Similarly 



where 





. c -1 



y = |y_ = b c a,"! (6) 



y ^a q q 2 



If the grid in the x-, y-coordinate system is to have constant grid spacing, 



all values of u and ]i will be constant (1 if Aa, = Ax and Aa„ = Ay) . 

 X y 12 



The constants a , b , c , a , b , and c for all the regions and 



p p p q q q 



the values of y and v at grid cell faces and centers are determined 

 X y 



using an interactive computer program called MAPIT. A plotting program called 

 CMSGRID is used to plot the grids to a suitable scale for overlaying nautical 

 charts such as those of the National Ocean Service (NOS) . 



10. Figures 3 and 4 show the two computational grids used to model 

 St. Marys Inlet and surrounding areas. Both grids extend for 141,670 ft 

 (23.3 nautical miles), in 90 grid cells, east-west and for 60,000 ft (9.9 nau- 

 tical miles), in 50 cells, north-south. The grids are oriented so that one 

 row of cells aligns with the navigation channel. The difference between the 

 two grids lies in the mapping of 17 rows of cells in the area which covers 

 St. Marys Inlet. Grid 1 (Figure 3) models the base condition of a 400-ft-wide 

 navigation channel. The smallest cell, located near Ft. Clinch, has dimen- 

 sions of 275 ft by 310 ft; while the largest cell, located at the eastern 

 boundary of the grid, is 5,200 ft by 4,170 ft. The 25th cell row from the 

 north edge of the grid overlays the channel. Grid 2 (Figure 4) models the 

 plan condition of a 500-ft-wide channel. The smallest cell in Grid 2, also 

 near Ft. Clinch, is 229 ft by 310 ft, while the largest cell size remains the 

 same. The 17 rows of cells in the area of St. Marys Inlet have been remapped 

 for Grid 2 so the 25th row again overlays the channel. This adjustment is 

 accomplished by mapping slightly larger cells just north of the channel and 

 slightly smaller cells just south of the channel. The mapping differences 

 between Grids 1 and 2 are so minor that no bath5Tnetic changes are required 

 other than those in areas affected by dredging for the plan conditions. 



13 



