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Modeling of Sediment Transport Process in Drainage Basins

Author(s): Menglu Qin; Daisuke Harada; Shinji Egashira

Linked Author(s): Menglu Qin, Shinji Egashira

Keywords: Basin-scale sediment transport modeling; Sediment sorting; Dam sedimentation; Bed variation

Abstract: Sediment deposits in dams reducing a large water storage capacity are a common issue in Japan. However, the dam also has a huge impact on the basin scale sediment transport causing environmental issues. This study develops a numerical model to estimate the sedimentation in the dam reservoir and the sediment transport from the catchment area to the downstream, which is expected to contribute to making a comprehensive maintenance plan for sediment issues in the dam reservoir and conserve the river environment. The upstream of the Oi-river basin in Japan is selected as the study area, where two large hydroelectric dams (Hatanagi-No.1 Dam: 326km2; Ikawa Dam: 459km2) located along the main stream and landslide and slope failure deposits widely cover the most upstream area. The model is developed by combining a grid-cell based rainfall runoff inundation model (RRI model) with a one-dimensional unit channel based sediment transport model. The dam is represented as one grid-cell in the RRI model and a single unit channel in the unit channel model, where defined as a sink for the sediment transported into it. The total storage volume of the dam is given as an initial condition and the outflow discharge must also be set in advance. So the capacity of the dam timely changes by calculating the difference between the inflow and outflow of water and sediment discharge. This study conducts a simulation of nine flood events (from 2011 to 2015) that occurred in the study area. The model simulates that in the main river channel, the bedload and suspended load increase from the upstream to downstream due to the discharge increases, while, the bedload decreases significantly when the flow reaches the reservoir correlates with the significant slope decreasing. In contrast, the suspended load is less sensitive to respond to such rapid slope decreasing and remains a high sediment concentration, thus suspended load is transported further and mainly deposits near the dam. And the bed material size finning and coarsening correlate with the bed aggradation and degradation, respectively. However, in both dams, the sedimentation amount in the reservoir fit with the observed data well in the first four floods, which contributed more than half of the sediment amount in total. After that, the sedimentation increases slowly, due to the sediment supply from the upstream significantly reduced induced by the degradation with bed material coarsening in tributaries. In conclusion, this model is suitable to estimate the flood event period sedimentation in the dam and the sorting process in the basin. For the long-term estimation, the sediment supply from the hillslope and bank should be incorporated into this model.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221422

Year: 2022

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