(6) Part of structure (trunk or head) 



(7) Angle of incidence of wave attack 



(8) Model scale (Reynolds number) 



(9) Distance below still-water level that the armor units extend down the 

 face slope 



(10) Size and porosity of underlayer material 



(11) Core height relative to still-water level 



(12) Crown type (concrete cap or armor units placed over the crown and 

 extending down the back slope) 



(13) Crown elevation above still-water level relative to wave height 



(14) Crest width 



Hudson (1959, 1961a, and 1961b), and Hudson and Jackson (1959), Jackson 

 (1968a), Carver and Davidson (1977), Markle and Davidson (1979), Office, Chief 

 of Engineers (1978), and Carver (1980) have conducted numerous laboratory 

 tests with a view to establishing values of Kt^ for various conditions of 

 some of the variables. They have found that, for a given geometry of rubble 

 structure, the most important variables listed above with respect to the 

 magnitude of Kr^ are those from (1) through (8). The data of Hudson and 

 Jackson comprise the basis for selecting K„ , although a number of limita- 

 tions in the application of laboratory results to prototype conditions must be 

 recognized. These limitations are described in the following paragraphs. 



(1) Laboratory waves were monochromatic and did not reproduce the 

 variable conditions of nature. No simple method of comparing monochromatic 

 and irregular waves is presently available. Laboratory studies by Oeullet 

 (1972) and Rogan (1969) have shown that action of irregular waves on model 

 rubble structures can be modeled by monochromatic waves if the monochromatic 

 wave height corresponds to the significant wave height of the spectrum of the 

 irregular wave train. Other laboratory studies (i.e., Carstens, Traetteberg, 

 and T^rum (1966); Brorsen, Burcharth, and Larsen (1974); Feuillet and Sabaton 

 (1980); and Tanimoto, Yagyu, and Coda (1982)) have shown, though, that the 

 damage patterns on model rubble-mound structures with irregular wave action 

 are comparable to model tests with monochromatic waves when the design wave 

 height of the irregular wave train is higher than the significant wave 

 height. As an extreme, the laboratory work of Feuillet and Sabaton (1980) and 

 that of Tanimoto, Yagyu, and Goda (1982) suggest a design wave of He when 

 comparing monochromatic wave model tests to irregular wave model tests. 



The validity of this comparison between monochromatic wave testing and 

 irregular wave testing depends on the wave amplitude and phase spectra of the 

 irregular wave train which, in turn, govern the "groupiness" of the wave 

 train; i.e., the tendency of higher waves to occur together. 



Groupiness in wave trains has been shown by Carstens, Traetteberg, and 



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