In this study, the following facts were made clear through on experiment. Firstly, the type of sediment yield in an area of an active volcano can be classified into that of slope failure or that of the generation of a gully, depending on whether or not the slope is covered with volcanic ash. In the latter type, it has a tendency to give rise to toe failure (slope failure) at the foot of walls of a gully. Next, the relation between the scale of the toe failure and the mobilization of its mass was made clear by using the theory of energy for a shear fracture. Falling ash not only influences the development of gullies directly, but also the action of sediment yield rapidly becomes active, and the chance of the occurrence of mud-debris flow becomes more frequnt.
Alluvial fans or cones formed by debris flow are investigated on shapes and areas using specific topo maps. The shapes are classified into tree. The relationship between fan areas and their drainage areas and one between the sizes of channels and fan areas are found. Debris flow depositional process is observed carefully with a scaled alluvial fan model. The depositional process and deposition areas depend on the amount of supplied sediment. A mathematical model is built to estimate the debris flow depositional processes. Applicability of the model and sensitivity of the coefficients in the model are examined comparing with the experimental results. The calculated result shows good agreement to the experimental result.
Many kinds of development have been so violently increased in upstreams of forested watersheds that natural own characteristics of watersheds have been changed and natural disasters have been frequently generated. Man induced destruction of nature has climbed up to upstreams of watersheds by road construction, water reservoirs, mining, agriculture, grazing, wood production and others. Therefore, the necessity of watershed conservation is strongly requied for effective land use. In undeveloped areas as Tohoku and Hokkaido, developments have been carried out in forested area of upstreams and changed natural characteristics as well as hydrological and erosional ones. Especially, the affects of wash load in the lower reaches of the river became a serious problem on applied hydrology. The managements of forested areas occupied the major portion of the river basin have an important role on the control of the wash load. The purpose of this paper is to clear up characteristics of forested watersheds related to water and soil conservation. Relationships of hydrological factors to erosion phenomena as surface erosion, landslides and debris movement were discussed from the previous results.