the Sound through the western tidal inlets and the Mobile Bay entrance. 

 Relatively stronger currents are again found in the western end of the Sound. 

 At the end of the tidal cycle, 96 hours, the velocity distributions are 

 somewhat similar to those at the beginning of the cycle. 



Detailed velocity distributions along a transect through the Mobile Bay 

 entrance are also shown at six hour intervals through the tidal cycle in 

 Figure 5.24. 



Tide-Induced Residual Currents 



Ideally, residual current can be defined as that part of the current that 

 is left after removal of all oscillatory tidal currents. In practice, 

 however, residual current is referred to as that part of the current that is 

 left after removal of diurnal, semidiurnal and higher frequency tidal signals. 

 The tide induced residual currents can be caused by a combination of the 

 non-linear inertia effect and the influence of complex geometry and bottom 

 topography. Of course, residual currents may also be generated by winds, 

 density gradients and river flows. 



Since the residual currents are usually a small fraction of the tidal 

 currents, it is not easy to accurately compute them from measured current data 

 except in such regions as the Bay of Fundy where residual currents on the 

 order of 80 cm/sec have been recorded. However, it is relatively easy to 

 compute the residual currents from a numerical model by time-averaging the 

 instantaneous tidal currents over a complete tidal cycle. Based on the tidal 

 currents described in the previous subsection, we have computed the 

 time-averaged currents over the 24-hour period which are shown in Figure 5.25. 

 The residual currents during this period are very weak and constitute only a 

 few percent of the instantaneous currents. However, stronger residual 

 currents may occur during other tidal periods. Since residual currents can 

 affect the long-term transport of contaminants within a coastal ecosystem, 

 further quantitative study on this subject is warranted. 



102 



