The relationship between geographical features, vegetations and shallow landslides by heavy rainfall in Shobara, Hiroshima in 2010

Ushio KUROKAWA and Yasuhiko OKADA

A very rare and heavy rainstorm occurred between 15 and 18 o'clock on July 16, 2010 and induced over 1,500 shallow landslides and debris flows in Shobara, Hiroshima. The three�]hour event was 173 mm of rain with a maximum hourly rainfall of 72 mm. In this study, the relationship between geographical features, vegetations and shallow landslides was investigated in relation to the landslides caused by the heavy rain. In order to analyze this disaster, aerial photographs, a1�]m�]mesh Digital Elevation Map created by airborne laser survey, and vegetation data created using a Geographical Information System were employed. It was determined that the landslides happened at altitudes between 300 and 650 m. The mean value of the slope gradient on which they occurred was 28.1 degrees, which is steeper than the average slope of the entire analysis area; landslides on slopes in excess of 40 degrees were widespread. Moreover, the slope�]failure�]area rate of the landslides was highest on northeast slopes. The vegetation coverage by area in the areas that experienced landslides existed in the following order: Sawtooth Oak (Quercus acutissima), Japanese cedar (Cryptomeria japonica) and Japanese cypress (Chamaecyparis obtusa). The landslide which was locating the Japanese oak existed in the steep place as compared with other forest type. 95% of the forest of the study area was less than the 16 age class and the forest of the 11 age class occupied the largest area. The slope�]failure�]area rate was very high for forests of age class 1, 2, and 6.