Laboratory measurements of the fall velocity of fine sediment in an estuarine environment

摘要

The study of the fall velocity variation of fine sediment in estuarine areas plays an important role in determining how various factors affect the flocculation process.Previous experimental studies have focused solely on the relation between the median fall velocity and influencing factors,while in the current study,the variation of the fall velocity in quiescent water also was examined.The experimental results showed that the vertical distribution of sediment concentration was more uneven,and larger variations occurred earlier during the settling process under higher salinity and/or sediment concentration conditions.The fall velocity initially increased then decreased over time,peaking at^20 min after settling began,and stabilizing at a value similar to that in freshwater,regardless of the initial sediment concentration and salinity combinations.Along the water depth,the fall velocity increased monotonically with a gradually decreasing gradient.The median fall velocity increased then decreased with increased salinity.The salinity at which the peak fall velocity occurred depended on the initial sediment concentration.The relation between the median fall velocity and initial sediment concentration displayed an obvious two-stage pattern(i.e.,accelerated flocculation and decelerated,hindered settling)at higher salinities;whereas the maximum median fall velocity was observed at two consecutive sediment concentration values under lower salinity conditions.Finally,an empirical equation estimating the median fall velocity of cohesive fine sediment was formulated,incorporating the effects of both salinity and sediment concentration.