Bridge- and tunnel-crossing studies will define current velocity 

 conditions which are encountered during construction and will determine 

 shoaling and other environmental factors. Qualitative shore erosion 

 studies are made to determine areas which may be subjected to scouring 

 velocities and to develop protection works (dikes, groins, etc.)- 



Waste-discharge studies will define zones of influence for existing 

 or proposed outfall locations and will determine optimum outfall loca- 

 tions. These studies are usually limited to tests of conservative dye 

 or thermal dispersion. 



Studies of freshwater supplies will determine whether or not the 

 supply is safe from saltwater contamination during low freshwater- flow 

 periods and will determine the effects of the freshwater withdrawal on 

 salinity conditions downstream. If regulation of the salinity of one 

 body of water is desired by introducing or regulating flow from another 

 water body, model studies will determine the size and location of the 

 control structure and the rate of the regulated flow required to achieve 

 the desired salinity. Upstream projects often alter the freshwater in- 

 flow to an estuary by regulating the flow (seasonally for flood control 

 projects or daily for hydroelectric projects) or by diverting freshwater 

 flow from or into the estuary. In any case, the effects of such projects 

 may have a significant impact on salinity and dispersion conditions in 

 the estuary. If large changes in salinity conditions are indicated, the 

 investigation should be expanded to determine the effects on shoaling 

 distribution. 



After model verification has been completed, the model can be used to 

 obtain more extensive and detailed data on existing conditions than are 

 available from the prototype. These data can provide a baseline for 

 use in evaluating the effects of proposed projects, and can also lead 

 to an improved understanding of prototype phenomena. For example, the 

 model can be used to determine the effects of tidal range, changes in 

 mean sea level, differences in sustained freshwater inflow, hurricane 

 surges, etc., on the extent of saltwater intrusion, salinity distribu- 

 tion, and general dispersion characteristics throughout the estuary. 



b. Advantages and Disadvantages of Scale Models . Modeling tech- 

 niques are an exceedingly important planning tool for a number of rea- 

 sons. It is easier and faster to initiate changes and test the effects 

 in a model than in the prototype. The testing of many alternatives in a 

 model represents only a modest financial investment; prototype testing of 

 the same alternatives would probably be a prohibitively expensive major 

 undertaking. Prototype testing of tidal-related problems is time consum- 

 ing, and the results are often of less than desirable quality or quantity. 

 To test a sufficient number of prototype alternatives to assure that the 

 final solution is the most desirable would usually be impossible (Simmons, 

 Harrison, and Huval, 1971). In addition, model testing of various alter- 

 natives may prevent irreversible damage to the estuary which might be 

 caused by an unsatisfactory design. 



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