数値解析による積雪条件の異なる融雪型火山泥流予測

抄録

We proposed a distributed model to simulate occurrence of snowmelt type volcanic mudflow in upstream hillslope areas. The model included processes of heat transfer between snow and erupted sediment layers, snowmelt, vertical infiltration of snowmelt water in the snow layer, slope failures at the snow-soil and soil-bedrock boundaries, erosion due to surface runoff including sediment (i.e., mudflow) and deposition by the surface runoff with excess sediment concentrations. Mudflow discharge from the upstream hillslope area was employed as an input in simulations of two-dimensional unsteady flow in the downstream area. Simulations were conducted for a steep catchment located at northwest of the summit of Mt. Yake, which is an active volcano, and its downstream area, Takahara River. Simulation results under uniform snow thickness and actual distributions of snow thickness in the hillslope area suggested that the spatial distribution contributed to differences in runoff timing of mudflows from the sub-catchments of the hillslope area. Snow density generally varies even within a winter season. Our simulation also revealed that relationship between snow density and mudflow volume was not always linear and showed a convex shape. The simulated mudflow discharge from the hillslope area and flow depths and sediment thicknesses at residential areas in the downstream area were maximum when the snow density was 330 kg/m³, which can be assumed snow conditions in mid-January.