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 struc- 

 ture subject to overtopping. As a general guide for overtopping condi- 

 tions, the minimum crest width should equal the combined widths of three 

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

 equation. 



WV/3 



B 



nk/ 



(7-107) 



where , 



B = crest width, ft. 



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



k^ = layer coefficient (Table 7-10) 



W = weight of armor unit in primary cover layer, lbs. 



Wj, = unit weight of armor unit, lbs. /ft? 



Tlie crest must be wide enough for construction and maintenance equipment 

 operated from the structure. 



Table 7-10. Layer Coefficient and Porosity for Various Armor Units 



b. Concrete Cap for Rubble-Mound Structures . Poured-in-place 



concrete has been added 

 breakwaters. Such use 

 the cover layer, on the 

 permits, to casting lar 

 This concrete may serve 

 crest; (2) to increase 

 access along the crest 



to the cover layer of rubble-mound jetties and 

 ranges from filling the interstices of stones on 



crest, and as far down the slopes as wave action 

 ge monolithic blocks of several hundred tons. 



any of three purposes: (1) to strengthen the 

 the crest height; and (3) to provide roadway 

 for construction or maintenance purposes. 



7-196 



