山腹崩壊と地形特性に関して-昭和50年5号台風による高知県下の山腹崩壊を対象として-

Abstract

The distribution of landslides in Kouchi pref. Caused by the heavy rain on Aug. 17 in 1975and the factors affecting them are discussed.Survey work showed that the collapse density (the number of collapse points per 2kmx2km)is in proportion to the gradient of slope (average gradient in the same area) till a certain criticalgradient in the A section where total rainfall is nearly constant, and in the B section where the gradi-ent is nearly constant, the collapse density is proportional to total rainfall (average total rainfallin 2kmx2km).From the results mentioned above, next equation is derived for estimating the number ofcollapse points;N(I, r)=Kir(I-io)p(r-ro)q with I=tan αwhere N(I, r): number of collapse points per unit area, Kir: coefficient, io: critical gradient foroccurrence of collapse, r: total rainfall in mm, ro: critical rainfall for occurrence of collapse inmm, α: average slope angle per unit area and p, q: constant to be determined from field data.Assuming that p and q equal 1 respectively and io equals 0 from the results obtained here, next approximate expression can be applied in practical use;N(I, r)=Kiri(r-ro).As to geomorphological aspect, the collapse density seems to be proportional to the densityof valley heads in A section.