Runoff characteristics and sediment yield in composite volcanic area

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In Kyushu, Japan, debris flow disasters have occurred frequently during rainy and typhoon seasons. The mechanism on runoff and sediment yield, and temporal variation of hydrological characteristics and sediment transported by debris flows in volcanic area of Kyushu are examined based on field study. The results are summarized as follow :1. Geomorphological conditions of basins was analyzed. As the result, distribution map of gradient, relief energy and drainage system were prepared, and were compared with topographical conditions and geological structure between volcanoes.2. In the hydrological observatories installed in the basins, discharge, temperature, and electrical conductivity of the stream water were measured continuously. Although these basins are located in the same flank, the runoff characteristics changes with the rates of gentle slope, the degree of weathered rocks, and the degree of dissection, etc.3. Vegetation was decreased sediment by sheet erosion. Median diameter of sediment by sheet erosion in slope covered with vegetation was about 50% of those by sheet erosion in bare land slope. Final infiltration capacity in soil and Manning's coefficient of roughness on slope were increased by cover of vegetation.