Table 2 

 Phase I Seawall Compared to Phase II Seawall for Hurricane Conditions 



swl 

 ft 



+9.5 



+8.0 



+7.0 



Percent Decrease in Q 



Phase I vs Phase II 

 30 to 70% 



43 



48 



54 



Percent Decrease in 

 Phase I vs Phase II 

 100% 



18 



24 



31 



the +9.5 swl and up to 60 percent for all lower swl's could be reproduced in 

 the Phase I seawall tests, relative performance of the Phase II seawall (where 

 wave heights up to 100 percent DWHAWB were reproduced) presented in Table 2 is 

 given first based on 30 to 70 percent data and finally on all data. Based on 

 these comparisons, it can be seen that the seawall geometry changes made from 

 Phase I seawall to Phase II reduced the overtopping rate between 18 and 

 54 percent depending on the conditions compared. Table 2 also shows that as 

 the swl increased geometry modifications had a smaller effect on reducing the 

 overtopping rate. This occurrence was expected since, as the water level was 

 increased, the waves became larger and began to inundate the wall more often. 

 In short, changes in seawall geometry are less effective at higher swl's. 



27. As mentioned earlier, the decision to lower the beach elevation in 

 model tests from +3.4 to +1.0 ft NGVD to test stability of the fronting riprap 

 also affected overtopping rates. For instance, a small change in the depth at 

 the structure toe d can significantly affect the magnitude of Q . By 

 lowering the beach elevation, d is increased, and this increase in turn af- 

 fects the local wave length L used in Equation 1. Also, as the water depth 

 near the structure becomes deeper, a larger wave can be supported. Thus, as 



d increases, H and L increase, causing the relative freeboard param- 

 s mo p 



eter F' to decrease. As can be seen in Figures 9 and 10, as F' decreases, 

 the overtopping rate Q increases exponentially. Therefore, by decreasing 

 the beach elevation from +3.4 to +1.0 NGVD the overtopping rate is, in effect, 

 increased. Overtopping rates for the +3.4-ft NGVD beach elevation were esti- 

 mated as explained in the following paragraphs. 



28. The relative freeboard versus Q plot was used to predict over- 

 topping rates for extrapolated values of d and/or F provided the 



25 



