Based on theoretical studies, ambiguous dependence of ultimate density of debris-flow mass on channel slope (debris flow path) and possibility of discontinuous increase of debris-flow mass density, when critical value of channel slope is exceeded, have been established. This allows to develop methods for calculating changes in characteristics of debris flow as it moves in a mountain valley and on a fan, to assess the effects of confluence of debris flow with surface and (or) underground water feeders, to develop optimal measures to reduce damage caused by debris flows by active impact on the rheological characteristics of debris-flow mass, to assess the risk of economic activity in debris-flow dangerous areas, and to develop optimal measures to reduce damage caused by debris flows.

In mountainous rivers, bed load, suspended load and washload are considered to flow at all depths. A Total Load Trapping Equipment system, capable of trapping all sediments within a unit width, was developed and placed at the Oshima check dam located upstream of the Abe River to measure sediment discharge. Based on four observations, the main results are as follows : (1) under normal flooding conditions, the ratio of suspended load plus washload to total load was nearly 80% or more and at times close to 100% in this river ; (2) the suspended load plus washload obtained was greater than that obtained at other rivers ; and (3) the bed load estimated using the Ashida-Takahashi-Mizuyama formula was greater than the measured amount in many cases, although the difference was smaller at peak flow. The bed load estimated using the Meyer-Peter-Muller formula was smaller than the measured quantity, but remained within one order when the sediment discharge increased.

In recent years, with the decline of forestry in Japan, unplanted areas left after clear-cutting have increased rapidly. As shallow landslides occur frequently in clear-cut and unplanted areas, this study newly constructed a landslide risk prediction model, an improvement of a previous model [Kuroiwa and Hiramatsu, 2004], that considers the forest condition, especially whether the forest root system is decayed or developed following forest harvesting. Using this model, we calculated the shallow landslide area ratio in the Miyagawa Dam basin. It was possible to predict the shallow landslide area ratio with high accuracy compared with previous model application. Thus, the proposed model successfully predicted changes in shallow landslide areas of the basin by considering forest harvesting and rainfall.