Effects of suspended sediment concentration and turbulence on settling velocity of cohesive sediment

摘要

Using a 5-MHz acoustic Doppler velocimeter (ADV),laboratory experiments were carried out to investigate the effects of suspended sediment concentration (SSC) and turbulence on the settling velocity (w s) of cohesive sediment. The measurement of w_swith the Clay Bank sediment showed that w_sincreased non-linearly with SSC in the range of 300–700 mg L~(-1), and that turbulence can increase w_sup to one order higher than w_s for nonturbulent conditions. This turbulence effect can explain why w_s derived by ADV is 1 to 3 orders higher than w_s estimated by Owen tube where the ambient turbulence is totally blocked.When the turbulent shear stress was higher than about 0.14 Pa,however, it contributed to tear apart flocs and reduce w_s. This study suggests that ADV is a useful tool to concurrently measure the instantaneous current velocities, SSC and w_s in turbulence-dominant environments without breaking up flocs and disturbing ambient flow.