the relative severity of wave attack on reef breakwaters. 

 Secondary stability factors 



21. Data analysis and observation of the laboratory tests indicate that 

 several factors other than the spectral stability number have a quantifiable 

 influence on the stability of reef breakwaters. Figure 9 will help identify 

 what will be referred to as secondary stability factors or variables. In 

 Figure 9 the damage trends for all five stability subsets are shown using sub- 

 jectively drawn curves. Figure 9 shows the relative crest height h /d (see 

 Figure 3) as a function of the spectral stability number. For intercomparing 



damage trends between subsets, the variable h /d is better than h /h' . 

 ° c s c c 



When various subsets are plotted using h /h' , the data trends tend to fall 



c c 



on top of each other, especially for N* < 8 . Using h /d to show damage 

 trends spreads the data out so that subsets can be distinguished and provides 

 better orientation by showing the swl. 



22. Relative exposure to wave action. One secondary stability factor 

 is the relative exposure of the structure to wave action. Submerged break- 

 waters are much less exposed to wave attack than breakwaters with crests above 

 the water level. Water overlying a submerged crest greatly dampens wave 

 impact forces and attenuates the lift and drag forces on the stone. This fac- 

 tor is illustrated in Figure 9 where structures with the greater initial rela- 

 tive height h'/d have their height reduced more rapidly with increasing N* 

 than structures with lower initial relative height. In Table 4, which can be 

 used with Figure 9 to evaluate the influence of secondary stability factors, 

 the average value of initial relative crest height h'/d is given by subset 

 along with two other secondary stability factors, the bulk number and the "as 

 built" effective reef slope C' , which are discussed below. Subsets 1 and 5, 

 which represent tests using the same stone size and water depth, illustrate 



the influence of h'/d on stability. Figure 9 shows that the wide dif- 

 c s 



ference in initial relative height of these structures is maintained until N* 



is about 6.0: however, when noticeable stone movement starts at about N* 

 ' s 



= 6 , the difference in relative heights for the reefs of the two subsets 

 tends to decrease with increasing value of N* . For the most severe condi- 

 tions at about N* = 17 , the difference in relative height between the two 

 s 



subsets is not very large. Based on analysis of all the data, it is concluded 

 that the greater the initial height of the reef the more vulnerable it is to 

 wave attack. 



18 



