It is important to take appropriate non]structural measures based on the Sediment]Related Disaster Prevention Act in order to reduce the number of victims from large]scale debris flow disasters\i.e., debris flow disasters induced by the failure of natural]dams or the deposits of volcanic ash]fallihereinafter called “volcanic debris flow"j. According to the Act, the national government is responsible for providing “Emergency Information on Sediment]related DisastersiEISDj” to the municipal and prefectural governments, which is supposed to be issued on the basis of “Emergency Investigation". Basically, EISD provides information about the areas that are hazardous and the periods for people in those areas to stay away. Technologies need to be developed to improve the precision of EISD, in particular information about the periods, in order to enhance the non]structural measures. The volcanic debris flows of active volcanoes, such as, Sakurajima and Mt. Shimmoe in the Kirishima mountain range, are fine examples that cause great concern to this day. The periods in which higher probability of volcanic debris flow occurrence are determined not only by rainfall intensity but also by multiple factors, including the physical and chemical characteristics of ash]fall, quantity of ash]fall, topography, surface geology, etc. This study, which aims to enhance the precision of EISD on volcanic debris flows, proposes a statistical analysis methodiSAVERjto be applied to the analysis of the relationship between the occurrence of volcanic debris flow and the index taking into account both of the rainfall and the ash]fall intensities. From the results of this study, it became possible to assess the risk of volcanic debris flow occurrence considering the effects of the ash]fall deposits. The method also enables the establishment of non]linear critical lines for volcanic debris flow risk that can be modified according to the intensity of ash]fall. Thus, the critical lines are highly compatible with the criteria for issuing the “Warning Information on Sediment]related Disaster" and, hopefully, the lines can also be used for determining the time of termination for “Emergency Investigation".

Key wordsFSediment]Related Disaster Prevention Act, Emergency Information on Sediment]related Disasters,statistical analysis method, debris flow, volcanic ash]fall