Evaluation Studies of Mountain Sediments and Supply Source in the Rokko District with Potentiality for Sediment Disaster

In Rokko district, many cities are developed between Osaka and Kobe, and these cities have suffered from numerous disasters caused by the sudden discharges of large amounts of sediments. To prevent and /or to forecast the disasters, geomorphological and geological characteristics as well as those of engineering technology were investigated in detail. A new conception "movement potentiality" and "occurrence potentiality" were established in order to present the "preventative geological map" and "hazard map". The results obtained can be summarized as follows.
The mountain sediments (torrent bed and slope sediment) were classified with the concept of movement potentiality. Concerning the cross section, the torrent bed sediments were divided into four kinds of depositional surfaces of S- I to S-IV according to the relative height from the present torrent bed. For the longitudinal-section, the torrent bed sediments were divided into three sediment stages (expressing the amount of sediment) and ten morphological types based on the bed inclination and the catchment area (expressing force of the stream). The slope sediments were classified into seven types based on the concept of scree.
A supply system of mountain sediments was examined mainly by evaluating the correlation between the geomorphological and/or geological condition and collapse in Rokko district. The lithologic character regulates grain size and resistivity against weathering, and the existence of fault regulates the scale and the occurrence position of the col-lapse. In the Rokko district, the weathering profile of granitic rocks has a piling structure of younger weathering zone upon the older weathering zone, the latter being formed more than several hundreds of thousands of years ago.
The movement of mountain sediments can be understood as the fluctuation of occurrence potentiality of sediment disaster through quantitative change of the torrent bed sediments in the longitudinal section. From this point of view, the occurrence potentiality of sediment disaster was determined by evaluating quantitatively the amount of sediment and potential energy of the mountain sediments.
The River Hirami area (catchment area 0.370k㎡) in Rokko district was taken as an example for establishing a preventative geological map. In this paper, hazard map of 1 : 2,500 scale was presented in which various factors such as slope inclination, geology, sediment, collapse land in the past (desolution condition ) and vegetation were taken into consideration.
For the mountain sediments on the fractionaized slopes, the occurrence potentiality of sediment disaster was evaluated. Based on this evaluation, the amount of sediment from the torrent bed and bank were determined. The calculations were made based on the sediment discharge at a probability of exceedent rainfall in 200 years according to the tractive formula of Meyer Peter.
Finally, significance of the presented "preventative geological map" for the prediction and forecast of the sediment disasters was discussed.