Abstract
Total rainfall of 193mm which included maximum hourly rainfall of 50mm, triggered 31 slope failures in a weathered granite area of 0.94km2 in Aburayama Citizen's Forest, Fukuoka, on June 29, 1999. Slope failures' average size and average sediment volume were estimated at 91m2 and 98m3 respectively. Relative cumulative frequencies of slope failures with respect to their area and sediment volume are expressed in the logarithmic normal distribution. Also, relative cumulative frequencies of slope failures with respect to slope gradients and shape ratio are expressed in the normal distribution. 93.5% of slope failures occurred on steep slopes that were artificially cut or filled for road construction. Cut slopes seem more susceptible to failure compared with filled slopes. 42% of slope failures occurred on concave slope, 32% on straight slope and 28% on convex slope. Flow types, which traveled more than two times of the length of rupture surface, were observed at 45.2% sites, and caused more damage.
