A method for delineating restricted hazard areas due to debris flows

ABSTRACT: To prevent debris-flow disasters in Japan, structural and non-structural measures for debris-flow disasters have been promoted efficiently. In April, 2001, Japan enacted a Sediment-Related Disaster Prevention Law to protect lives and bodies from landslides and debris flows. The purpose of the law is to promote measures for preventing sediment-related disasters, arrange warn-ing and evacuation systems in hazardous areas, and regulate specific types of development within areas likely to be susceptible to landslides and debris flows. At the end of 2006, around 9300 re-stricted areas at risk from debris flows were designated under the law. In this report, we state the idea of delineating hazardous areas through verification of past debris-flow events. We propose methods of identifying areas subject to debris-flow inundation and areas debris flow will not reach. Using our methods, we identify areas where warning and evacuation systems should be established. We also identify areas where development should be restricted by considering the range of forces likely to be exerted on buildings within the inundation zones and comparing those forces to the strengths of the affected buildings (wooden structure).