Numerical simulation of debris flows focusing on the behavior of fine sediment

Abstract

Debris flows generally includes a wide range of grain sizes, in which fine sediment behaves as a fluid phase rather than as a solid phase and enlarges the pore fluid density. Although in existing models fine sediment constantly behave as fluid phase from initiation to deposition, previous researches have reported that behavior of fine sediment can vary through debris-flow propagations depending on the kinematic conditions (i.e., relation of turbulence and the settling velocity of the particles). To test the effects of this transitional behavior of fine sediment and compare with existing models, we conduct numerical simulations of debris flows with bidisperse granular materials, employing two models for the behavior of small particles: (i) all small particles constantly behave as a fluid phase (Model I); and (ii) the ratio of small particles behaving as a fluid phase varies depending on the kinematic conditions (Model II). In the simulations, we used an inclined channel with erodible bed at the upper stream end of the reach, where debris flows initiate by supplying water. Varying the inclination from 15° to 20°, we measured the time series of discharges, flow depths, sediment concentrations and pore fluid densities at the downstream end. Hydrographs of the two models are significantly different at higher slopes, with a sharp peak at the front of debris flows in Model I and relatively moderate peak in Model II. These differences are caused by higher pore fluid densities from the front to the tail of debris flows in Model I, in contrast to lower pore fluid densities in Model II, where not all of small particles behave as a fluid phase. This infers that discharge rate of debris flows can be overestimated especially at higher slopes if the transitional behavior of fine sediment is not considered.