Low-Submergence Effect on Incipient Sediment Motion: Fid-Bau Portal

Low-Submergence Effect on Incipient Sediment Motion

Cheng, Nian-Sheng / Tang, Hongwu / Yan, Jing

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 $θ_{c}$ 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 $θ_{c}$ 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 $θ_{c}$ with increasing $d_{50} / h$ or $S$ . The theoretical formula agrees reasonably with experimental data collected from eight sources in the literature.