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IntroductionThe environment in many coastal and estuarine areas is characterized by the presence of large amounts of fine-grained cohesive and noncohesive sediment. The transport and fate of these sediments determine the bathymetry and stability of these areas, affect its water quality and ecological health through turbidity levels and adherence of contaminants and sediment composition, and are a nuisance to port authorities, who are often forced to undertake frequent maintenance dredging operations to safeguard navigation. Farther offshore, for instance, on continental shelves, large transports of sediment are encountered when the slopes of these shelves destabilize, generating huge turbidity currents.Nowadays, engineers and scientists commonly use threedimensional numerical models to study and predict the transport and fate of these fine-grained sediments. An important issue in such models is the interaction between the suspended sediment and the turbulent water movement. The present paper describes a study on this interaction in open-channel flow, discussing its implications and modeling requirements.The water-sediment mixture is treated as a single-phase fluid in which all particles follow the turbulence movements except for their settling velocity. Uittenbogaard [1994] In this study, distinction is made between the behavior of noncohesive and cohesive sediments. Noncohesive particles form a rigid bed upon deposition, at which turbulence production is always possible. As a result, an equilibrium condition exists, also referred to as capacity flow, which is the basis of many sediment transport formulae. In the case of cohesive

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The purpose of this paper is to establish a relation between a few measurable quantities (the socalled ζ potential, organic matter content, and shear rate) and the flocculation behavior of mud. The results obtained with small-scale flocculation experiments (mixing jar) are compared to results of large-scale experiments (settling column). The mud used for all experiments has been collected in October 2007 in the lower Western Schelde, near Antwerp, Belgium. From this study, it was found that the mean floc size and the Kolmogorov microscale vary in a similar way with the shear rate for suspensions with different pH and salt concentrations. The size of flocs at a given shear rate depends on the properties of the suspension, which affect the electrokinetic properties of the sediment; these can be described by means of the ζ potential. The main findings of this paper are: (1) In saline suspensions at pH = 8, the mean floc size increases when the salt concentration and the ζ potential increase. (2) For a given ζ potential, the mean floc size at low pH is larger than observed at pH = 8 for any added salt. (3) The mean floc size increases with increasing organic matter content. (4) Mud with no organic matter at pH = 8 and no added salt flocculates very little. The response of mud suspensions to variations in salinity and pH is similar to that of kaolinite. This suggests that a general trend can be established for different and complex types of clays and mud. This systematic study can therefore be used for further development of flocculation models.