Cohesive sediment is a mixture of clay, silt, sand, organic material, water, and, sometimes, gas. The clay, silt, and organic particles can form flocs, sometimes also incorporating some larger sand particles. These flocs contain large amounts of water (70% to 95% by volume) and are much larger than the individual floc-building particles. As a result, the settling velocity of flocs is much higher than that of the individual clay, silt, and organic particles. Moreover, the flocculation process affects the density of fluid mud or slurries formed by these cohesive sediment flocs.

One of the first researchers who studied flocculation processes in the natural environment was Krone.1,2 He suggested that flocs are built up in a hierarchical way: large third-order flocs would be formed by a number of smaller second-order flocs, which themselves would consist of a number of even smaller first-order flocs, etc.

In sanitary engineering, Argaman and Kaufman3 and Parker et al.4 studied the flocculation process in wastewater. They found that floc sizes first increase with increasing turbulence shear, and then decrease in size. This would imply a variation in floc size over the tidal cycle in natural systems. Indeed, Van Leussen5 measured floc sizes 2.8 m below the water surface in the Ems estuary, The Netherlands, and found large variations in settling velocity, as shown in Figure 12.1. Further to these ideas, Dyer6 proposed the conceptual diagram of Figure 12.2, indicating floc size increase with increasing suspended sediment concentration and an increasing-decreasing trend in floc size with increasing turbulence intensity.

©2005 by CRC Press 249

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