does not generate sufficient turbulence to reproduce the proper degree of 

 mixing, and the models tend to be more stratified than their prototype 

 counterparts. To obtain proper model salinity mixing, oscillating fans 

 are mounted on stands and positioned to blow down on the model water sur- 

 face in a pattern designed to prevent the establishment of unnatural sior- 

 face eddy patterns (Fig. 3-31). The number and position of the fans are 

 determined by trial and error to achieve the proper vertical salinity 

 gradients. The mixing index (or salinity ratio) is determined for both 

 model and prototype at various locations throughout the estuary by divid- 

 ing surface salinity by bottom salinity and multiplying by 100 to convert 

 the result to a percentage. Examples of the degree to which the mixing 

 index has been reproduced are shown in Figure 3-32 and in Table 3-2. 





Table 3-2. Lake Pontchartrain mixing indices.' 





Station 



Prototype 



Model 



Station 



Prototype 



Model 



GC-11 



100.0 



100.0 



P-7 



95.0 



97.0 



T-5 



98.0 



85.0 



P-8 



100.0 



100.0 



T-6 



101.0 



83.0 



P-9 



100.0 



100.0 



T-7 



101.0 



100.0 



P-10 



101.0 



90.0 



T-8 



98.0 



100.0 



P-11 



99.0 



97.0 



A-6 



99.0 



96.0 



P-12 



100.0 



89.0 



A-8 



99.0 



88.0 



P-13 



99.0 



100.0 



A-10 



100.0 



91.0 



P-14 



94.0 



69.0 



M-1 



100.0 



91.0 



P-15 



91.0 



91.0 



R-1 



97.0 



93.0 



P-16 



100.0 



100.0 



P-6 



100.0 



97.0 



P-17 



99.0 



98.0 



Avg. 



98.7 



93.4 



'Salinity ratios (U.S. Army Engineer Waterways Experiment Station, 1963). 



The first step in the long-term salinity verification is also deter- 

 mination of the proper source salinity. Periods of high freshwater dis- 

 charge may result in a considerable dilution of salinities immediately 

 offshore of the estuary entrance. Thus, the model source salinity may 

 liave to be varied on a seasonal basis to achieve proper salinity veri- 

 fication in the estuary. These tests usually are conducted using a 

 repetitious mean tide. The freshwater inflows are set to reproduce 

 prototype conditions at the beginning of the survey, and the model is 

 operated until salinity conditions for that prototype day are achieved. 

 At that point, reproduction of the freshwater inflow hydrographs in the 

 main stream and all tributaries is started and continued to the model 

 time scale. The model hydrographs are usually stepped on a weekly 

 (prototype) basis. 



The results of the long-term salinity verification can be presented 

 as the variation of salinity with time at given points, as salinity pro- 

 files along the channel at various times, or as plots of constant isochlors 

 or isohalines at various times on a map of the estuary. An example of the 



06 



