Low-Submergence Effect on Incipient Sediment Motion

Abstract

Low submergence is usually observed over a channel bed of a steep slope. It is equivalent to large-scale roughness characterized by high ratios of grain diameter to flow depth, d5/h. This study shows that the critical Shields stress θc for incipient sediment motion can be theoretically formulated either as a function of d5/h or the channel bed slope S for fully rough beds. Different from previous studies, the derivation is conducted by involving the friction factor that applies for open-channel flows subjected to large-scale roughness. The analysis also takes into account other factors including grain density, sediment uniformity, and intergrain friction. The results show that for natural gravel beds, if d5/h<1 or S<.6, the incipient sediment motion is controlled by the bed resistance and therefore θc increases with increasing d5/h or S. Otherwise, the incipient motion is dominantly driven by the streamwise component of the grain gravity, which causes a reduction in θc with increasing d5/h or S. The theoretical formula agrees reasonably with experimental data collected from eight sources in the literature.