“HOKAI-DANCHI” (DEBRIS-AVALANCHE CONCENTRATED AREAS) AND THE MECHANISM OF THEIR OCCURENCE

Abstract

The western part of the maturely dissected Tanzawa Mountains nearly 100km. west of Tokyo is well characterized by a great number of debris-avalanches. Some of them, however, are very densely distributed in a limited number of small areas. The writer of this paper names such areas as “hokai-danchi” (debrisavalanche concentrated areas) and sought for the cause of the occurence of “hokaidan-chi”. His research was primarily concerned with slope distribution, soil texture, soil depth, micro-relief and so forth in and around the areas. As a result, the following facts were discovered. The immediately lower areas of “hokai-danchi”, which are always located at the heads of streams, have steeper slopes, coarser soil textures, thinner soil depths, and more rugged textures than in the case of “hokai-danchi”. Furthermore, there always exist remarkable knickpoints on the boundaries between “hokai-danchi” and the immediately lower areas. Thus, he came to the following conclusion. These “hokai-danchi” have been protected for a long time from vigorous down-deepening and accompanied denudation because of the existence of such krick points. For this reason, the soil in “hokai-danchi” is now thicker than that of the immediately lower areas and also shows homogenous depth in general, causing almost homogeneous reaction thi oughout the “hot ai-danchi” against heavy rains or big earthquakes both of which are motives of debris-avalanches. Such a process, the writer presumes, may be a major cause of the existence of “hokai-danchi”.