Assessment of debris-flow hazards of alluvial fans

A strategy is presented for the assessment of debris-flow hazards on alluvial fans on the basis of a case study carried out on the southern foot of a stratovolcano named Mt. Yatsugatake. Transformation of commercial forests into a golf course was planned at a corner of the Kikkakezawa fan. The case study involves an assessment of hazards due to debris flows of different magnitudes and their recurrence intervals. The environmental conditions for the recurrence of these debris flows are discussed as well as the extent of the areas affected by them. In order to generalize the case study, concepts of hazard potential, hazard and risk for debris flows on alluvial fans are established. Accordingly, the hazard potential is the possible hazards at any location on an alluvial fan in an indefinitely long time period, which can be assessed on the basis of hydrological and other geoscientific investigations. Hazards associated with a particular land use can be evaluated on the basis of the hazard potential considering the location and the time period associated with the land use. Risk can be further assessed considering the life styles in this land and the social conditions.