Abstract
The heavy rain disaster occurred in July 2017, in northern Kyushu, Japan. The river environment greatly changed due to sediment moving accompanied by erosion. It is important regarding disaster prevention to localize watersheds where sediment transportation is active. In this study, the sediment dynamics were discussed on the basis of our investigation about the stored sediment, water-flow, and silica fluxes from sub-basins. As a result, the sediment survey revealed that many sediments consist of sand or gravel and were moved secondarily by water-flow. By the hydrological survey, it was confirmed that the trend of the dissolved silica concentrations varied between the right and the left bank tributaries. It was suggested that the left bank tributaries have a various process of water-flow. Considering the distribution of collapsed slopes, the right bank tributaries have more collapsed slopes than the left bank tributaries. As suggested by the results, the range of the silica flux is wider at the left bank tributaries because the sedimentation shows various distributions. On the other hand, the right bank tributaries were estimated topographically stable since the value of the silica flux is about the same.
Highlights
The July 2017 disaster occurred during heavy rain, in northern Kyushu of Japan, causing serious damage to houses and river structures and producing large amounts of sediment and driftwood.More than half of the 300 reported cases of damage due to the heavy rainfall were caused by debris flows [1]
As the sediment had already been artificially moved in November, the evaluation of the erosion and the characteristics of the flood sediment were based on the results of the August and September surveys
This study investigated the distribution of sediment and silica fluxes in the Otoishi River basin
Summary
The July 2017 disaster occurred during heavy rain, in northern Kyushu of Japan, causing serious damage to houses and river structures and producing large amounts of sediment and driftwood. It was clear that excessive sediment had been supplied from the upstream area of the Otoishi River and the damage had spread to the river valley plain due to a blockage in the river Such a phenomenon occurred due to the over deposition because the river was clogged with the disaster waste such as driftwood and sediments. As a fundamental study to predict the occurrence of the sediment disasters the cannotand be predicted by statistics in research sites which have no data past disasters. As a fundamental study to predict the occurrence of the sediment disasters due to topography and water-flow. Sediment deposition and slope failure locations were investigated and the. Deposition and/or and slope failure locations were investigated and the distribution of silica fluxessediment in the mainstream tributaries were observed in the Otoishi.
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