The crest of a structure designed to be submerged, or that might be 

 submerged by hurricane storm surge, will undergo the heaviest wave action when 

 the crest is exposed in the trough of a wave. The highest wave which would 

 expose the crest can be estimated by using Figure 7-69, with the range of 

 depths at the structure d , the range of wave heights H , and period T , 



n 

 and the structure height h . Values of -r— , where r\ is the crest 



H Q 



elevation above the still-water level, can be found by entering Figure 7-69 



H J d 



— =■ and — -r 



gT^ gT' 



with — =■ and — =■ . The largest breaking and nonbreaking wave heights for 



which 



d < h + H - n (7-114) 



e 



can then be used to estimate which wave height requires the heaviest armor. 

 The final design breaking wave height can be determined by entering Figure 



d ""a 



7-69 with values of — ^r , finding values of -rr- for breaking conditions, and 



selecting the highest breaking wave which satisfied the equation 



d = h + H - n (7-115) 



a 



A structure that is exposed to a variety of water depths, especially a 

 structure perpendicular to the shore, such as a groin, should have wave 

 conditions investigated for each range of water depths to determine the 

 highest breaking wave to which any part of the structure will be exposed. The 

 outer end of a groin might be exposed only to wave forces on its sides under 

 normal depths, but it might be overtopped and eventually submerged as a storm 

 surge approaches. The shoreward end might normally be exposed to lower 

 breakers, or perhaps only to broken waves. In the case of a high rubble-mound 

 groin (i.e., a varying crest elevation and a sloping beach), the maximum 

 breaking wave height may occur inshore of the seaward end of the groin. 



c. Hydraulics of Cover Layer Design . Until about 1930, design of rubble 

 structures was based only on experience and general knowledge of site 

 conditions. Empirical formulas that subsequently developed are generally 

 expressed in terms of the stone weight required to withstand design wave 

 conditions. These formulas have been partially substantiated in model 

 studies. They are guides and must be used with experience and engineering 

 judgment. Physical modeling is often a cost-effective measure to determine 

 the final cross-section design for most costly rubble-mound structures. 



Following work by Iribarren (1938) and Iribarren and Nogales Y Olano 

 (1950), comprehensive investigations were made by Hudson (1953, 1959, 1961a, 

 and 1961b) at the U.S. Army Engineer Waterways Experiment Station (WES), and 

 a formula was developed to determine the stability of armor units on rubble 

 structures. The stability formula, based on the results of extensive small- 

 scale model testing and some preliminary verification by large-scale model 

 testing, is 



7-204 



