The critical rainfall conditions triggering topsoil failures on slopes are presented with functional relationships include the strength parameters (C, Φ), saturated coductivity (Ks), steepness (θ), soil depth (D) and length (L) of the hillslopes. On the basis of the virtical one-dimensional slope stability analysis and the sectional two-dimensional kinematic-wave runoff analysis with Darcy's low, rainfall thresholds are derived as follows. Value of the cumulative rainfall threshold (Rc) depends on the C, Φ, D, θ and effective porosity (w') . Value of the rainfall intensity threshold depends on C, Φ, Ks, D, θ and L. The threshold curve on cumulative rainfall-intensity relationships derived with the initial water loss medel, accounting water absorption through virtical unsaturated infiltration, can be fit firly well in the other threshold curve which had been derived from the response analysis for slope failures in Japan using the antecedent rainfall index.
Sakurajima Volcano has erupted continuously with emission of ash since 1955. The frequency of eruption has varied with time. Effects of the temporal variation of eruption on rainfall for occurring the debris flow, peak discharge and total runoff of the debris flow in the volcano were examined. The rainfall intensity for occurring the debris flow is smaller during the more frequent volcanic eruptions than during the fewer eruptions. The rainfall onditions for occurring debris flow depend on the volcanic activity. Both the peak discharge and the total runoff of debris flow are larger during the more frequent eruptions than during the fewer eruptions under the same conditions of rainfall. The runoff characteristics of debris flow in the volcano depend on not only the rainfall but also the volcanic activity.
A process to estimate effective rainfall in a small mountain watershed as a premise for estimating the flood hydrograph using the Unit-hydrograph method was studied. The effective rainfall of each flood can be estimated through deducting infiltration losses from the rainfall in the watershed. The method to estimated infiltration capacity is based on Horton's equation of infiltration capacity curve, where the initial infiltration capacity f0, final infiltration capacity fc, and extinction coefficient k were analitically estimated according to the ydrological data. As a result, f0 can be regarded as equal to rainfall intensity. The value of fc revealed as constant. But varies over a long period by the recover of vegetations. Simultaneously, it was found that the coefficient k varies with rainfall intensity and duration. As the recovery in infiltration capacity during a drought is thought to be due to the depression of soil water content, the equitable value of the extinction coefficient β is estimated by a simulation method through setting up an equation between the soil water content and the infiltration capacity of the watershed. According to the consideration of these results, an estimate process of the effective rainfall in time series was proposed. By applying the proposed estimate process of hydrological analysis concerning the effective rainfall in the Ashio experimental basin, the process demonstrated reasonable results.
A lot of slope failures and debris flows occurred at the northwestern district of Hiroshima Prefecture, Japan, because of the storm rainfall on July 20 and 21, 1988. The authors have investigated this disaster from the many aspects. In this paper, the effect of Sabo works on the prevention of debris flood and the sediment budget in these area will be discussed. The results are followings: These debris flows had the tendency of the high mobility and the inclination of the sediment at most of check dams was much lower than the expected values. The mobility of the debris flows depended on the geological characteristics and/or on the poor frequency of the sediment run-off. The number and the scale of the Sabo works in this area are too small to prevent the next debris movement in future.