V. STORM SURGE SIMULATION 



1. Hurricane Carl a . 



Hurricane Carla was an immense, slow-moving, and meandering storm 

 which struck the gulf coast south of Port Lavaca, Texas, at 2000 G.m.t. 

 on 11 September 1961. The radius to maximum winds was 50 kilometers. 

 The atmospheric pressure drop across the storm was 75 millibars. The 

 maximum sustained winds were approximately 51 meters per second. 



For numerical stability a time step of 180 seconds was used in the 

 computations. The surge simulation was performed for a 66-hour period 

 from 1800 G.m.t. on September to 1200 G.m.t on 12 September. The 

 boundary conditions are those given in Section III. The wind fields 

 are analytically deformed in a manner previously discussed. The ob- 

 served (squares) and computed (solid line) water levels at selected 

 grid points along the coast are shown in Figure 59. The observed 

 water levels have been corrected for the astronomical tide. At the 

 start of the computations, the water surface along the coast from 

 Sabine Pass to the Matagorda Bay area was elevated by approximately 

 1 meter. Consequently, for comparison the computed water levels 

 have been raised to correspond to the local sea level condition. The 

 water surface topography in 0.5-meter contour increments at 46, 50, 

 and 54 hours are shown in Figure 60. The computed maximum coastal 

 surge profile corrected for the astronomical tide is presented in 

 Figure 61. Indicated on the figure are the observed high-crest values 

 from tide gages and water marks. The depression of the water surface 

 to the left of the storm track (as viewed from the sea) and the 

 low-peak surge in the Aransas Pass area results from the offshore 

 wind and from simulating the coast as a wall. It should be emphasized 

 that the surge model does not consider the coastal flooding from the 

 shelf surge nor includes the influence of the increased communication 

 between the semi enclosed bays and the open sea. While the computations 

 agree fairly well with those observed for the peak coastal surges, 

 especially for the area to the right of storm landfall, and for the 

 Galveston hydrography, there is some discrepancy for regions far from 

 the storm track such as Sabine Pass. This might be attributed to the 

 problem of proper deformation of the wind field (both speed and 

 direction) to reflect the influence of land. 



Considerable erosion of beaches and adjacent offshore areas along 

 the Texas coast occurred during Hurricane Carla (U.S. Army Engineer 

 District, Galveston 1962). Figure 62 shows the water velocity 

 V = Qs*i + Qt*J)/D in centimeters per second at selected grid points 

 for the same times as that in Figure 60. Of practical importance in 

 assessing the erosion potential and transport of the material is the 

 alongshore current, Qg*/D . In shallow water, the current is uniform 

 from the surface to the sea bed. Values of the current in centimeters 

 per second as a function of time at selected grid points are presented 



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