GRAVEL MOBILIZATION AND CHANNEL EVOLUTION DUE TO SAND COVERING TO GRAVEL-BEDS

Abstract

Armored gravel bed formation in rivers downstream of dams may be serious problems caused by the cutoff of sediment supply. Releasing flows large enough to generate shear stresses adequate to mobilize the coarse gravels in the armor layer is not always appropriate, because other problems such as generation of flows with high turbidity may arise. On the other hand, when fine sediment (i.e. sand) is present in streambeds composed of coarse gravels, the gravel can be more easily removed as compared with gravel beds without fine sediment (e.g. Ikeda, 1984). Gravel augmentation using fine gravel has been recently demonstrated as a viable alternative to mobilizing coarse surface layers. However, the effect of gravel mobilization on channel evolution was hardly discussed. In this study, we investigate the effects of sand supply to the armored gravel bed associated with sand covering on gravel mobilization and channel evolution. A subsurface layer in an armored bed may contain not only gravel but also sand. We also examined the effect of sand in the subsurface layer on gravel mobilization experimentally.