Simple geomorphologic approach to estimate debris-flow entrainment. applications to the pyrenees and the alps

Abstract

The basal incorporation of material, also called entrainment, is a common characteristic of debris flow dynamics. The volume of a debris flow can considerably increase when compared to the initial volume, due to the erosion of material produced along the travel path. This study is a preliminary attempt to establish a simple geomorphologic approach for the calculation of volume to be entrained in a torrent, if a granular debris flow event would occur. The methodology presented has been developed using data obtained by comprehensive field surveys, carried out in six torrents affected by granular debris-flow events. The torrents, divided into 43 reaches, are located in the Pyrenees and the Alps. The application of the methodology requires data to be measured along the flow path (geologic and geomorphologic factors). As a result, a predicted erosion rate for determined reaches can be estimated in function of these factors, properly weighted. Although the results indicate a high scatter, the total predicted erosion volume calculated for each test site using the proposed approach coincides rather well with the volumes observed in the field.