Comparison and analysis of the influence of geological differences on sediment runoff volumes from watersheds -case study of plutonic and metamorphic rocks in two sediment disasters in Japan-

This study examined the sediment characteristics of areas where landslides occurred due to heavy rains between 2014 and 2019. A total of 5 types of geology in two geographic regions in Japan were examined using LiDAR LP topography data before and after the disasters occurred to estimate the changes in elevation. In addition, the volume of sediment runoff for each case was estimated for watershed areas ranging from 0.01 up to 0.1 km². The influence of geological differences on the sediment runoff volume within the basin using indicators such as the density of landslide occurrence, landslide volume, and watershed erosion intensity was also assessed. The results showed that, for all geology types, as the watershed area increases, the relief ratio decreases and the sediment runoff volume increases; however, the magnitude of this increase in sediment runoff volume differs depending on the underlying geology. In addition, the density of landslide occurrence was high in plutonic and metamorphic rocks. The landslide volume and the total eroded sediment volume within a watershed can be regressed using the linear equation y=ax. Since the average total eroded sediment volume within a watershed is approximately twice that of the landslide volume, there is a proportional relationship of 1:2. The relationship between the relief ratio and watershed erosion intensity shows that the watershed erosion intensity increases gradually as the relief ratio increases, and the rate of increase is larger in plutonic rocks (granite and granodiorite) than in the other groups. Metamorphic rocks had a relatively low watershed erosion intensity; these geological differences are reflected in differences in the degree of erosion of stream beds and banks by flood flows.