Massive sediment movements take place in steep areas when an external factor such as rainfall, volcanic activity, or seismic motion exceeds some critical level, and can cause sediment disasters of potentially high cost. On the other hand, even if a large size debris flow takes place, nobody will recognize the occurrence of a disaster in the area if there are no human activities. Japan, located along the tectonically active circum-Pacific island arcs, is a mountainous country about seventy percent of which is steep and hilly areas, with a rainy season and frequent typhoons yielding severe...