Flume experiments and numerical simulation focused on fine sediments in stony debris flow

Abstract

In stony debris flow, it has been considered that the gravels move like laminar flow, but the interstitial water behave as turbulent flow. Moreover, fine particles can behave with the interstitial water as fluid and many previous studies call this process of fine sediment as shifting solid phase to fluid phase, “phase-shift”. Phase-shifted sediment affect the fluidity of debris flow. Therefore, it is necessary to consider fine sediments behavior to describe run-out processes of debris flow. However, the hydraulic conditions that fine sediment can behave as a fluid are not well understood. Here, we analyzed this hydraulic condition through flume experiments and numerical simulations. We examined effects of grain size distribution on the equilibrium sediment concentration, which has been defined as the sediment concentration that in which there is neither erosion nor deposition on the experimental flume bed. We found that for the same hydraulic conditions the equilibrium sediment concentration differed due to variations in the grain size distribution. Based on these experimental results, we tested the following three models for describing the conditions that fine sediment can behave as a fluid. First, we fixed fine sediment concentration in interstitial fluid (Model 1), then, we fixed the maximum diameter of phase-shifted sediment (Dc) (Model 2). In Model 3, Dc is assumed to be variable according to the ratio of the friction velocity to the settling velocity of Dc. As the result, the experimental relationship between grain size distribution and longitudinal gradient of deposited sediment surface under steady-state condition can be described by using the Models 2 and 3, but Model 1 could not describe.