Abstract
Identifying the risk of debris flow for each torrential stream is an effective disaster prevention measure and should be useful for determining the sites to take countermeasures and the order of priority among the sites. However, methods that are now proposed need precise site surveys and complicated computation and are inappropriate for determining the risk for a number of torrent stream channels. It is more efficient to estimate the risk of debris flow using desk check data and not performing site surveys, and the grounding of such a determination should be easy to understand. In this study, the authors investigated the topographic conditions of the stream channels that suffered debris flows in July 1982 in Nagasaki City using topographic maps, and prepared a database consisting of topography, geology, rainfall, and disaster records. The rough set theory was then applied to the database to derive objective rules of debris flow generation and non-occurrence, and the validity of the rules was verified. The authors also attempted to determine the risk of debris flow for a steep torrent channel that was found to be in conformity with so extracted rules of occurrence and non-occurrence of debris flows.
