This scale was selected based on the size of available tribar units for 

 the tests, compared with the estimated size of the required prototype 

 armor unit, using equation (6-4b) and a value of K = 10, and the capa- 

 bility of the wave generator compared with the wave heights required in 

 the model tests. Although the selected design wave for the proposed 

 structure was 13 feet in height with a period of 11 seconds, test waves 

 larger than the design wave should always be used to obtain sufficient 

 damage to indicate the degree of risk involved in the use of the selected 

 armor unit weight. The design wave selected is equal to the significant 

 wave. In a natural wave spectrum, the height of the significant wave is 

 exceeded by 13 percent pf the waves which may reach heights as much as 

 1.87 (Hjfe) . After the linear scale was selected, the required model 

 limestone armor units were obtained from the Bloomington quarry and sawed 

 to model sizes and shapes estimated from a survey of the available stones 

 stockpiled at the quarry. The specific weights of the water and the lime- 

 stone armor units were the same, model-to-prototype, according to the 

 similarity requirements based on Froude's law. However, the specific 

 weight of the molded tribar armor units was 140.4 pounds per cubic foot; 

 the specific weight of the concrete armor units in the prototype was 

 assumed to be 150 pounds per cubic foot. The relations between the 

 weights and specific weights for these units, model-to-prototype, were 

 determined from the following derived transference equation: 



«. (^4 



(N„ W„\^ 



(H 



- 1 



ei 



-1 



(6-39) 



where 



subscript m = model quantity 



subscript p = prototype quantity 



W^ = weight of armor unit in pounds 



Y = specific weight of armor unit in pounds per 



cubic foot 



Sg^ = specific gravity of an armor unit relative to 



the water in which the breakwater is situated 

 (i.e., S^ = Ya/Yw where y^ is the specific 

 weight of the water) 



Details of the test section using the tribar armor units are shown in 

 Figure 6-18. Although seven different sections were tested where the 

 limestone blocks were used as armor units, the major differences in the 

 sections concerned the placing method of the armor units. Figure 6-19 

 shows a section of uniformly placed limestone blocks; Figure 6-20 shows 

 a section of randomly placed limestone armor units. Table 6-2 gives the 

 weights of the tribar and limestone armor units, the underlayer units, 

 and the core stone. 



372 



