土石流の実態とその発生条件に関する研究 : 高知災害を中心として

Abstract

土石流の実態をその発生条件を中心に調べた。対象としたのは昭和50年8月の高知災害における荒廃流域および過去に土石流が多発した花崗岩地帯である。その主な結果は次の通りである。1) 高知災害では構造性大崩壊にもとずく土石流も散在するが大部分は急勾配 (30°以上) 1次谷が崩落し, それと2次谷上部を発生源として流下し, 5° - 10°の個所で堆積した。2) 花崗岩地帯でも1次谷に始点を持ち2次谷上部を発生源として流下し5°以下の個所で堆積する。3) 花崗岩地帯の1次谷の土石流発生率は1災害で40 - 60％に達する。高知では20％前後で高次谷ほど当然発次率は高くなる。急勾配渓流が多いほど発生率は勾配の影響が強くなる。4) 土石流を発生した流域には明らかに低次の谷, 山腹斜面の崩壊が多発している。土石流発生条件の一つとして検討すべきであろう

In this paper, we examined the conditions for mud flow occurrence, illustrating the mud flows which occurred in Kōchi (1975) and in granite regions. The results were as follows: 1) Examining the mud flows at two catchment areas where many mud flows had occurred, it were made clear several characteristics of them in Kōchi and could be presumed the sediment yield to be brought with the mud flow and other erosion. 2) The mud flow was made with the severe erosion or the collapse of deposits in the lower part of 1st order stream or the upper part of second order. And it eroded the bed and side of 2nd order stream, then stoped at the areas of the gradient less then 10°(in Kōchi) or 5°(in granite regions) 3) In granite regions, the rate of numbers of mud flow occurrence in 1st order streams was 40 or 60%. In Kōchi, this rate was nearly 20%, and the rate of 2nd order streams was 40 or 60%. The rate of numbers of mud flow occurrence depended strongly on the gradient of stream. 4) It was recognized the occurrence of many collapses of the slopes and 1st order streams in upper part of mud flow. It will be necessary to investigate the role of the collapse for occurrence of mud flow.