The factors of the high risk of the sediment disasters in the Hiroshima district and the problems concerning the transmission of the precipitation information for the warning and evacuation

In Hiroshima Prefecture, there are a lot of danger areas of slope failure disasters and debris flow disasters, and their numbers tend to increase every year. As the natural causes, we can guess that (1) weathered granite and decomposed granite soil cover very widely on the mountainous slopes, (2) vegetation and surface soil layer are still not enough rich, and (3) a lot of unstable sediments are rested on the slopes without being eroded and washed out because of little annual precipitation. Therefore, in Hiroshima area, slope failures and debris flows occur easily even with less storm-rainfall than that in Kyushu and southern Shikoku districts. As a social cause, we can point out that many people live unwittingly in the danger areas of sediment-related disaster. Recent typhoon disasters suggest that the failure prone and debris flow prone situations still continue near residential areas. In order to do successfully the disaster prevention and mitigation, we must give appropriately the rainfall information to people, let them understand the degree of danger, and then lead them to warn and/or evacuate. For this reason, it is very useful that the rainfall situation is plainly represented every moment on the map with real-time observed data