60% of the sediment particles are concentrated within the size range of 

 13.5 urn to 42.8 vm. Thus, for cohesive sediments in a coastal environment, 

 the dynamics of sediment particle size distribution as influenced by the 

 coagulation process have to be addressed in a comprehensive sediment transport 

 model. Due to the sharper particle size distribution, however, relatively 

 fewer particle groups need to be resolved in a salt water environment. The 

 relatively stronger coagulation leads to frequent interactions among the 

 various particle groups. Although the coagulation process is quite 

 complicated, and a comprehensive working model is not yet available at the 

 present time, we will assess the present state of the art of our understanding 

 of the important parameters affecting this process. This would be the basis 

 for future model development. 



6.3 Cohesion of Sediments 



In contrast to non-cohesive sediments such as quartz sand, which 

 generally exists as individual particles, the inter-particle forces lead to 

 cohesion among the fine-grained cohesive sediments. Cohesive sediments are 

 comprised primarily of colloidal clay particles and fine silt which possess 

 colloidal properties to a lesser extent. In general, cohesive sediments from 

 a coastal environment also include a certain amount of organic materials, 

 bacteria, benthos and their fecal materials. 



According to the difference in layer lattice crystal structures, clay 

 minerals can be classified into three main types as kaolinite, illite, and 

 smectite, with increasingly stronger clayey features. Within the individual 

 crystals, positively charged cations occupy interior layers, and the negative 

 charged hydroxyl and oxygen atoms occupy the platy surfaces. Positive charges 

 are exposed at the crystal edges. The cations in the crystal lattice may be 

 substituted by other ions of lower valence, thus producing a net charge 

 deficiency which makes the surface negative charge even greater. With a small 

 concentration of free ions in fresh water, electrostatic repulsive forces will 

 be suppressed in favor of attractive van der Walls forces among individual 

 clay particles. With increasing ion concentration in salt water, cohesion 

 among particles is further enhanced and larger floes can be formed. 



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