Abstract

Salt flocculation causes small clay- and silt-sized particles in estuarine waters to coagulate, resulting in much higher settling velocities than that undergone by the original single particles. Thus, the flocculation process changes the behavior of suspended cohesive sediment as it enters the estuarine environment. In the Gradyb tidal area of western Denmark, it can be shown that suspended concentration of cohesive sediment controls about 80 per cent of the variation in median settling velocity of cohesive suspended sediment; higher concentrations give increasingly higher settling velocities. In addition, wind speed influences the concentration level of suspended sediment. As a rule, settling velocities will generally be higher during periods of high wind speed and this will enhance the efficiency of settling and scour-lag processes. Over a three week monitoring period, measurements show an ebb oriented net transport during calm weather periods and flood oriented net transport during periods of high wind speed. This shift is thought to be due to the formation of larger sediment flocs during adverse weather periods. Such a mechanism may be an important controlling factor on the transport of cohesive sediments in shallow, microtidal areas with fairly high suspended concentrations of cohesive sediment.