Qualitative analysis of the overtopping-induced failure of noncohesive landslide dams: Effect of material composition and dam structure on breach mechanisms

Landslide dams are composed of wide-graded materials characterized by nonuniform structures that govern breaching mechanisms. However, investigations of the failure characteristics of single -structure dams with different material compositions and inverse grading structure dams remain insufficient. In this study, a series of flume experiments are conducted to investigate the influences of noncohesive dam materials and inverse grading structures on the breaching mechanisms, hydraulic characteristics and residual dam parameters during and after landslide dam failures. The results indicate that the dam breach process is controlled by the material composition and dam structure. A coarse-grained dam remains stable with seepage, a medium-grained dam fails by headcutting and backwards erosion, and a fine-grained dam fails due to layered erosion. An inverse grading dam with coarse-grained overburden features backwards erosion or a combination of sliding and backwards erosion, while a dam with medium-grained overburden fails by headcutting and backwards erosion. The maximum erosion rate occurs at the accelerated breaching stage for single -structure dams and at the initial overtopping or accelerated breaching stage for inverse grading structure dams. Four longitudinal evolution patterns are extracted based on the breach process and erosion characteristics. In addition, the outflow discharge during dam failure can be estimated by measuring the breach width, which is defined as the straight line distance between the ends of the breach crest at the overflow face. Both the peak discharge and residual dam parameters for single -structure dams are sensitive to the median diameter of the material. These parameters of inverse grading structure dams fall within the range of values observed for dams formed by the top layer material and the bottom layer material. The initial overtopping and backwards erosion stages account for 10% - 35% and 36% - 66% of the total breach duration for single -structure and inverse grading structure dams, respectively. Serious errors in the prediction of breach parameters can occur when top layer materials are considered to characterize the material of inverse grading structure dams.