Flocculation and settling velocity of fine sediment

Abstract

This paper describes field and laboratory measurements of floc size and settling velocity of cohesive sediment in an estuarine environment and its modelling. The measurements and modelling showed a considerable variation in floc size, hence settling velocity over the tide, and over the spring-neap tide cycle. The simple flocculation model describes the evolution of floc size at one fraction only. The measurements, however, revealed the existence of a pronounced distribution in floc size, which varies over time. For instance, at the beginning of a flood period, hardly any macroflocs were observed in the field, whereas later in the tide, the fraction of macroflocs increased to about 45 to 80 %. Also the structure of the flocs appeared to vary over the tide, though this may be caused by advection of sediment from other locations and/or bed erosion. In the laboratory experiments floc sizes and settling velocities similar to those observed in the field were obtained.