Low-Submergence Effect on Incipient Sediment Motion

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, d(50)/h. This study shows that the critical Shields stress theta(c) for incipient sediment motion can be theoretically formulated either as a function of d(50)/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 d(50)/h < 1 or S < 0.6, the incipient sediment motion is controlled by the bed resistance and therefore theta(c) increases with increasing d(50)/h or S. Otherwise, the incipient motion is dominantly driven by the streamwise component of the grain gravity, which causes a reduction in theta(c) with increasing d(50)/h or S. The theoretical formula agrees reasonably with experimental data collected from eight sources in the literature. (C) 2018 American Society of Civil Engineers.