六甲山地における崩壊の年次変化と崩壊無効雨量にもとづく土砂生産量の推定

Abstract

Characteristics of collapse phenomena in the Rokko area were analysed, based on the data in-troduced into the database system composed on the digital map of the area with 50 meters mesh.Analyses of three remarkable collapse events in 1961, 1965, and 1967 caught by the aerial photo-graphs before and after show that the collapse occurrence is proportional to the power, usually 2, of slope gradient, and that the critical rainfall to trigger the collapse is ca. 130 mm/day. Averagerate and the possible maximum value of sediment yield from slopes can be estimated based onthese results. Taking into account of the average depth 1.7 m of the collapses in this area, themaximum value is estimated to be 51, 000 m3/km2 under 400 mm/day rain-fall for the slope of 1.0gradient, and the average depth of erosion over 35 years is to be 1.2 mm/year for 30 degree slope.It should be stressed conclusively that the threshold value of precipitation discussed here is a kindof the potential ability of the nature preventing the occurrence of hazards by slope collapse phe-nomena.