Two boundary' conditions were specified for the model. Month-long tide gauge 

 records provided the water-surface elevations at the offshore boundary located 

 approximately 2 miles offshore. Averaged flow measurements of the freshwater influx 

 specified the flow boundary condition situated across the Choctawhatchee River Delta on 

 the eastern border of the bay. 



Figures 20 and 21 depict channel currents during spring flood and spring ebb. 

 Velocity vectors are overlaid on contour plots of the velocity magnitude. The solid black 

 lines in the figures indicate the location of the maintained channels. Magnifying the 

 areas of interest, Figures 22 through 25 depict the currents over the shoaling hot spots. 



In Chapter 3, the DMS-Manual identified two possible shoal classifications for the 

 shoaling hot spot at the channel bend. These were shoaling caused by horizontal 

 expansion of the channel (through shoreline recession) and shoaling caused by vertical 

 expansion of the water column (i.e., shoaling from currents crossing the channel at an 

 angle to the channel's axis). Figures 22 and 23 illustrate current patterns in the vicinity 

 of this shoal. Examination of the gradient of the velocity-magnitude contours as the flow 

 enters the inlet on flood tide (Figure 22) tests the accuracy of the horizontal expansion 

 classification. Figure 22 shows a marked decrease in velocity magnitude traveling north 

 through the inlet. The velocity magnitude decreases from a value of almost 4 ft/sec near 

 the jetty tips to less than 2.5 ft/sec at the top of the channel bend. This sharp velocity 

 decrease over a relatively short distance (-2,000 ft) verifies the choice of this shoal 

 classification. 



To verify the second classification (vertical expansion) requires examination of the 

 current patterns over the channel. The velocity vectors indicate the existence of a few 

 areas where currents enter the channel at an angle (e.g., in Figure 22, the flood currents 

 enter the channel south of the bend at an angle to the channel axis). However, the 

 perpendicular component of these vectors is small. Also, as currents cross into the 

 channel, no noticeable decrease in velocity occurs. This observation suggests that the 

 dominant mechanism causing shoaling in this area is the horizontal expansion of the 

 channel related to the recession of the banks on either side of the channel. 



The second area where the manual identified more than one possible shoal type is in 

 Old Pass channel. In this area, one of the possible classifications was Vertical 

 Expansions: Cross Channel Flow. Testing of this classification involves examination of 

 the current patterns and velocity magnitudes in the channel vicinity. Figures 24 and 25 

 illustrate the inlet hydrodynamics in this area. Both figures show the currents cross the 

 channel at almost right angles to the channel axis. In addition, the figures also illustrate 

 the marked decrease in velocity after currents enter the channel (the shift from green to 

 blue). Both these behaviors reinforce the selection of this classification. 



A further investigation of the shoaling behavior at East Pass involves a simple 

 treatment of sediment transport. The RMA2 model provides the velocity and water depth 

 at each node in the finite-element mesh. From this information (together with a 

 representative sediment size), the sediment-transport rate at each node can be calculated 

 with an empirical sediment-transport function. 



40 Chapter 4 DMS-Analytical Toolbox 



