Estimation of dam breach parameters and simulation of flood propagation due to overtopping failure in long-existing landslide dams

Overtopping is the most common failure mode of landslide dams, potentially causing flood hazards in the downstream region. In this study, the breach parameters of a long-existing landslide dam are estimated using empirical equations. To assess the accuracy of the empirical equations, the breach geometries of 28 historical landslide dams are compared with the results of applying the equations. The depth of the breach can be estimated more accurately than the top and bottom widths of the breach, with an error of less than 8% in half of the cases. To obtain a breach hydrograph, a physically-based model, developed for landslide dams is applied. Breach hydrograph is calculated by varying the erosion coefficient and overflow depth. The peak discharges obtained from the physically-based model vary between 200 and 500 m3/s which corresponds to the lower range of results of the regression-based models between 150 and 1190 m3/s. A 2D hydrodynamic model is also run to simulate flood propagation. The calculated hydrographs with high, medium, and low erosion rates are applied as the input to the flood model. The calculated flood discharge, the arrival time of the peaks, and the intensity are presented. The simulated floods due to dam breach impact the downstream areas up to at least 20 km from the dam failure site.