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RELATIVE FREEBOARD. F' 



Figure 19. Overtopping rate versus the relative freeboard for the 

 Cape Hatteras vertical seawall (configuration CH-3) 



are shown with shaded symbols which follow a trend distinctly lower than that 

 from the data collected at two slightly higher water levels. 



25. During the Roughans Point study, a standard multilayered riprap 

 revetment was built against the seawall with the top of the revetment near the 

 top of the wall just below the recurve. The revetment reduced the overtopping 

 rates by about 45 percent over the same seawall configuration without a revet- 

 ment. Figure 20 shows a comparison of the performance of the two configura- 

 tions using data trend curves developed from regression analysis. Standard 

 riprap revetment was found to be less effective in reducing overtopping than 

 was originally thought. Two possible reasons for the disappointing perfor- 

 mance of the riprap were identified: 



a. When the water levels are high, waves ride over most of the 

 revetment without much attenuation. 



b. When water levels are low, the standard revetment provides a 

 ramp for waves to ride up and over the wall without encounter- 

 ing a major discontinuity to their flow. 



The second factor suggests that it might be better not to build the revetment 



very high against the wall but to use a profile having a berm. This type of 



28 



