Abstract
The purpose of this study is to forecast the area of landslide occured by heavy storm and the author consider the mechanism of landslide as follow.
Phenomena raised by the stored water infiltrating through rainfall such as the increase of soil weight, decline of soil strength and porepressure are some kind of shocks acting sloped. When many various shocks are accumulated in the solpe and mechanical change reaches up to the critical stage, landslide will break out.
If the shocks happen inn proportion with precipitation, at random in watershed composed with the unit slopes that are independent each other, the state of shock occuring in the unit slope would be approximated by the Poisson Process in the probability theory.
Consequently, the probability F that an unit slope may slide under total precipitation r, is equal that shock number X(r)=j reaches upto the critical value C, and given by the next gamma distribution model.
F(X≥C, r)=1-∑C-1j=0(λr)f/j!e-λr
where λ is the shock rate 0.01(mm-1) and C is the resistance index characteristic to region.
Such mechanism acts in every slope, therefore this equation means the relation between total precipitation r, resistance index C and the theoretical landslide area ratio F of watershed.
This relation was confirmed in results of surveying landslide area by using aero-photographs which scale ratio is 1:10, 000 at Fujiaka, Niyodo, Minami Izu and Nozoki region suffered from heavy storm recently.
In the case of forecasting landslide area, next equation based upon the idea that many slopes are adapted to the regional peculiar storm in each watershed, is useful to estimate the resistance index C,
C=0.0433J
where J is the mean monthly precipitation in July.
