at the nearshore grid boundary east of Oregon Inlet agreed completely, indi- 

 cating that this nearshore boundary was indeed far enough from the inlet. 



55. Hurricane Donna also was simulated in th.? nearshore grid with ';he 

 inlet closed. This test served to answer the question "What is the maximam 

 radius of influence at which Oregon Tnlet ;an affect a peak storm surge?" A 

 comparison of nearshore computations with existing conditions with the closed 

 inlet case illustrates the effects of closure. Plates 39 and 40 show the two 

 cases tor the sites in Figure 8. At the closed inlet the surge differences 

 are, of course, large; but they diminish rapidly away from the opening. The 

 difference is only 1 ft at the Roanoke Sound channel gage, a site 3 milts from 

 the center of the inlet. Figure 13 shews the peak surge contours around 

 Oregon Inlet for the closed inlet and for natural conditions. The surg'- 

 differences become nil near Rodanthe south of the inlet (13 miles), Manteo 

 north of inlet (12 miles), and 10 miles west of the inlet. So, the closed- 

 inlet computation with the nearshore model indicates that the maximum radius 

 of influence cf the inlet is about 10 miles but changes of a foot or more only 

 will occur within a radius of A miles from the inlet's center. For example, 

 water elevations at the Tea Island Coast Guard Station and the Oregon Inlet 

 Marina increased by 2.4 ft and 1.4 ft, respectively, under closed inlet con- 

 ditions. Any ronradical changes to the inlet's hydrodynamics (such as 

 jetties) will exert a minimal influence on peak surge levels, as the jetty 

 computations of this study will show. 



Storm Surge Tests 



56. All of the storm surge simulations with the jetties were computed 

 in the nearshore model. Hurricane Donna was simulated with the 2,500- and 

 5,000-ft structures. The 5,000-ft structure changed the surge levels and peak 

 velocities very little from the nearshore simulation with existing conditions 

 at the inlet, while the 2,500-ft structure slightly raised the surges and 

 velocities at the peak of Hurricane Donna. 



57. Plate 41 shows the flew patterns through Oregon Inlet at the peak 

 (hour 22) of the high velocities that occurs nearest the north jetty, due to 

 the presence of shallow water near the south structure. Plates 42 and 43 com- 

 pare marigrams of the 2,500-ft structure to existing conditions for the sites 

 of Figure 8 while Plates 44-47 compare the corresponding velocity records. The 



37 



