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

Linked Author(s):

Keywords: Gravel riverbed; Low-flow channel; Sand cover; Gravel mobilization; Channel evolution

Abstract: Cutting off the sediment supply to rivers downstream of dams may cause channel degradation, bed surface armoring and channel immobilization. Many studies have been conducted to restore such rivers. Since the existence of fine sediment (e. g. sand) in streambeds composed of coarse sediment (e. g., gravel) causes a reduction in the friction angle of coarse sediment, fine sediment supply to gravel bed may cause channel evolution through gravel mobilization. This phenomenon may indicate that the sand supply to gravel beds is a viable alternative for mobilizing coarse surface layers and may cause bed deformation and channel evolution. In this study, we investigate the effects of sand supply to armored gravel beds associated with sand cover on gravel mobilization and bed evolution. In particular, the effect of sand cover thickness on the mobilization and evolution was discussed. As an active layer near the surface of the gravel deposit in an armored bed may contain both gravel and sand, we therefore examined the effect of sand in the active layer on gravel mobilization. Two types of gravel bed were discussed in this study. One was the bed with straight low-flow channel and the other was the bed with meandering low-flow channel. The results are summarized as follows: (1) Gravel in streams with armored beds can be easily mobilized by supplying sand so as to cover the gravel. The fraction of sand in the active layer of the bed can also promote the mobilization of gravel in the active layer. (2) Sand cover causes low-flow channel evolution through the widening and shallowing of the low-flow channel by the degradation of the sand cover area and aggradation of the low-flow channel beds. (3) The low-flow channel width and depth, and cross-sectional area depend on the thickness of sand cover. The ratio of the sand cover thickness to gravel size of almost over 1 may be effective for active channel evolution in the straight low-flow channel. (4) The inner banks in the meandering low-flow channel were eroded compared with the outer banks due to the straightness of the flow.

DOI: https://doi.org/10.64697/978-90-835589-7-4_41WC-P2025-cd

Year: 2025