侵蚀速率系数k与粒度分布参数μ和D 之间的关系也提出了_c，相关系数R ²达到0.965。研究结果为泥石流溃坝洪水的预测提供了重要的理论依据，有助于防灾减灾的循证决策。