(5) Under similar wave conditions, the head of a rubble structure 

 normally sustains more extensive and frequent damage than the trunk of 

 the structure. Under all wave conditions, a segment of the slope of the 

 rounded head of the structure is subject to overtopping. A part of the 

 head is usually subject to direct wave attack regardless of wave direc- 

 tion. A wave trough on the lee side coincident with maximum runup on the 

 windward side will create a high static head for flow through the struc- 

 ture. 



Based on available data and the discussion above. Table 7-6 pre- 

 sents recommended values for K^. Because of the limitations discussed, 

 values in the table provide little or no safety factor. The experience 

 of the field engineer may be utilized to adjust the K/) value indicated 

 in Table 7-6, but deviation to less conservative values should be fully 

 evaluated. A two-unit armor layer is recommended. If a one-unit armor 

 layer is considered, the K^ values for a single layer should be ob- 

 tained from Table 7-6. The indicated K^^ values are less for a single- 

 stone layer than for a two-stone layer, and will require heavier armor 

 stone to ensure stability. More care must be taken in the placement of 

 a single armor layer to ensure that armor units provide an adequate cover 

 for the underlayer and that there is a high degree of interlock with 

 adjacent armor units. 



These coefficients were derived from large- and small-scale tests 

 that used many various shapes and sizes of both natural and artificial 

 armor units. Values are reasonably definitive, and are recommended for 

 design. 



The values given in Table 7-6 are indicated as no-damage criteria, 

 but actually consider up to 5 percent damage. If some degree of damage 

 to the cover layer is acceptable, slightly larger values of Kn can be 

 used for design. The deliberate selection of a larger value of % 



than recommended in Table 7-6 may be partly justified by the fact that 

 settlement of the structure and readjustment of the interlocking between 

 armor units can result in a more stable structure than the original 

 structure. It is possible that structural damage will occur to indi- 

 vidual concrete armor units during movement and rekeying of the units. 

 However, a structure designed to resist waves of a moderate storm, but 

 which may suffer damage without complete destruction during a severe 

 storm, will have a lower annual cost than one designed to be completely 

 stable for larger waves. Values of K^ as a function of percent damage 

 to the rubble structure have been determined for several of the armor 

 unit shapes. (See Table 7-7.) These values, together with statistical 

 data concerning the frequency of occurrence of waves of different heights, 

 should be used to determine the annual cost as a function of the accept- 

 able percent damage without endangering the functional characteristics 

 of the structure. 



Table 7-7 shows the results of damage tests where H/Hj^-^ and K^j 

 are functions of the percent damage D for various armor units. H is 

 the significant wave height corresponding to damage D. H^-n is the 



7-177 



