296 / 2024-02-29 09:40:27

Gravel mobilization and bed evolution due to sand supply to gravel-bed

gravel bed,armoring,sand-covering,gravel mobilization,bed evolution

Cut off the sediment supply to rivers downstream of dams may cause not only the channel degradation and bed surface armoring but also the channel immobilization. Therefore, many works have been conducted in order to make restoration of such rivers. Since the existence of fine sediment (i.e. sand) in streambeds composed of coarse sediment (i.e. gravel) causes the reduction of the friction angle of coarse sediment, fine sediment supply to the gravel bed may cause channel evolution through the gravel mobilization. In this study, we investigate the effects of sand supply to the armored gravel bed associated with sand-covering on gravel mobilization and channel formation.



Experiments were conducted in a straight open channel with a length of 16 m and a width of 0.5 m. Three kinds of uniform sediments of Gravel A (grain size d_g = 7.1 mm), Sand A (d_s = 1.4 mm), Sand B (d_s = 0.52 mm), and the mixture of Gravel A and Sand B were used in the experiment. The mobile-bed was an armored bed composed of the mixture. The longitudinal right half of the mobile-bed area was covered with Sand A before commencing experiment. Five kinds of the thicknesses of the cover layer D was provided. The amounts of Gravel A and Sand B which moved from their original positions were measured sequentially during the experiment. The bed surface elevations were measured in a prescribed timing. In order to clarify the effects of the sand-covering and subsurface sediment on the gravel mobilization and channel formation, the experiments without the sand-covering and/or without Sand B as subsurface sediment (only Gravel A) were also conducted.



The results are summarized as follows:

1. Gravel in streams with armored beds can be relatively easily mobilized by supplying sand as a sand-covering. The fraction of sand in the subsurface layer in the bed can also play a role in the reduction of the fraction angle of gravel.

2. The amount of gravel run-off increases with increasing the thickness of the sand-covering layer, whereas the efficiency of the sand-cover decreases when the height of surface of sand-cover exceeds the height of gravel bed surface.

3. Sand supply to the armored beds can produce the bed variation and channel formation through the gravel mobilization. In particular, the bed variation area is extended to the area without sand-covering in case of the value of D/d_g is over approximately 0.6.