SPECTRAL STABILITY NUMBER Ns« 



Figure 10. Reef breakwater response slope versus the spectral 

 stability number for stability subsets 1, 3, 5, 7, and 9 



since the natural angle of repose for gravel is about 45 deg, giving C = 1.0 

 for a triangular reef cross section with side slopes of IV on 1H. Equation 7 

 can be compared to the observed data in Figure 10. It is surprising that the 

 response slope of the reef, stone size and density, and severity of wave 

 attack can all be linked with a relation as simple as that in Equation 7. It 

 is difficult to add secondary stability variables to an equation like Equa- 

 tion 7 and improve the ability to predict the response slope over Equation 7 

 very much. At the same time it is clear from Figure 9 that secondary stabil- 

 ity factors have some influence on reef stability. After trial and error the 

 following equation was developed which includes one secondary stability vari- 

 able and does a better job of predicting the response slope of the reef: 



C = — 7y = exp 



N* 



0.0676 + 0.0222 



h' 



(8) 



where the relative "as built" crest height of the reef h'/d was added to an 



c s 



equation like Equation 7 to improve the predictive ability. Equation 8 



23 



