significant bottom shear stress and hence entrain the bottom sediments. 

 Usually, complicated transport-deposition-entrainment cycles lead to large 

 spatial and temporal variations in sediment distribution within any given 

 coastal environment. Hence, a comprehensive impact analysis must address not 

 only the short-term effect, but also the long-term influence of a coastal 

 project. 



Sediments containing a large fraction of clay minerals are generally 

 cohesive in nature and can form floes of various sizes. The particle size 

 distribution is greatly influenced by the turbulence and the salinity and as 

 such, can vary appreciably with time and location. Deposition and entrainment 

 of the cohesive sediments at the bottom of coastal waters are quite different 

 from those for the non-cohesive sand particles. Consequently, a comprehensive 

 sediment transport model should contain quantitative information on these 

 processes. 



1.2 Coastal Ecosystem 



Coastal currents and sediment transport can significantly affect the 

 other components within a coastal ecosystem. Some examples are given here. 

 Of primary importance is the direct influence of coastal currents on the 

 transport of a variety of water quality parameters and biota. The fine 

 cohesive sediments, entering from rivers or dredged from the navigation 

 channels, may contain nutrients or other undesirable materials adsorbed onto 

 the particles. Depletion of oxygen within the lower water column may occur 

 due to the formation of a sharp density gradient such as the thermocline. 

 Properties of the bottom sediments may be greatly influenced by the 

 bottom- feeding organisms, organic matter, and bacteria. Feeding action of the 

 benthos may also influence the diffusion of chemical species within the bottom 

 water and the surfacial sediments. 



A diagram illustrating the complex interactions occurring among the 

 various components of a coastal ecosystem is shown in Figure 1.1. Rectangles 

 indicate the hydrodynamic components while circles indicate the other 

 components. Interactions among the various non-hydrodynamic components may 

 occur both within the water column and below the sediment-water interface. It 

 is apparent from this diagram that a comprehensive understanding of the 



