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

Abstract

 The heavy rain in July 2018 caused debris flow and river flooding in Higashi river, Shimata river system in the Osogoe area of Iwakuni City, Yamaguchi Prefecture. In this study, we conducted a record of the disaster situation and field survey using the UAV after the disaster in the Osogoe area. In addition, we conducted ortho-images production at the time of disaster, debris flow analysis, runoff analysis, and flood analysis. As a result, it is clarified that the flood in Osogoe district was caused by significant aggradation of riverbed due to the sediment and driftwood carried by the debris flow into the river and the obstruction of the flooding water due to driftwoods being caught in the pier. Moreover, there is a high possibility that river flooding would not occur if debris flow did not occur. Therefore, our results suggest that we need river management that takes into account the possibility of river flooding due to the complex disaster in river sections, where debris flow is assumed to flow directory into river.