steady-state is on the order of 5 days. If the dimensions of the estuary are 

 increased, a longer adjustment time is required. 



Results of the above simulation imply that in any realistic simulation of 

 density-driven currents, the specification of the initial density distribution 

 is extremely important. For simulations on the order of a few days, the final 

 salinity distributions and density-driven currents are extremely sensitive to 

 the initial values of the simulations. During the previous tidal simulations 

 of the Mississippi Sound, salinity data was only collected within the Sound 

 and no data was available in the offshore waters. Due to the relatively short 

 simulation period (5 days), meaningful realistic simulation of the salinity 

 field could not be performed without knowing the detailed offshore salinity 

 distribution at the beginning of the simulation period. However, available 

 salinity data indicate rather uniform salinity distribution within the Sound, 

 in both the vertical and horizontal directions. The magnitudes of the 

 density-driven currents are thus expected to be much smaller compared to the 

 tide- and wind-driven currents. This is confirmed by the excellent agreement 

 between the model 's computed and measured currents, as shown in the previous 

 sections. Density-driven currents should only be important during period of 

 strong river inflows, particularly within the Mobile Bay. 



Salinity and temperature data were collected from the transect stations 

 of the Mississippi Sound on 9/02/80, 9/03/80, 9/08/80, and 9/04/80. The 

 results at 1 meter depth were interpolated onto our numerical grid and are 

 shown in Figure 5.34. It is apparent that during this period spatial 

 gradients in temperature and salinity should have a negligible effect on the 

 currents. Vertical gradients in temperature and salinity are also wery small 

 during the same period. 



5.4 Return Currents in Bottom Waters 



It was demonstrated in Section 5.3 that return currents in the bottom 

 water could be caused by fresh water inflow into a density stratified 

 environment. 



117 



