The results in Table 6.3 indicate that, under normal conditions in a 

 coastal environment, sediment particles with radii less than 100 urn will 

 completely follow the eddy motions. Under storm conditions, however, only 

 particles with radii less than 30 um will completely follow the eddy motions. 

 Considering the particle distribution shown in Figure 6.2, it is clear that a 

 majority of the cohesive sediments from the Mississippi Sound will follow the 

 turbulent eddy motions. At high turbulence during storm conditions, bigger 

 particle sizes resulting from coagulation may exceed the critical radius and 

 hence may be partially decoupled from the turbulent eddies. 



Basics of Collision/Coagulation Model 



Turbulence can affect the particle coagulation by increasing the 

 collision frequency among particles in various size groups, through the 

 turbulence induced shearing, turbulence induced acceleration, and 

 gravitational settling. Since we are generally concerned with particles 

 bigger than 1 ym in radius, the effect of Brownian motion on particle 

 collision is much smaller than the turbulent contribution, and hence can be 

 neglected. The total collision rate per unit volume between two groups of 

 sediment particles with radius (r^, r^) and number density (n, , n2 ) can be 

 expressed as: 



N = n^ n2 V (r^, r^) (6.1) 



where v is the collision kernel and can be expressed as (Saffman and 

 Turner, 1956): 



2 2 2 2 2 



V = (v^ + v^ + Vg) e (6.2) 



where v v„ , and v, are the contribution from turbulent shearing, turbulent 



12 3 ^ ' 



acceleration, and differential settling. These are expressed as: 



(S) 2 

 Vj = AV^2 ^ ^2 



^ =*^ (V9) ^^[l^ ^ ^2 



131 



