Analysis of debris flow behavior with a one dimensional run-out model incorporating entrainment

Abstract

Estimating the magnitude and the intensity of rapid landslides like debris flows is fundamental to evaluate quantitatively the hazard in a specific location. Dynamic run-out models are able to characterize the distribution of the material, its intensity and define the zones where the exposed elements will experience an impact. These models can provide valuable inputs for vulnerability and risk calculations. However, most dynamic run-out models assume a constant volume during the motion of the flow, ignoring the important role of material entrained along its path. Consequently, they neglect that the increase of volume can enhance or reduce the mobility of the flow and can significantly influence the size of the potential impact area.Limited work has been done to quantify the entrainment process and only a few have proposed physical explanations for it. One of the reasons is that material entrainment is a complex process and an adequate understanding of the phenomenon is needed to facilitate the development of appropriate dynamic models. A proper erosion mechanism needs to be established in the analyses of debris flows that will improve the results of dynamic modeling and consequently the quantitative evaluation of risk.The objective of this paper is to present and evaluate the performance of a 1D debris flow model with a material entrainment concept based on limit equilibrium considerations and the generation of excess pore water pressure through undrained loading of the in-situ bed material. The debris flow propagation model is based on a one dimensional continuum mechanics approach using a depth-integrated approximation based on the shallow water assumption (Saint–Venant equations). The flow is treated as a laminar one-phase material, in which behavior is controlled by a visco-plastic Coulomb–Bingham rheology. The model parameters are evaluated and the model performance is tested on a debris flow event that occurred in 2003 in the Faucon torrent (Southern French Alps).