Effects of freshwater salinity on suspended sediment settling during different high-turbidity events

Understanding the changes in suspended sediment settling during high-turbidity events (HTEs) under different salinity conditions-from freshwater to mildly saline (0-2 ppt)-is important for assessing risks to aquatic life. Hydrostatic settling experiments were conducted with different salinity levels (electrical conductivity [EC] = 0.3, 0.4, 0.8, 1.2, and 2.0 mS/cm) and initial suspended sediment concentrations (SSCs; turbidity = 400, 800, and 1200 NTU) to evaluate the effects of salinity on sedimentation after HTEs. Compared to the control set (EC = 0.3 mS/cm), increasing initial salinity inhibited the settling velocity of suspended sediment particles (SSPs) for 44-600 min during the early settling stage. In this stage, the hindering effects of liquid density and viscosity dominated the settling rather than flocculation. A similar trend was observed when SSC was the single variable: compared to the 400 NTU set, the increase in the cumulative turbidity reduction ratio in the 800 and 1200 NTU sets was delayed by 40-400 min. Overall, increasing salinity level and initial SSC accelerated SSP settling throughout the experiments. The total mean turbidity reduction rate was positively correlated with salinity level and initial SSC (0.07-0.94 NTU/min). However, at high initial SSC (turbidity ≥1200 NTU), the acceleration effect of slightly saline levels (EC ≤ 0.8 mS/cm) was negligible.