Some considerations about the simulation of the breach channel erosion on landslide dams

The analysis of the flood hazard related to the areas downstream of landslide dams is one of the most interesting aspects of studying the formation and the failure of natural dams. The BREACH code, simulating the collapse of earthen dams, both man-made and naturally formed by a landslide, was chosen in order to analyse the case of the Valderchia landslide dam (central Italy). The bed-load transport formula used in BREACH (Meyer-Peter and Muller, modified by Smart) is based on flume experiments with well-sorted sediments. Such a methodology probably makes this equation not very suitable for describing the sediment transport peculiar to a landslide body presenting a very poor material sorting. The Schoklitsch formula was implemented into the programme as an alternative to the Smart equation. However, because the landslide deposits may often have a strongly bimodal grain-size frequency curve, the percentile D50 (the typical granulometric parameter requested by bedload sediment transport formulas) can sometimes correspond to one of the grain-size classes which are really present to a lesser degree. To consider this phenomenon, the BREACH programme (version 7/88-1) was implemented with a new procedure that calculates two granulometric curves, one for each mode of the original distribution, and evaluates transport of the landslide material separately. Results of the analysis show that the model is very sensitive to the bed-load equation and that the procedure implemented to consider the eventual bimodal distribution of the dam material simulates the armouring phenomenon (which can stop the erosion of the dam during the overtopping phase).