Tjirum (1966), Johnson, Mansard, and Ploeg (1978), and Burcharth (1979), to 

 account for higher damage in rubble-mound or armor block, structures. 

 Burcharth (1979) found that grouped wave trains with maximum wave heights 

 equivalent to monochromatic wave heights caused greater damage on dolosse- 

 armored slopes than did monochromatic wave trains. Johnson, Mansard, and 

 Ploeg (1978) found that grouped wave trains of energy density equivalent to 

 that of monochromatic wave trains created greater damage on rubble-mound 

 breakwaters. 



Goda (1970b) and Andrew and Borgman (1981) have shown by simulation 

 techniques that, for random-phased wave components in a wave spectrum, 

 groupiness is dependent on the width of the spectral peak (the narrower the 

 spectral width, the larger the groupiness in the wave train). 



On a different tack, Johnson, Mansard, and Ploeg (1978) have shown that 

 the same energy spectrum shape can produce considerably different damage 

 patterns to a rubble-mound breakwater by controlling the phasing of the wave 

 components in the energy spectrum. This approach to generating irregular 

 waves for model testing is not presently attempted in most laboratories. 



Typically, laboratory model tests assume random phasing of wave spectral 

 components based on the assumption that waves in nature have random phasing. 

 T/4rum, Mathiesen, and Escutia (1979), Thompson (1981), Andrew and Borgman 

 (1981), and Wilson and Baird (1972) have suggested that nonrandom phasing of 

 waves appears to exist in nature, particularly in shallow water. 



(2) Preliminary analysis of large-scale tests by Hudson (1975) has 

 indicated that scale effects are relatively unimportant, and can be made 

 negligible by the proper selection of linear scale to ensure that the Reynolds 

 number is above 3 x 10 in the tests. The Reynolds number is defined in 

 this case as 



where v is the kinematic viscosity of the water at the site and k is the 

 layer coefficient (see Sec. III,7,g(2)). 



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

 armor units in the laboratory is unlikely to be duplicated in the prototype. 

 Above the water surface in prototype construction it is possible 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 K values. 



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

 only limited test results are available for plunging waves. Values for these 

 conditions were estimated based on breaking wave tests of similar armor 

 units. The ratio between the breaking and nonbreaking wave K 's for 

 tetrapods and quadripods on structure trunks, for example, was used to 

 estimate the breaking wave K 's for tribars, modified cubes, and hexapods 



7-209 



