急勾配山地からの土砂流出に関係する流域諸要因

抄録

Topographic, geologic and rainfall factors affecting the rate of sediment discharge from steep mountain basin are investigated by analyzing the data of deposited sediment in sediment control dams in mountain area of Northern Kanto, Central Japan. Almost linear ralations can be recognized between average annual volume of accumulated sediment per unit drainage area (q_s) and mean gradient of basin (S) in log-log papers with each geological division. It can be derived that the erodibility of plutonic rocks is half as large as that of young volcanic body and four times as large as that of Paleozoic rocks by vertical distances of the regression lines. Computed values by the regression equations of which variables are drainage density (D), valley bed slope of upper reaches of dam (I) and S agree quite well with observed values. By using the close relations between S and q_s in short periods when heavy rains occurred, it is inferred that the maximum rainfall intensity for three hours (R₃) has the largest effect on sediment yield. The equations for estimating sediment yield of which variables are S, D and R₃ are presented.