general effects of wave and ship action on the resuspension of sediments. 

 Although wind-induced currents and water level setup can be reproduced, 

 it is generally not economically feasible to construct a large estuary 

 model in a wind tunnel with the capability of generating winds from 

 various directions. Coriolis forces can be simulated, but comprehensive 

 estuary models cannot be rotated unless they are constructed to very small 

 scales. 



The rise and fall of a tide or surge and its progression upstream can 

 be accurately modeled. Not only can the magnitude, phasing, and direction 

 of currents be reproduced at a particular point, but the longitudinal, 

 lateral, vertical and temporal velocity distributions are reproduced. The 

 same is true for salinity. Thus, the physical model provides a time- 

 varying, three-dimensional representation of the hydraulic and salinity 

 regimens of the estuary. Presently, only physical models are capable of 

 providing such three-dimensional representations. 



Although it is possible to model the dispersion of pollutants in three 

 dimensions and with time, the model tracer is usually a conservative dye 

 (i.e., no decay with time). The physical model results must, therefore, 

 be treated analytically before they can be applied to field conditions. 

 For dispersion of thermal and other pollutant discharges, far-field dis- 

 persion can be modeled, but the near-field dispersion cannot be accurately 

 reproduced in a distorted-scale model, if vertical exchange in the dis- 

 charge jet is important. 



A qualitative reproduction of shoaling patterns and distributions can 

 be achieved by the empirical development of a model operating technique. 

 However, the technique does not reproduce the changes in cohesion of 

 deposited materials, the effect of internal shearing of the flows on 

 aggregation or dispersion of suspended sediments, suspended-sediment 

 concentrations, flocculation, or resuspension of sediments by wave action 

 inside the estuary. 



The various problems or projects often investigated in fixed-bed 

 physical models include: 



(a) navigation channels (existing, new, enlarged, or realined) ; 



(b) navigating conditions (current velocities and patterns) ; 



(c) sediment traps and turning basins; 



(d) manmade inlets or canals; 



(e) training works (jetties, groins, dikes, channel constric- 

 tions, etc); 



(f) port facility siting; 



(g) landfills (commercial or industrial sites, highways, 

 airport rianways, diked disposal areas, etc.); 



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