Figure 8 presents another method of analyzing the same information. 

 Starting with a trial design which is definitely less stable than the most 

 economic, curve a represents the additional investment required for various 

 more stable designs and curve a is the corresponding average annual repair 

 cost. Curve b is the incremental reduction in average annual repair cost. 

 Curve e is curve d divided by curve b and represents the effective return 

 on each incremental investment. The economical design is selected as the 

 one beyond which an additional increment of investment fails to offer an 

 attractive return. 



It is realized, of course, that such procedures involve low, if not 

 negative safety factors under extremely adverse conditions. Although this 

 is a logical philosophy for severe but infrequent conditions like destruc- 

 tive earthquakes, bomb blast, or severe storm waves, catastrophic failures 

 must be considered and avoided in such calculated-risk designs. 



b. Armor Design - The required tetrapod weight was determined by 

 extrapolation of U.S. Army, Corps of Engineers, Waterways Experiment 

 Station (1955), tests conducted for the Crescent City breakwater design. 

 Using the modified Iribarren formula, 



W = . ,3 (S, - S.)3 



ill cos a — sin af "^ J 



where 



W = required weight of individual units 

 of armor material in pounds 



K^ = dimensionless coefficient, 



w = unit weight of fresh water, 



Sj- = specific gravity of armor material, 



S£ = specific gravity of fluid, 



H = wave height for no damage, 



y = coefficient of friction of armor 

 material , 



X = angle, measured from horizontal of 

 breakwater slope, 



values of K^ = .0223 and y = 1.10 were used. A principal reason for the 

 laboratory wave tests of the west face revetment was to verify these 

 values, which were confirmed. 



28 



