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

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

Keywords: Sediment supply; Sediment replenishment; Non-uniform sediment; Numerical simulation; Hinogawa River

Abstract: For the conservation and restoration of river environment, a sediment replenishment technique, which conveys a part of the sediments excavated from reservoirs to the river below dams, is developed and has been implemented tentatively in several dams in Japan. Sediments placed as replenishment can be flushed out and transported downstream by floodwater or dam releases. In recent years, this technique has been considered to apply to middle and lower reaches of a river as a countermeasure against sediment shortage in river channels. Since excavated sediments may often contain large gravel and stone, it is necessary to consider how to use them as the replenishment sediment. Practical use of large gravel and stone is also valid for utilization of the excavated sediments in the river improvement works. On the other hand, when fine sediments (i.e., sand) are present in streambeds composed of gravels, the gravel can be more easily removed from its original position, compared with gravel in a streambed without fine sediment but otherwise under the same hydraulic conditions. Therefore, the effective sediment augmentation will be available by combining fine sediments in reservoirs and large gravels and stones in excavated sediment in riverbeds. Since the river width of middle and lower reaches is generally wide compared with upper reach near below dam, the location of replenishment sediment for its effective flushing can also be discussed. In this study, we investigated a flushing efficiency of a replenishment sediment consists of sand and gravel mixtures by using flume experiments. A two-dimensional numerical model was also developed to further investigate the effects of sediment augmentation. We also discussed the effectiveness of replenishment sediments with large gravel components in Hinogawa River in Tottori Prefecture, Japan by using the numerical model. The results are summarized as follows: (1) Gravel components in replenishment sediments composed of large gravels and fine sediments can be easily moved even though the tractive force is smaller than the critical tractive force of the gravel. This means that the sediments composed of different grain sizes can be used as a replenishment sediment. (2) Replenishment sediment located at away from a river bank can be easily flushed compared with it located at near the river bank. Therefore, the replenishment sediment may be available at the high flow velocity zone in the middle and lower reaches of rivers. (3) We proposed the numerical analysis model for calculating flush and transportation processes of replenishment sediment composed of large gravel and fine sediment. The calculation results showed roughly the tendency of the experimental results. (4) Flushing and transportation processes due to an estimated flow rate can be predicted by using the proposed model. We also illustrated an effective sediment supply measures using sediment augmentation.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022966

Year: 2022