the computation for H' is illustrated in the following table. 



TABLE 31 - Computation of H and H' for various values of d 



H^ = 17, L^ = 18U b^b = 0.81i 



d 



d/L^ 



smh .^jr- 



(h/h'q) 



«s-^'^ 



H'* 



13 

 20 

 30 



0.070^ 



0,109 



0.163 



0.7823 



1.076 



1.5U6 



O.970I4 

 0,9263 

 0,9130 



13.9 

 13.2 

 13.1 



0.1; 

 2.6 

 1.2 



* H and H' are plotted on Figure I30 



ii02. Assuming that 6-ton stone is available and will be placed in 

 the section between the elevations of -13 and -20 feet, and that quarry- 

 run stone averaging l/2 ton would be used below elevation -20 feet, 

 stable slopes may be found from Plate 9a, Appendix D with K' = 0,0l5, 

 With the 6 ton stone, W/K' = 8 x 10^ and with H' = 5. it feet (d = 13 feet) 

 the stable slope is found to be about 1 on 1-l/lj.. With the l/k ton stone 

 W/K' = 3.3 X 10^ and with H« = 2.6 feet (d = 20 feet) the stable slope 

 is found to be about 1 on l.U. 



[i03. In order to obstruct the vjaves effectively the breakwater crest 

 should be at an elevation of about 19 l/2 feet (H x 1-1/2) above still 

 water level. If only partial obstiniction is mandatory the crest elevation 

 may be set somewhat lower. The choice between the two must be made on 

 a use and economic basis. Crest widths are determined mainly by the in- 

 tended use of the breaktjater or cans traction methods emplojred; in this case 

 (say) 20 feet. Landward side slopes ordinarily should not be less than 

 1 on I-I/2 to the depth of one wave height (13 feet), thence 1 on 1-l/U to 

 the bottom. With these criteria and the computed sea side slopes of 1 on 

 1.75 to ele.vation -13, 1 on 1.20 to elevation -20, and 1 on l.I; to the 

 bottom, a stable -profile may be dra-wn as in Figure 131. Note that end 

 slopes should be designed in the same manner as sea side slopes. 



hOU. Composite Type Brekkwaters , A composite type breakwater is one 

 comprised of two or more types and materials or of two or more materials. 

 There have been m.any types of composite breakwaters designed and constructed 

 x\There it has a-opeared that a saving in time, material, or cost would toe 

 effected thereby. One type of composite breakvrater consisted of a shale 

 or earth core incased in a rubble-mound covering. Another type is comprised 

 of a concrete caisson i>ra,ll set on a rubble-mound base. 



I4.O5. As wave action is affected by the profile of the substructure, 

 the characteristics of the wave which act on the superstructure must be 

 determined accordingly. Should it be desired to place the base of the 



195 



