Author(s): Takashi Wada; Yuki Kajikawa; Hiroshi Miwa

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Keywords: River sediment dynamics; Riverbed variation; Riverbed surface grain size; Planar two-dimensional riverbed deformation calculation; Typhoon Lan

Abstract: In Japan, since many dams and weirs are interrupted the sediment transport in a river channel, the river sediment dynamics have inactivated. This reduces the frequency of the topographical changes in a river channel, resulting in the fixing of the low-flow channels and sandbanks. Solving these problems, we must first understand the mechanisms of sand and gravel movement in a river channel during a flood. In this study, we investigated time-series changing processes of riverbed deformation and surface grain size distribution during a large flood through planar two-dimensional riverbed deformation calculations and field surveys. The study area encompasses the Tenjingawa and the Mitokugawa rivers in the middle part of the Tottori Prefecture, which suffered significant riverbed deformations due to severe flood in August 2023. The results also qualitatively reproduced the trends in the spatial distribution of mean grain size on the surface of the three sandbanks measured with photographs. These suggest that our calculation model can qualitatively reproduce river sediment dynamics caused by the flood. In addition, our calculated results indicated that riverbed deformations can terminate within the period of flood hydrograph curve formation, and that the spatial differentiation of riverbed surface grain size is formed during the flood attenuation period. The calculated changed thickness of riverbed with considering vegetation was slightly larger than that without considering vegetation whole the study area. This may be due to small flow resistance due to the vegetation because the vegetation in the study area was dominated by herbaceous. However, the flow resistance of herbaceous has larger effect on the changes in the riverbed surface grain size than that on the riverbed deformations. Therefore, the calculation with considering the flow resistance due to the vegetation in a river channel is needed to predict the sediment dynamics.

DOI: https://doi.org/10.64697/978-90-835589-7-4_41WC-P2070-cd

Year: 2025