c. Salinity . Depending on the nature of the tests to be conducted 

 in the model, salinity verification is achieved in either one or two 

 phases. The first, and often the only, phase involves the reproduction 

 of the salinity conditions which were obtained during the quasi-steady- 

 state conditions of a single tidal cycle; the second phase involves repro- 

 duction of transient, long-term salinity conditions as affected by sea- 

 sonal variations in freshwater inflow. 



The first step in the quasi-steady-state verification process is to 

 determine the proper source salinity. Selection of the proper salinity 

 for the ocean water supply system is usually based on the maximum bottom 

 salinity observed during the prototype metering program at the deepest 

 station on the range closest to the ocean. If subsequent model tests 

 show that the maximum salinity at this location is affected by freshwater 

 inflow, the source salinity will have to be increased until the proper 

 maximum salinity is reproduced. Because the model salinity conditions 

 are completely artificial and temporary at the time model operation is 

 initiated, the model should be operated until salinity conditions have 

 stabilized before making detailed salinity measurements in the model. 

 After stability has been achieved, samples are withdrawn from the model 

 at hourly (prototype) intervals at the stations and depths for which 

 prototype data are available. As in the velocity verification, a repre- 

 sentative tide from the prototype survey period is reproduced in the 

 model; however, because tidal range has little, if any, effect on numer- 

 ical salinity concentrations in many estuaries, it is often unnecessary to 

 adjust salinities observed during different tides as long as the fresh- 

 water inflow conditions are the same. Drastic changes from the tidal 

 range of the previous day should be avoided. In this manner it is possi- 

 ble to demonstrate that the effects of tide on short-term (one tidal cycle) 

 phasing and fluctuations in concentration of salinity are accurately repro- 

 duced. Further refinements to the model roughness to achieve a satisfac- 

 tory salinity verification are usually unnecessary. However, modification 

 of the skimming weirs may be necessary to remove excess freshwater from 

 the surface of the model ocean to prevent dilution of the source salinity. 

 Note that quasi-steady-state conditions in the model are losed to represent 

 prototype conditions which are actually transient. When prototype salin- 

 ity conditions are changing rapidly in response to significant variations 

 in the freshwater inflow, the model cannot be expected to accurately re- 

 produce these conditions with a quasi-steady-state test. Other unusual 

 conditions during the prototype survey may also result in a rather poor 

 verification; e.g., heavy winds immediately preceding or during the pro- 

 totype survey may result in a higher than normal degree of mixing. How- 

 ever, local rainfall may result in a surface layer of freshwater over a 

 large area of the estuary. An example of quasi-steady-state salinity 

 verification is shown in Figure 3-30. 



In broad, shallow bays with low tidal ranges (common along the Gulf 

 of Mexico), tidal currents are generally of insufficient strength to gen- 

 erate a high degree of mixing between saltwater and freshwater. However, 

 these estuaries are typically well mixed by wind-generated wave action. 

 In estuary models of this type, the artificial roughness normally used 



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