INVESTIGATION OF THE COMPLEX DISASTER CAUSED BY DEBRIS FLOW AND FLOOD IN THE HIGASHI RIVER, SHIMATA RIVER SYSTEM DUE TO THE HEAVY RAIN IN JULY 2018

抄録

 The heavy rain in July 2018 caused debris flow and river flooding in the Higashi River in the Shimata River Basin in the Osogoe district of Iwakuni City, Yamaguchi Prefecture. In this study, we conducted a record of the disaster situation and field survey using an unmanned aerial vehicle (UAV) after the disaster in the Osogoe district. In addition, we conducted the orthoimages production, runoff analysis, and inundation analysis. The results showed that the causes of the inundation in the Osogoe district were 1) the significant rise in the riverbed due to the flowing of sediments and driftwoods carried by the debris flow and 2) the driftwoods blocking up the river flows at the bridge. In addition, The situation of the inundation analysis shows that the inundation would not have occurred without the influx of the debris flow. The area where the debris flow occurred was included in the landslide warning area. Therefore, the management strategy for afflicted areas like the Osogoe district needs to include predicting the direct inflow of the debris flow with a lot of sediment and driftwoods to the river segments.