Gravel Mobilization and Bed Evolution due to Sand Supply to Gravel-Bed

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. In this study, we investigated the effects of sand supply to armored gravel-beds associated with sand cover on gravel mobilization and bed evolution. 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. The results are summarized as follows: (1) Gravel in streams with armored beds can be easily mobilized by supplying sand as a sand cover. The fraction of sand in the active layer in the bed can also promote the mobilization of gravel in the active layer. (2) Gravel removal increases with increasing thickness of the layer of sand cover. The efficiency of the sand cover to mobilize gravel decreases, however, when the top of the sand cover exceeds the height of the surface of the gravel-bed. (3) Sand supply to an armored beds can produce bed variation and channel formation through gravel mobilization. When the bed is only partially covered with sand the area over which the bed varies can be extended to the area without sand cover when the thickness of the sand cover is greater than 0.66 times the gravel size.