date of 10 November 1982 agreed with these mean ranges and so represented an 

 average tide for the study area. Since the prototype tides measured on 

 10 November 1982 represented an average tide range, these tidal elevation sig- 

 nals were used as boundary conditions in WIFM. The prototype range current 

 data were used to verify the velocity computations. 



76. Plate 5 shows the prototype tide records for 10 November 1982. The 

 sampling rate for the records was 5 min, and these data were spline filtered 

 to remove high-frequency noise. Tides measured at Gages 2, 3, and 4 served as 

 boundary conditions to the Amelia River, St. Marys River, and Cumberland Sound 

 boundaries of the model. The signal from Gage 1, located at the south jetty 

 of the inlet (Figure 10) , was used as the boundary condition at the eastern 

 edge of the computational grids. However, the travel time for a gravity wave 

 between the eastern boundary and the actual location of Gage 1 is 25 min, so 

 the boundary condition was phase shifted 25 min to account for this distance. 

 The lateral ocean boundary conditions were interpolated between this offshore 

 signal and the tide signal at the inlet (Gage 1). The boundary condition at 

 St. Marys River (Gage 2) was also phase shifted 7 min to account for gravity 

 wave travel time between the mouth of the river (the grid boundary) and the 

 site of the prototype gage farther upstream. 



77. The zero datum shown in Plate 5 represents the mean for each 

 measured tide record rather than a geophysical datum such as the National 

 Geodetic Vertical Datum (NGVD) of 1929 . The elevations of the tide recorders 

 used in this study were not referenced to a benchmark, so the relationships 

 between the gage means are not known. The lack of a common datum caused 

 numerous problems during calibration, since WIFM requires all elevations to be 

 measured from a common datum. Since the tide gages were all fairly close to 

 one another (less than 2 miles apart) , even minor changes in elevations caused 

 gradients great enough to change the flow patterns within the study area. 

 These elevation adjustments were determined during the model calibration. 



78. Permeability of jetties. Since both jetties at St. Marys Entrance 

 are awash at high tide and known to be permeable, the tidal model has to prop- 

 erly simulate this effect on the velocity patterns. From field measurements 

 taken by Florida Coastal Engineers in 1975, "it was estimated that up to 



28 percent of the total [tidal] flood flow enters [the inlet] through the per- 

 meable jetties rather than at the ocean terminus of the structures." (Parchure 

 1982, p. 27). Since the widths of the jetties are small compared with grid 



46 



