recommended that the toe stone be exposed to wave attack; therefore, the wall 

 was tested with a fronting slope intersecting the toe of the riprap at 

 +1.0 NGVD (Figure 6b). 



19. Initially, a berm with a median stone weight of 250 lb (Figure 5b) 

 was tested and found to be unstable (Photo 1), Based on Goda's stability 

 theory (Goda 1985 and Tanimoto 1982) and engineering judgment, a 1,000-lb 

 median stone weight (Figure 6c) was selected for testing. Stability of this 

 berm was acceptable for all test conditions (Photo 2) . 



20. Observations made during the tests suggested that the fronting rip- 

 rap toe reduced wave overtopping, especially at swl's of +7.0 ft and less. 

 This conclusion is based on the observed energy dissipation as waves propa- 

 gated over the berm at the lower swl's. A quantitative description of the 

 influence of the fronting riprap on overtopping rates at low water levels is 

 difficult; however, it is apparent that at swl's of +8.0 ft and above the 

 fronting riprap caused little reduction of overtopping rates at the seawall. 

 To accomplish a significant decrease in the overtopping rates, the berm width 

 would have to be increased considerably before its dissipating effect on the 

 long-period storm waves which were tested would be noticeable. Nevertheless, 

 presence of the 10-ft-wide fronting berm could be advantageous in other re- 

 spects. The riprap may help minimize undermining at the toe of the structure 

 and could help to reduce erosion of the beach adjacent to the structure by 

 absorption of incident wave energy. Since the Virginia Beach seawall was de- 

 signed with a steel sheet-pile cutoff wall to prevent undermining of the 

 structure, inclusion of a fronting riprap berm may not be necessary. 



Analysis of Overtopping Parameters and Trends 



21. The dimensionless relative freeboard parameter which consolidates 

 the data into a single trend was first developed and used for the Roughans 

 Point seawall /revetment study for US Army Engineer Division, New England 

 (Ahrens and Heimbaugh 1986) . The relative freeboard parameter Is defined as 

 follows: 



F' - '-jj^ (1) 



(«' ^ ) 



\ mo p/ 



18 



