the natural environment, measurements were carried out in a settling cylinder 

 without adding any dispersant to break up the floes. Typical grain size 

 analyses of the sediments, however, are carried out in a settling tube 

 containing distilled water and 6.5 gA of sodium hexametaphosphate, a 

 dispersant used to prevent coagulation. The settling speeds determined under 

 such a condition are generally much smaller, reflecting the abundance of 

 smaller micron size particles. Using the same sediments as those used in 

 Figure 6.2, settling speeds determined in this fashion are compared with those 

 determined earlier. As shown in Figure 6.4(a), the median settling velocity 

 determined by the grain size analysis is on the order of 0.002 cm/sec, 

 compared to 0.05 cm/sec in sea water (30 ppt) and 0.02 cm/sec in fresh water 

 (0.2 ppt). Similar results obtained for Site-4 sediments are shown in 

 Figure 6.4(b). 



As indicated in the previous sections, sediment particle size 

 distribution and hence the settling speed distribution depend on the 

 coagulation dynamics which is influenced by the turbulence and salinity of the 

 surrounding water. Turbulence increases the collision, while salinity 

 increases the cohesion, among particles; thus, both tend to shift the 

 settling speed distribution towards larger values. Most settling speed 

 measurements, however, have been carried out in low turbulence laboratory 

 settings. Wei 1 -designed experiments are urgently needed to quantify the 

 influence of turbulence on the settling speeds of sediment particles. 



At higher concentration of sediment particles, settling speed may be 

 reduced due to hydrodynamic interference among the particles. This so-called 

 "hindered settling" was studied by Batchelor (1972) in a dilute suspension of 

 identical small rigid spheres with random positions in a Newtonian fluid. The 

 settling speed is equal to (1-6.55 C) w^, where C is the concentration and w^ 

 is the single particle settling speed in an unbounded fluid. 



135 



