Geomorphological Analysis on the Occurrence Factors of Slope Failure around Shimoda City

Shizuoka Prefecture

Abstract

In order to estimate the influence of geomorphological factors to the occurence of slope failure, geomorphological classification of head hollow slopes and comparison between geomorphological factors of collapsed slope and that of non-collapsed slope is done in the Ochiai river basin (Shimoda City, Shizuoka Prefecture) where many slope failures occurred in September, 1991.
Firstly, by gradient change lines of slopes, head hollow slopes of the area are classified into three categories: gentle slope on the ridge top, valley side steep slope, head hollow bottom surface. In the collapsed slope, collapse generally occured in the lowest part of valley side steep slope just above gradient change line of the slope.
Secondly, some geomorphological factors are confirmed to be important to the occurrence of slope failure. These are, 1) valley side steep slope gradient, 2) difference of gradient between valley side slope and head hollow bottom surface, and 3) curvature of contour. In regard to collapsed area, negative correlation on logarithmic scale between W_M/W_E and A_F/A_B, is clearly confirmed (W_M: maximun width of the catchment basin, W_E: width at the end of the catchment of basin, A_F: collapsed area, A_B: catchment area).