(12) core height relative to Stillwater level, 



(13) crowri type (concrete cap or armor units over the crown and 

 extending down the back slope), 



(14) crown elevation above Stillwater level relative to wave height, 

 and 



(15) crest width. 



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

 numerous laboratory tests with a view to establishing values of K^ 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 Kp are those from (1) through (8). 

 While the angle of wave approach may be important in the stability of 

 armor units especially when the waves are breaking directly on the struc- 

 ture, sufficient information is not available to provide firm guidance on 

 angle effect of the stability coefficient. The data of Hudson and Jackson 

 comprise the basis for selecting K^i, although a number of limitations in 

 the application of laboratory results to prototype conditions must be 

 recognized. These are: 



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

 variable conditions of nature. Laboratory studies by Ouellet (1972) and 

 Rogan (1969) have shown that action of irregular waves (wave spectrum) on 

 model rubble structures can be modeled by monochromatic waves if the mono- 

 chromatic wave height corresponds to the significant wave height of the 

 spectrum. The validity of this comparison depends somewhat on the shape 

 of the wave spectrum, with the best agreement for a narrow band spectrum 

 (narrow range of frequencies or periods) when the wave heights are dis- 

 tributed according to a Rayleigh distribution. (See Section 3.2.) 



(2) Preliminary analysis of large-scale tests has indicated that 

 the scale effect is probably unimportant, and can be made negligible by 

 the proper selection of linear scale for the tests (Reynolds Number, 



Re ^ 6 X 10"+). 



(3) The degree of interlocking obtained in the special placement 



of armor units in the laboratory is unlikely to be duplicated in the pro- 

 totype. Above the water surface in prototype construction, it is possi- 

 ble to place armor units with a high degree of interlocking. Below the 

 water surface, the same quality of interlocking can rarely be attained. 

 It is therefore advisable to use data obtained from random placement in 

 the laboratory as a basis for Kp values. 



(4) Numerous tests have been performed for nonbreaking waves, but 

 only limited tests are available for plunging waves. Limited test re- 

 sults for breaking waves indicate that the K^ value for breaking waves 

 is proportional the Kj) value for nonbreaking waves. Therefore, K^ 

 values for armor units not tested for breaking waves have been obtained 

 by applying a reduction factor to the Kp value for nonbreaking waves. 



7-176 



