Estimations of danger of landslides based on a geomorphological investigation of the landslides in Amakusakamishima Island, Kumamoto Prefecture

Abstract

The Geographical Survey Institute made an investigation of the landslides in Amakusakamishima Island caused by the heavy rainfall of July, 1972, mainly from geomorphological viewpoints and made a quantitative estimation of danger of landslides by a statistical method. The report is summarized as follows.1. The heavy rainfall caused about 4, 600 small debris avalanches and very few large slumps of bedrock on mountain slopes of the eastern part of the island. In the aerial photographs taken in 1965, 240 scars are observed and 70 of those were scarred again due to this rainfall.2. These debris avalanches originated a large number of debris flows. Where a hard arkose sandstone of Palaeogene or volcanic rocks are present, debris flows became large and destructive owing to the very large blocks of these rocks.3. Most of the debris avalanches and flows occurred when the rainfall was extremely intensive, between 11 and 13 o'clock of July 6, while the large slumps seems to have occurred after the rain.4. The relation between distribution of debris avalanches and the age of mountainslope was studied. Knick-lines on which the slope gradient changes discontinuously, from the upper gentle to the lower steep, were read in a stereo-airphotographs and were used as erosion fronts for classifying the slope age. An erosion front indicates the borderline of an erosion area which spreads itself with time. The age of those in the south of Amakusa is divided into four.1st, originated in the middle and late Pleistocene.2nd; originated in the late Pleistocene.3rd; originated during the regression of the late Pleistocene and early Recent.4th; originated during the Recent transgression.Debris avalanches intensively occurred in the 2nd and 3rd erosion areas, while not so many occurred in the 4th. It is probable that the 4th area had not had much detritus to move down due to its short periodicity of erosional activities.5. The smaller limit of a watershed above a scar and its average gradient were found to be 40-50m2 and around 1/4 by measuring 1/2, 000 topographic maps.6. An equation for estimating the magnitude of debris avalanche was built by means of multivariate analysis in the area A where the precipitation was very high. Six factors were adopted as the predictor variables, i.e. rock, slope, drainage density, thickness of detritus, vegetation, and orientation of slope. The equation was applied to an estimation for another area B in the island, where the precipitation was much less than in A. The estimated magnitude is much larger than the real one.