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

Linked Author(s): Takashi Wada, Yuki Kajikawa, Hiroshi Miwa

Keywords: Replenishment Sediment; Trees in River Channel; Two-Dimensional Computation of Flow and Bed Deformation; UAV Photo Surveying; Mean Annual Maximum Flow Rate

Abstract: In the Hinogawa River, which flows through the Western Tottori Prefecture in Japan, the specific gravity separation of iron sand using the river flows was predominant about 300 years ago. Therefore, a large amount of sediment transport was flowed down in the river, which greatly advanced the shoreline of the Kaike coast to the vicinity of the river mouth. However, after the specific gravity separation work was ended about 100 years ago, the amount of sand flowed down in the river decreased. At the moment, the shoreline has receded to a maximum of approximately 300 m. Therefore, several replenishment sediment have been attempted for the enrichment of the sediment transport in the river channel and the maintenance of the shoreline. In this study, using the two-dimensional numerical model of flow and riverbed deformation which are based on the model developed by Kajikawa and Hinokidani (2013), we investigated the transport volumes of the replenishment sediment composed of sand and gravel mixtures derived from excavated riverbed sediment. In our investigation, the flow resistance of trees in the river channel was considered. The flow resistance was estimated with the composite roughness model based on tree heights proposed by Tomidokoro et al. (2001). The tree heights were measured using UAV photo surveying in the actual river channel. Our results suggest that the representative particle size (d60) and the sand content (fs) of replenishment sediment are the suitable parameters for the optimum composition which allows for sufficient transport. By adjusting these parameters with removing the maximum grain size component from the materials, it is possible to identify the effective composition of replenishment sediment for the target flow rate. In addition, this suggests that it is possible to roughly predict the transported volume approximately by comparing the dimensionless tractive forces for the target runoff rate around the replenishment sediment and the dimensionless critical tractive forces of their representative particle sizes. Our results also indicated that the flow resistance of trees on the fixed sandbank disturbed the transport of replenishment sediments when the runoff scale was larger than the mean annual maximum flow rate, while the resistance has an insignificant effect on the transport when the runoff scale was smaller than the mean annual maximum flow rate. Thus, our method can predict the reasonable transportation process of replenishment sediment with several runoff scales under actual river conditions, such as the distribution of tree heights on sandbanks.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022847

Year: 2022