Study on sediment disasters in Tennou, Kure city, Hiroshima prefecture and sediment and flood damage in downstream area

Abstract

Due to heavy rains in July 2018, many sediment disasters occurred in western Japan. Not only were there direct damage caused by debris flows, such as destructed buildings, but some places several kilometers downstream from the landslides and debris flows also experienced flooding that lasted several hours and sediment deposition from the rivers. In this study, we focused on Tennou, Kure City in Hiroshima Prefecture, aimed to study the disaster situation in detail, and applied simulations. In Tennou, debris flows occurred and large boulders moved upstream. Sediment movement occurred but did not cause flooding outside the midstream river. Deposition and flooding occurred outside the downstream river. Downstream, deposition occurred at the upstream side of the box culvert but did not occur downstream; thus, it was assumed that blocking due to sediment occurred at the culvert. In the simulations, we considered the culvert blocking, and also considered sediment runoff continuing for many hours, which was different from shorttime debris flow. From the simulation results, the flooding and deposition outside the river first occurred at the blocked culvert point. However, as the runoff continued, the riverbed rising from deposition became larger in the upstream side. Flooding and deposition outside the river also occurred in the upstream area from the culvert.