in the marsh area south of Lake Borgne, and a high water mark of 4.8 

 meters along the eastern side of the Mississippi River levee. A 

 part of the discrepancy between the observed and computed surges re- 

 sults from not considering the flooding of low coastal areas and the 

 open communication of Mississippi Sound with Lake Borgne which is, 

 in turn, connected with Lake Pontchartrain. The tide gage at Shell 

 Beach located on the southern end of Lake Borgne crested at 3.4 

 meters. Another test simulation was conducted with the hurricane 

 track moved 18 kilometers to the east. The results showed greater 

 seaward circulation in Chandeleur Sound reducing the setup along the 

 levee by approximately 1.5 meters. The location of the peak surge 

 was shifted to the east but the surge magnitude was not reduced. 



3. Hurricane Gracie. 



Hurricane Gracie crossed the coast near Beaufort, South Carolina, 

 at 1600 G.m.t. 29 September 1959. The radius to maximum winds was 

 25 kilometers. The atmospheric pressure in the eye was 950 millibars. 

 The maximum sustained winds were approximately 50 meters per second. 



A Cartesian grid for simulating the surge caused by Gracie is 

 shown in Figure 76. The reach and grid spacing of this system are 

 comparable to the curvilinear grid shown in Figure 32. The thin 

 dashlines are a part of the trans form- generated coast and seaward 

 boundary curve (180-meter depth contour) . The surge computations are 

 performed with both grid systems using the same boundary conditions 

 and forcing functions. The stairstep boundary representing the coast 

 is the heavy solid line in Figure 76. Another difference between the 

 models other than the coastline representation is that the Cartesian 

 system's boundary, j = 1, extends farther seaward into deeper water 

 than that of the curvilinear system. 



For numerical stability a time step of 150 seconds was used in 

 the computations with the curvilinear grid. The time step in the 

 rectilinear system was taken as 75 seconds. The surge simulation was 

 performed for a 48-hour period with zero hour corresponding to 1200 

 G.m.t. on 28 September. 



The observed and computed water levels along the coast in the 

 curvilinear (solid) and rectilinear (dashed) systems are shown in 

 Figure 77. The inserts on Figure 76 show the simulated hydrograph 

 locations. Figures 78 and 79 present the water surface topography in 

 the curvilinear and rectilinear grids, respectively, at the same times 

 and for the same contour increments. The depression of the water 

 surface to the left of the storm track exhibited in both figures is 

 similar to that from the Carla simulation. At the Savannah River 

 entrance, the observed water level corrected for the astronomical 

 tide is depressed after 27 hours reflecting the offshore-directed 

 wind. The effect on the coastal surges and water surface topographies 



122 



