Surface failure caused by snow melt occurred in April, 1997, on the Kohirasawa, Yunotani village, Niigata prefecture. In this study, a model for predicting surface failure caused by snow melt are proposed using the heat balance method, saturated unsaturated infiltration analysis method and infinite slope stability analysis method. And then this model is applied to the mountain slope in Kohirasawa to consider the process and mechanism of surface failure caused by snow melt. The results obtained are as follows (1) Snow and safety factor distribution obtained by numerical computation are consistent with actual distribution .(2) The effect of snow melt to the surface failure is that the water is supplied to the surface layer constantly and it gets ground water level high easily by rainfall.
We estimated the effect of forest by simulating on surface failure caused by snowmelt at the slopes of forested and unforested areas in the Kobira River basin. The simulation model consisted of a snow model and a surface failure model . The snow model estimates snow depth distribution and snowmelt, using the heat balance method. The surface failure model estimates the ground water level and slope stability, using analyses of saturated-unsaturated infiltration and infinite slope stability model.Generally, landslides are thought more likely on unforested slopes, because of increased snowmelt. In the Kobira River basin, however, forest prolonged snowmelt causing surface failures when heavy rainfall in early April coincided with the peak of snowmelt. The model verified that the presence of forest changed the landslide potential . This will make it possible to predict surface failures and debris flows during the snowmelt period . These results provide fundamental information for planning warning and evacuation systems to prevent sediment disasters.
In Rokko mountain range, by the Hyogo-ken Nanbu Earthquake to occur on January 17th, 1995, many cracks occurred on a mountain slope. In this study, scale and distribution situation of cracks formed on a mountain slope, was grasped. As considering these results, then, numerical experiments were executed and influence of cracks against slope stability was evaluated using the numerical simulation model for predicting hillside landslides. With these results, in the slope downstream cracks, it was confirmed to ascend suddenly in ground-water level and increase 10-55 cm grade in maximum ground-water level, in comparison to the case without cracks. And, in the case that cracks exist on a mountain slope, appearance time of a collapse slope gets earlier and collapse slope total number increases 10% grade, than the case that cracks don't exist. Furthermore, in order to be as an index to plan effective land erosion control master plan after earthquake, influence of topographic shape of a place where cracks exist on hillside landslides, was evaluated quantity-like. The typical results were as follows:(1) In the case that cracks were formed on parallel and convex shapes than concave shapes, slope stability falls more remarkably.(2) Because influence of cracks on sediment yield potential is different by a difference of topographic shape of a place where cracks exist, in case that land erosion control master plan will be reconsidered after earthquake, it is necessary enough to consider a topographic shape of a place where cracks exist.
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