Debris-flow prone basins estimated using distribution maps of slope movements

－Geomorphic thresholds considering rainfall characteristics－

Abstract

  For the reduction of debris-flow disasters spawned by heavy rainfall, the relationship among geomorphic quantities of the mountainous small drainage basin, the arrival of the debris flow to the basin outlet, and rainfall characteristics was examined by using the distribution maps of slope movements and digital elevation models (DEM) for seven disasters. The geomorphic thresholds for the debris-flow arrival become smaller with an increase in rainfall amount in granite areas, which increases the debris-flow risk in many basins. The geomorphic thresholds are shown to vary significantly with lithology. The threshold decreases in a slope area with the coarser soil and the soil-layer structure that soil water is restricted to moving deeper into the soil below the potential slip plane. The arrival ratio of debris flow to the basin outlet becomes larger with an increase in the drainage area.