higher than for rubble slopes (see Section 11,1, and Figs. 7-19 and 7-20). 

 The selected crest elevation should be the lowest that provides the protection 

 required. Excessive overtopping of a breakwater or jetty can cause choppiness 

 of the water surface behind the structure and can be detrimental to harbor 

 operations, since operations such as mooring of small craft and most types of 

 commercial cargo transfer require calm waters. Overtopping of a rubble 

 seawall or revetment can cause serious erosion behind the structure and 

 flooding of the backshore area. Overtopping of jetties can be tolerated if it 

 does not adversely affect the channel. 



The width of the crest depends greatly on the degree of allowable 

 overtopping; where there will be no overtopping, crest width is not 

 critical. Little study has been made of crest width of a rubble structure 

 subject to overtopping. Consider as a general guide for overtopping 

 conditions that the minimum crest width should equal the combined widths of 

 three armor units (n = 3). Crest width may be obtained from the following 

 equation. 



B = nk /fV/^ (7-120) 



where 



B = crest width, m (or ft) 



n = number of stones (n = 3 is recommended minimum) 



k. = layer coefficient (Table 7-13) 



A 



W = mass of armor unit in primary cover layer, kg (or weight in lb) 



3 3 



w = mass density of armor unit, kg/m (or unit weight in lb/ ft ) 



The crest should be wide enough to accommodate any construction and main- 

 tenance equipment which may be operated from the structure. 



Figures 7-116 and 7-117 show the armor units of the primary cover layer, 

 sized using equation (7-116), extended over the crest. Armor units of this 

 size are probably stable on the crest for the conditions of minor to no 

 overtopping occurring in the model tests which established the values of K^^ 

 in Table 7-8. Such an armor unit size can be used for preliminary design of 

 the cross section of an overtopped or submerged structure, but model tests are 

 strongly recommended to establish the required stable armor weight for the 

 crest of a structure exposed to more than minor overtopping. Concrete armor 

 units placed on the crest of an overtopped structure may be much less stable 

 than the equivalent quarrystone armor chosen using equation (7-116) on a 

 structure with no overtopping. In the absence of an analytical method for 

 calculating armor weight for severely overtopped or submerged structures, 

 especially those armored with concrete units, hydraulic model tests are 

 necessary. Markle and Carver (1977) have tested heavily overtopped and 

 submerged quarrys tone-armored structures. 



7-233 



