An examination of the settling behaviour of a flocculated suspension

Abstract

An analysis of particle settling behaviour was carried out in one of the University of Hamburg's mesocosm carousel flumes, using a flocculated suspension from the Elbe Estuary. After fourteen days of stirring with a simulated tidal-current oscillation to a maximum of 24 cm·s⁻¹, the material in suspension was allowed to settle for an extended period. Results obtained from image analysis of silhouette photos taken in situ, and from total SPM and particle size analysis of the inorganic constituent grains of water samples collected, showed different stages in the settling of the flocculated material. Inorganic-rich flocs began to settle out approximately one minute after the paddles in the flume had been stopped. This initial settling period was followed by an increase in the modal diameter and concentration of organic-rich flocs to a maximum after five minutes. The increase in floc diameter preceded a rapid clearing of the water column during which the shape of the settling flocs changed from roughly spherical to elongate stringers. Fifty percent of the material in suspension at 1 m below the surface had settled by eleven minutes after stirring had stopped and by one hour the water column appeared to have reached an equilibrium concentration of small flocs representing only ten percent of the original concentration. The implications of this rapid flocculation on results obtained from in situ settling columns are discussed.