Entrainment maps considering hydrological conditions for mass movement runout modelling: Application to debris-flow bulking at Pizzo Cengalo

¹ Swiss Federal Research Institute for Forest, Snow and Landscape Research WSL, CH-8903 Birmensdorf, Switzerland
² Geological Institute, ETH Zurich, CH-8092 Zurich, Switzerland
³ WSL Institute for Snow and Avalanche Research SLF, CH-7260 Davos, Switzerland
⁴ Institute of Environmental Engineering, ETH Zurich, CH-8093 Zurich, Switzerland

^* Corresponding author: jacob.hirschberg@erdw.ethz.ch

Abstract

Debris flows entrain sediments and water along their flow path and grow significantly in size. Because the entrainment process isn’t well understood and data is rare, hazard and risk assessment with numerical models is challenging. It is known, however, that both for debris flows and rock avalanches, interstitial pore water in flow path substrate can cause increases in pore water pressures when overridden by the flow, which enhances erosion. The entrained water likely also plays a role in the process transition from rock avalanche to debris flow, like in the Pizzo Cengalo event in 2017. We present a framework for producing entrainment maps serving as an input for runout modelling, here illustrated using RAMMS. The entrainment maps consist of spatially-distributed entities with properties such as max erosion depth, and soil water content inferred from land cover and lithology maps. This study serves as a basis for producing duration curves of subsurface water available for entrainment and include it into the entrainment maps. Such hydrologically-informed entrainment maps will be useful to assess the probability of certain runout distances.