5:15 PM - 7:15 PM
[HGM04-P05] Relationships between landslides in Hiroshima and the geology and topography during July 2018 West Japan floods
Keywords:Granite, 2018 West Japan floods, Landslide, Hiroshima
Due in part to the effects of global warming, many parts of Japan have experienced unprecedented heavy rainfall disasters in recent years. In the Chugoku region in particular, landslides with human casualties have occurred in large numbers due to the geological characteristics of the region, where easily weathered granite is abundant, and the geographical characteristics, where many residents live on slopes with few flat areas in relation to the population. The aim of this study was to investigate the geology, precipitation, slope and elevation of slope failure sites in the areas of Etajima City, Kure City, Saka Town, Kumano Town, Kaita Town and Aki Ward and Minami Ward in Hiroshima Prefecture that were severely damaged by July 2018 West Japan floods, in order to understand the tendency of landslide disasters.
Most of the areas where slope failures, which are the main cause of mudslide disasters, occur are located on steep slopes, and field surveys are extremely difficult immediately after a disaster due to the risk of secondary disasters. For this reason, more than 2,000 collapse sites were identified using aerial photo interpretation of pre- and post-disaster data from Geographical Survey Institute maps and organised using ArcGIS Pro 3.3. These were overlaid on the Geological Survey of Japan's 1:200,000 seamless geological map of Japan V2 to determine the location of each collapse and its geological characteristics.
The geology with the highest number of failure points was Late Cretaceous massive granite (86.3±0.5-1,005,000 years ago). However, the collapse rates per geological feature were higher for rhyolite and dacite geological features. The distribution of dip angles ranged from a minimum of 1.175° to a maximum of 60.606°, but more than 70% of the collapse sites had dip angles in the 20°-40° range. It has also been noted that the likelihood of collapse increases as the slope angle increases above 30° in both granite and sedimentary rocks, and this study is consistent with such studies.
Most of the areas where slope failures, which are the main cause of mudslide disasters, occur are located on steep slopes, and field surveys are extremely difficult immediately after a disaster due to the risk of secondary disasters. For this reason, more than 2,000 collapse sites were identified using aerial photo interpretation of pre- and post-disaster data from Geographical Survey Institute maps and organised using ArcGIS Pro 3.3. These were overlaid on the Geological Survey of Japan's 1:200,000 seamless geological map of Japan V2 to determine the location of each collapse and its geological characteristics.
The geology with the highest number of failure points was Late Cretaceous massive granite (86.3±0.5-1,005,000 years ago). However, the collapse rates per geological feature were higher for rhyolite and dacite geological features. The distribution of dip angles ranged from a minimum of 1.175° to a maximum of 60.606°, but more than 70% of the collapse sites had dip angles in the 20°-40° range. It has also been noted that the likelihood of collapse increases as the slope angle increases above 30° in both granite and sedimentary rocks, and this study is consistent with such studies.