Journal of the Japan Society of Erosion Control Engineering, Vol.70, No.5, 2018

Proposal of a new idea for designating hazard areas based on travel distance of sediment and woody debris discharged from rainfall|induced landslides in Aso caldera walls

Osamu SHIMIZU, Ryuuhou KODAMA and Mio KASAI

The northeastern caldera walls of Aso Volcano in Kumamoto, Japan have a relief of approximately 300m. The walls form long steep hillslopes, in which the upper part has a slope of more than 30degrees and the lower part has a slope of 20 to 25 degrees. During an extreme rainstorm in July 2012, many landslides were initiated from the upper parts of the hillslopes and the hillslopes collapsed over a long distance. Eroded sediment and large woody debris reached approximately 200m from the toes of the hillslopes, causing serious damage in residential areas. The present criterion for designating landslide hazard areas does not include the hillslopes in which the horizontal distance between the upper steeply inclined segment (more than 30degrees in slope) and the footslope residential areas exceeds 50m. A new criterion applicable to long]runout landslides in this kind of hillslope is required. We researched the distribution of sediment and woody debris in depositional areas of five selected landslides using LiDAR DEMs and aerial photos, analyzed sediment depth and wood size that can affect damage to houses, and discussed a new idea for designating the hazard area and for partitioning the area based on the hazard ranks of house damage. The landslide hazard area should be designated as the area within a distance of 1.5 to 3 times the hillslope height from a concave break line in slope. On this occasion, it is important to designate the hazard area regardless of the maximum distance 50m in the present criterion even if the height of the hillslope is large. The hazard area can be partitioned into three hazard ranks based on the depth of sediment deposited and the amount of large wood transported.

Key wordsFshallow landslide, long runout, hazard area, woody debris, Aso Volcano

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