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

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(s) with the Clay Bank sediment showed that ws increased non-linearly with SSC in the range of 300-700 mg L(-1), and that turbulence can increase w(s) up to one order higher than w(s) for non-turbulent 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 ws. 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.