In addition to the dependence on salt concentration, the cohesion among 

 particles also depends on the surface charge density, the water temperature, 

 the inter-particle distance, the valency of cation in water, the pH of water, 

 and the kind of anions in water. The parameters of primary importance are: 

 (1) total salt concentration, (2) cation exchange capacity which is an 

 effective measure of the clayey activity, (3) sodium absorption ratio which is 

 proportional to the ratio of exchangeable sodium to calcium plus magnesium 

 ions found in the diffuse layer of sorbed ions near the clay surfaces, and 

 (4) pH of the water. 



Studies on the effects of various parameters on cohesion are generally 

 carried out in low-turbulence laboratory settings. Although cohesion is the 

 primary cause of particle coagulation in a low-turbulence environment, 

 flow-induced collision among particles should play a more dominant role in 

 determining the state of coagulation in generally turbulent coastal waters. 

 Wind-induced turbulence in shallow coastal waters significantly increases the 

 collision frequency among particles and brings about much enhanced coagulation 

 among particles. Hence, to accurately resolve the sediment particle dynamics, 

 turbulence must be accurately predicted. Sophisticated mathematical models 

 (e.g., Sheng, 1982) are available for the prediction of turbulence in coastal 

 waters. The roles of turbulence in affecting the particle collision and 

 coagulation are described in the following. 



6.4 Turbulence and the Collision/Coagulation Process 



Dissipation Eddies in Coastal Waters 



Turbulence in coastal waters consists of the random motion of eddy 

 structures, ranging from the largest energy containing scales to the 

 dissipation scales where molecular viscosity comes into play. Due to the 

 drastic difference in the vertical and horizontal dimensions in the coastal 

 environment, we are generally concerned with the vertical eddies. Hence, the 

 largest scale is on the order of the largest macro-length L, such as the depth 

 of the water column, while the smallest scale is on the order of the 



3 



micro-scale X defined in terms of the turbulent energy dissipation e =q /A as 



, 3 ,1/4 " 

 AQ = (n /e) , where q is a representative turbulent velocity and n is the 



molecular kinematic viscosity. Similarly, a micro-time characterizing the 



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