Runoff analysis of debris flows at Mt. Unzendake volcano, Japan

A simulation method is developed to predict debris-flow runoff from the slopes of Mt. Unzendake volcano. In this runoff prediction, a kinematic wave method is employed on mountain slopes, and a dynamic wave method in the reach of mountain rivers. The study area is the drainage of the Mizunashigawa River on the bare slopes of Mt. Unzendake volcano. To model the debris flow runoff, we divide the catchment into mountain slopes and river reaches. For the mountain slopes we use a momentum equation of uniform flow and a continuity equation of unsteady sediment-water mixture flow. Assuming that the erosion rate in the continuity equation is flow velocity times a function of sediment concentration, which is also assumed to be a constant, we can estimate the inflow rate of sediment-water mixtures from the slopes to the river reaches. For the river reaches, we use a momentum equation for unsteady mixture flow and two continuity equations of unsteady flow for both the sediment and water phase. The estimate of the sediment transport rate and river width is necessary to solve these equations. In the calculation we use a sediment discharge equation unified for various transport types, which is appropriate for wide range of bed slopes. We also use an empirical equation of river width, which is obtained from the investigation of relationship between river width and drainage basin area at any stations in various rivers. Overall, our calculated results of mixture-flow discharge and flow surface level agree with the field measurements in the Mizunashigawa River.