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.
Improper forest management frequently causes serious damage to hill slopes and torrents. This study examined the influence of annual variation of deforested and reforested areas on slope failure in Miyakawa-dam basin, Mie prefecture and Nagasawa-dam basin, Kochi prefecture. The followings were observed in this study: 1) Collapsed area is proportional to the non-forested area and inversely proportional to the reforested area. 2) Slope failure frequently occurred after 5-7 years of deforestation in both the basins. In reforested sites of Miyakawa-dam basin, slope failure declined after 20 years of reforestation. On the other hand, many slope failures occurred in non-forested sites even after 24 years of deforestation. In Nagasawa-dam basin, there occurred little slope failures after 10 years of deforestation. Using these findings, a slope failure prediction model considering precipitation and forest management conditions was developed, and the model was applied to Miyakawa-dam basin and Nagasawa-dam basin over a 30 years simulation period. The comparison of the measured and simulated collapsed area ratio showed that the model was adequate for predicting the slope failure areas of both the basins.
Landslide hazard prediction can provide very useful information or technical supports to regional landslide mitigation and control. Now there are many methods and examples of landslide prediction, but statistics methods are usually used methods, in which, quantification II approach is the most used. On the other hand, being an effective tool, GIS has been used in many regions now, including landslide prediction. In this paper, we present a combination of quantification II approach and GIS techniques to predict landslide hazard in HAYACHINE Mountain ranges, Japan. The details are showed as follows: (1) Based on the recognition, delineation and analysis of existing and past landslides (derived from aerial photograph interpretation) and factors impacting on regional landslide (termed “causal factors”, such as slope gradient, slope aspect, geology, soil, precipitation, slope type, vegetation and elevation, they were extracted and analyzed with the help of GIS techniques), landslide and 8 causal factors grid maps were produced respectively, each of them has the total of 1024*1266 pixels and pixel size is 9*9m ; (2) 372 samples (pixels) were selected from each grid map. Based on these samples, the regional landslide prediction model was obtained by quantification II approach. Through testing with the overall pixels in research area, the precision of landslide prediction showed 80.49%; (3) the landslide hazard zones were classified by the probability of landslide occurrence in the research area.
There is a tendency that the dangerous area of slope failure is increasing cause of the residential land development nearby mountains district part around city. An enormous cost and time are necessary to take construction of countermeasure facilities to every dangerous area of slope failure. Therefore, The establishment of objective priority order of construction plan is needed. In this paper Authors proposed the objective establishment method on priority order of construction by using Data Envelopment Analysis.And Validity of the method is proved engineering instances of the construction.