consisting of a wedge of salt water, move upstream in Perdido Bay during a 

 flood tide. These upstream currents encounter the usual downstream currents 

 and create counterclockwise bottom currents in the upper basin of Perdido 

 Bay. Figures 18 and 19 show flood- and ebb-tide current patterns in Perdido 

 Bay. 



The velocities of flood ti J es in Perdido Bay have been measured by the 

 U.S. Army Corps of Engineers to average 0.8 m/s (2.5 ft/s) and reach a maxium 

 of 1.2 m/s (4.0 ft/s). Ebb-tide velocities average 0.6 m/s (2.0 ft/s) and 

 reach a maximum of 0.7 m/s (2.3 ft/s). 



Mobile Bay and Mississippi Sound 



Information on current patterns in Mobile Bay has been collected by 

 several researchers, using both field observations and computer modeling. 

 Austin (1954) was the first to map flood and ebb tide circulation patterns in 

 Mobile Bay. Story, et al . (1974) traced circulation patterns in a portion of 

 Mobile Bay by the use of fluorescent dye markers. Several publications by 

 Schroeder (1976, 1977, 1979a, 1979b) discuss circulation patterns in Mobile 

 Bay. His studies involved current data collected while anchored, drogue 

 studies, and inferring circulation patterns from salinity patterns. 



Computer models of Mobile Bay have been developed by April et al . (19 76), 

 April and Raney (1979), Pitts and Farmer (1976), and Gaume et al . (1978). 

 The major difference between these models is how they model the openings of 

 the bay, which are complicated by Dauphin and Little Dauphin Islands (Biloxi 

 quadrangle) (0 * Neil and Mettee 1982). Figures 20 and 21 show flood and ebb 

 tide current patterns in Mobile Bay as predicted by April et al . (1976). 



The general circulation pattern in Mississippi Sound is induced by the 

 tides but wind has a significant effect on the currents. Horn Island Pass 

 (Biloxi quadrangle) is the natural dividing point for tidal currents in the 

 Mississippi Sound. During flood tide the currents bifurcate in the Horn 

 Island Pass area and flow westward and eastward. From Horn Island pass to 

 Mobile Bay, currents flow in through the passes and eastward on the flood 

 tide, then westward and out during ebb-tide (U.S. Army Corps of Engineers 

 1983). 



Recent numerical modeling by the U.S. Corps of Engineers (1983) on 

 current patterns in Mobile Bay and Mississippi Sound indicate that wind has a 

 significant effect on circulation patterns. East and west winds have a 

 pronounced effect with north and south winds having minimal effects. 

 Variance in wind velocities also exerted pronounced effects on circulation 

 patterns (U.S. Corps of Engineers 1983). 



The effect of the wind on the tidal currents is mainly to superimpose a 

 wind induced current on the Sound that shifts the bifurcation of the currents 

 either toward the east or west. The eastward or westward movement of the 

 current depends upon the direction of the wind and whether the tide is in ebb 

 or flood. An east wind component induces a westward current in the Sound, 

 which causes a shift in the current dividing point to the east during flood- 

 tide and to the west on the ebb-tide. Winds with a component from the west 



174 



