stability characteristics and thus its stability coefficient will not change 

 (disregarding any changes brought about by additional testing) . The designer 

 is therefore dealing with a relatively constant characteristic of the armor unit. 

 Figure 3 demonstrates how the preceding armor unit costs factors influence 

 selection of a design level. The figure shows how the total average annual cost 

 of a rubble structure varies with design wave height. If a large design wave 

 height is selected, a more massive structure design results with a corresponding 

 high first cost. The probability of the large design wave being exceeded in any 

 given time period is relatively small and, therefore, the need to repair damage 

 caused by waves larger than the design wave will be relatively small; conse- 

 quently, the annual cost for maintenance and repair will be low. Also, since a 

 large structure will be designed, the amount of protection afforded the area in 

 its lee will be high, thereby providing greater economic benefits. In contrast, 

 if a low design wave height is selected, a relatively cheaper, smaller structure 

 will result from the design. This structure will have a lower first cost, but 

 the probability of the low design wave being exceeded in a given time period 

 will be relatively high. The average annual cost of maintaining and repairing 

 the structure will also be high since the design wave height may be exceeded 

 frequently. In addition, the smaller sized structure may not offer much pro- 

 tection to the area behind it because of the frequent damage. Benefits realized 

 by the project may therefore be lower, or equivalently as shown in Figure 3, the 



First Cost (onnual basis) / 



Avg Annual Mointenonce ond 

 ^ Repoir Costs / 



\ 



/ 



y 



Benefits Foregone 



Design Wove Heigtit 



Figure 3. Relationship between first costs, 



maintenance and repair costs, benefits 

 foregone, and design wave height. 



12 



