Effects of debris-flow and bed composition on erosion and entrainment

Department of Physical Geography, Utrecht University, Utrecht, The Netherlands

^* Corresponding author: l.roelofs@uu.nl

Abstract

Erosion and entrainment of material by debris flows determine debris-flow volume growth and therefore hazard potential. Recent advances in field, laboratory, and modelling studies have distilled two driving forces behind debris-flow erosion; impact and shear forces. A third factor influencing the (relative) importance of these forces is the viscosity and abundance of the interstitial fluid in the debris flow and the bed. However, how erosion and these forces depend on the composition of the debris flow itself and the composition of the bed remains unclear. Here, we present results of small-scale flume experiments with a loosely packed erodible bed that highlight the far-reaching effects of debris-flow and bed composition on erosion processes and magnitude. We quantify the effects of gravel, clay, and solid fraction in the debris flow on bed erosion. In addition, we quantify the effects of water and clay content of the unconsolidated bed on erosion by a debris flow. We show that debris flow erosion increases linearly when the gravel fraction of a debris flow is increased, which is linked to an increase in both impact and shear forces. We find that debris flow erosion, and the related forces, are non-linearly impacted by the clay and water content of the debris flow and those of the bed. For both the clay content of the debris flow and the bed, an optimum in erosion exists around a specific clay percentage that does not directly relate to an optimum in either shear or impact forces. When the water content of the bed and/or the debris flow is increased, erosion becomes largest when supersaturated conditions occur. These conditions are unrelated to the magnitude of the two erodible forces. This shows that both clay and water content affect erosion by affecting the transfer of pore pressures from the debris flow to the bed. We can therefore conclude that impact and shear forces dictate debris flow erosion in most cases but that their (relative) importance is significantly altered by the means and effectivity of pore pressure transfer from the debris flow to the bed. The latter is highly influenced by the viscosity and abundance of the interstitial fluid of the debris flow and the composition of the bed.