Hydrological and geomorphic study on immunity of hillside slope against slope failure

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In Japan, hillside landslides occur frequently in rainy seasons and give the great damage to human life and public property. Hillside landslides are triggered mainly by rainfall. This study was carried out to clarify causes of differences and variations of possibility of sediment yield. Cohesion and bulk density of forest soil got exponentially small and porosity of forest soil got large, in proportion to years after a weathering. In order to understand effects of forest roots (living-root and decayed-root) on hydrologic processes, that is, water movement in soil layer, field sprinkling experiments were carried out on a hill slope in the non-rainy area. The following matters were confirmed by these field experiments. Rainwater concentrates on the surroundings of forest roots. And, runoff from surrounding of forest roots in steady state increases approximately with rainfall intensity and follows Manning's low. On the other hand, runoff inside porosity of soil follows Darcy's low.Furthermore, Evaluation model of dangerous condition to hillside landslides considering both of glowing of thickness of soil layer and possibility of rainfall was made. And this model was applied to small slope in the rainy area and non-rainy area. As the result of numerical simulation, slope stability decrease, in proportion to amount of rainfall and years after a weathering in both area. The critical amount of rainfall in rainy area was large 300mm/day from that in non-rainy area. On the other hand, the index of slope stability in rainy area was almost the same as that in non-rainy area. As a result of this numerical simulation, it was clarified that the immunity against slope failure in rainy area was larger than that in non-rainy area.