[H-GM03] Geomorphology

[HGM03-P02] Relationship between three-dimensional morphology of debris-flow fan and affected area of debris flow -A case study of the 2014 Hiroshima torrential rain disaster-

*Yuna Yazawa¹, Takuro Ogura², Shunsuke Harada¹, Tsuyoshi Hattanji², Shoji Doshida³, Tanaka Yasushi⁴ (1.College of GEOSCIENCE, School of Life and Environmental Sciences, University of Tsukuba , 2.Faculty of Life and Environmental Sciences, University of Tsukuba., 3.National Research Institute of Fire and Disaster, 4.Faculty of Letters, Komazawa University)

Keywords:Debris flow-fan, Debris flow, Microtopography, Artificial structure, 1m DEM

This study aims to clarify the relationship between the three-dimensional shape of debris flow fans and the extent of debris flows and depositions by visualizing artificial structures and micro-topography on the debris-flow fans using 1m DEMs. This study targets two debris flow fans in the Midorii area, Asaminami district, Hiroshima City, Hiroshima Prefecture where debris flows caused by heavy rainfall damaged residential areas on August 20th, 2014. Although the geology of source areas and magnitude of the debris flows are similar, the sizes of debris fan are different. This study measured the topographic features of the debris flow fans using aerial photographs and DEMs. The topographic changes before and after the disaster were calculated from the pre- and post-event DEMs. In addition, ideal fan surface DEMs were created to visualize the micro-topography on the debris flow fan surface. The actual DEMs were compared with ideal fan surface DEMs to describe the characteristics of their landform. As a result of the topographic analysis, the debris flow moved on the fan surface in the direction of a relatively lower elevation. In the middle part of the fan, the debris flow selected a steeper road as a flow path. Furthermore, the debris flow also spreads in the direction perpendicular to the flow direction by entering into the gentler part at the edge of the fan. These results show that movement and structures of the debris flow depend on the distance from the top of the fan. The most important fact determining the direction of debris flow is the shape of the micro-topography near the top of the fan. It may also have been affected by the widening of the road and the flattening of the land due to the construction of residential areas. In addition, artificial structures such as houses impeded the movement of the debris flow and promoted the change of the flow direction. This pattern of debris flow movement was observed in both of the two debris flow fans. The direction of the debris flow was regulated to some extent once it entered into a road. The debris flows selected the road with a steeper slope close to the original flow direction. This is because of the relative difference in elevation between the roads traversing the debris-flow fan and residential areas. In addition, two points may have contributed to the stoppage of the debris flow. First, the fan surface has a gentle slope approaching the fan edge. Second, the difference in elevation between roads and houses tends to become smaller.