Hydrological models, which simulate rainfall infiltration and groundwater generation at forested hillslopes, have been proposed for predicting shallow-landslides. However, model performances were not adequately evaluated in comparison with observed rainwater infiltration processes. These models require soil hydraulic parameters, which describe water retention characteristics and hydraulic conductivities, as input data. While these parameters were usually determined in a laboratory by using soil samples, it was suggested that in-situ infiltration processes are not adequately described by using the laboratory-determined hydraulic parameters. In this study we examined three methods to determine hydraulic parameters; the traditional laboratory methods for measuring retention characteristics and saturated hydraulic conductivity with estimating unsaturated hydraulic conductivity by Mualem's model (Method 1), the combination of the laboratory and field methods (Method 2), and the field method which uses vertical profiles of water content and matric pressure observed in-situ during a storm event (Method 3). The accuracy of each method was evaluated by comparing observed and predicted water contents and matric pressures for 10 storm events. Results showed that Method 1 tended to overestimate water storage capacity of soil and under estimate hydraulic conductivity, resulting in a delayed wetting front propagation. On the average, the error between simulated and observed water content changes for Method 1 was 5.84 times greater than that for Method 3. Method 2 produced better prediction results than Method 1, but performed worse than Method 3. In conclusion, Method 3 was the suitable method to determine hydraulic parameters for describing rainwater infiltration processes at forested hillslopes.
Scientific and practical education method for sediment-related disaster prevention was presented for schoolchildren. This method was based on the combination of seminar in debris flow prone torrents and teaching hydraulic apparatus use in a classroom. We conducted three-hours classes in May and June, 2005, based on the above-mentioned method for each three elementary schools in the Appetsu River watersheds, Hokkaido, where was damaged by the 2003 typhoon No.10. We taught the schoolchildren conception of watershed, occurrence of sediment-related disaster due to slope failure and debris flow in watershed, role and its criteria of Sabo facilities, critical rainfall amount for debris flow generation, importance of early evacuation. Changes in the schoolchildren understanding of the contents of the class were investigated through three times questionnaire.
In these years, they construct many sediment discharge control dam (slit sabo dams) in Hokkaido, but cases drift woods blockade slits of the dams during floods within the scope of the sabo plan are be confirmed. To reduce this troubles, we need devices to prevent drift woods from blockading slits with dams itself, and structures trapping drift woods. In this study, we devised new type dams based on the actual conditions of slits blockade by drift woods. And we estimated the function through hydraulic model tests, achieved mentioned below conclusions. 1) We can obtain the equivalent effect of sediment control of an usual slit dam, with the new type dam whose lower about fifth of slit width was doubled and other higher slit width was reduced to half. 2) We can reduce slit blockades by drift woods with the new type dam when the water level is over the top of wider slits. Because drift woods sink and flow downstream through the wider slit. We need the wider slit is over drift woods to prevent blockading slits for certain.