is therefore only meaningful in the range of 0-3 percent primary armor dis- 

 placement. Nielsen and Burcharth (1983) have indicated that measurements 

 of very low levels of displacement or rocking (0-3 percent) are less reliable 

 than those of higher levels. This trend relates to the resolution of mea- 

 surement techniques as well as the repeatability of the experimental results 

 themselves. 



59. Given a relatively consistent and precise method of measuring 

 displacement, Ahrens (198^1) has proposed a useful dimensionless parameter for 

 systematic quantification of breakwater damage: 



where Ap is the average eroded cross-sectional area for a specific length of 

 model breakwater (Figure 10). Van der Meer and Pilarczyk (1984) applied the 

 following dimensionless damage parameter Sg in their model tests of quarry- 

 stone, which was mentioned previously (Equations 5 through 8) in the discus- 

 sion of their conclusions regarding stability: 



S„ = 1—^^ (13) 



2 



Kso)' 



It is also important to identify erosion of the underlayers or core that may 

 coincidentally occur with erosion of the armor layer. 



Analytical Damage Prediction 



60. Scale model studies reported by Jackson (1968a) and Carver and 

 Dubose (in preparation) have addressed, to a limited degree, the level of dam- 

 age to breakwater armor layers experienced when the design wave height is ex- 

 ceeded. This information was applied to formulate Table 7-9 in the SPM (1984) 

 which predicts the percent damage 7»D for various armor types as a function 

 of the design wave exceedance ratio H/Hj where H is a monochromatic inci- 

 dent wave height which is greater than the design wave height H , . The re- 

 serve stability trends, or tendency for damage levels to increase with design 

 wave exceedance ratio, can also be characterized by a function of the follow- 

 ing form: 



37 



