Numerical simulation for evaluating the phase-shift of fine sediment in stony debris flows

Abstract

To predict hazard-endangered areas and debris-flow velocity, a variety of physically-based numerical simulation models have been developed. In these models, the relatively large sediment particles such as boulders move as a laminar flow, but the interstitial fluid between sediments behaves like a turbulent flow. Moreover, several recent models assumed that fine sediments act as a fluid. This behavior of fine sediment is referred to as the “phase-shift” of fine sediment. However, because it is difficult to observe the phase-shift of fine sediment in the field, adequate data on the phase-shift of debris flow are still lacking. In the last two decades, intensive monitoring for debris flow has been conducted all over the world, and observations have dramatically increased. For example, in the Illgraben catchment, Switzerland, observations of bulk density, pore pressure, flow depth, front velocity, and temporal and spatial patterns of erosion due to debris flows are available. So, we used these data for model input conditions. We applied the numerical simulation model Kanako-LS to evaluate the phase-shift concept for describing a variety of debris flow properties and behaviors at the Illgraben, Switzerland. Here we successfully describe a variety of observed debris flow behaviors, such as erosion and deposition pattern and shape and velocity of debris-flow fronts. However, if we ignored effects of phase-shift, the deposition volume was overestimated and flow velocity was underestimated.