23. Influence of reef bulk. Subsets 1 and 5 can be used also to 

 illustrate the influence of size or bulk of the reef on stability. Even 

 though the difference in relative height for the two subsets narrows with 

 increasing N* , the crest heights of the reefs of subset 5 always are higher 

 than those of subset 1. In fact, Figure 9 shows that the relative position of 

 the trends for subsets 1, 3, and 5 are maintained such that the larger struc- 

 ture always has a greater crest height than the smaller structure for a given 

 value of N* . In order to intercompare the stability of all subsets, a 

 general measure of breakwater size is needed which will be consistent with the 

 data trends shown in Figure 9. Within this context, the variable which best 



characterizes the size of the reef breakwater is called the bulk number B 



n 



and is defined as 



A A 



B ~ " T73 " "f (3) 



W 50\ So 



where 



2 

 A = area of breakwater cross section, cm 



t 3 



w = unit weight of stone g/cm 



d _ = dimension of stone, cm 



24. Bulk number can be described as the equivalent number of median 



stones per median stone width in the breakwater cross section. Equivalent is 



used because B does not include the influence of porosity which is about 

 n 



45 percent for the two stone gradations used in this study. The value of the 

 bulk number lies in its ability to explain the rather straightforward behavior 

 of the relative location of the damage trends for subsets 1, 3, and 5 in Fig- 

 ure 9. It also explains the rather anomalous behavior, such as that of the 

 trend for subset 9 crossing the trend for subset 1. At first it seems sur- 

 prising that the reefs of subset 9 degrade faster than those of subset 1, con- 

 sidering that the reefs of subset 9 have the greater cross-sectional area (see 

 Table 2). However, when the bulk number is used to measure the size of the 

 reef rather than the cross-sectional area, the relative behavior of the damage 

 trends for subsets 1 and 9 seems more plausible. Subsets 1 and 9 have bulk 

 numbers of 337 and 222, respectively, indicating that the reefs of subset 1 



20 



