height of vertical, concave curved, or re-entrant face walls above the 

 low water datum may be set at 



h^ = ht -^ 0.7 H^ (15) 



if the wall is located in or landvjard of the breaker zone, and 



h^ = h^ + 0,6 H (16) 



if the wall is located seaward of the breaker zone 

 where h^ is the wall crest elevation above mean low water; 



h-|^ is the highest still water level above mean low water 



expected at the structure; (this may be due to tides, seiches, 



wind set-up; etc.); 

 H is the wave before breaking (in the absence of any structure); 

 H-)-) is the wave height at breaking (in the absence of any 



structure) . 



122 o Stepped or sloping faced seawalls should have a soirewhat higher 

 crest elevation as their shape is conducive to permitting water to overtop 

 them with an effective horizontal momentum. For these, the wall crest 

 elevation may be set at 



he = h^ + 1,3 Hb (17) 



123, Relative effectiveness of vertical and curved re-entrant face 

 walls of heights less than required in equations (l5 ) and (I6) may be 

 found by reference to Figure 57. Considering a wall designed as in 

 equation (l5) and (j.6) as being 100 percent efficient in preventing over- 

 topping, the percent of incident wave height transmitted by walls of lesser 

 height is shown by the solid lines of figure (57). It must be noted that 

 these curves are approximate only. 



12ij., Bulkheads so located as to have a permanent beach berm to protect 

 them from the direct impact of the waves may have their crest height re- 

 duced to a minimum of 2 feet above the height of maximum wave uprush or to 

 the height of fill the bulkhead is designed to retain. 



125. Determination of Ground Elevation Before the Wall . - Seawalls 

 are usually built to protect the upland area from the effects of continuing 

 erosion as well as to protect shore improvements from daiaage by wave attack. 

 Although the exact nature of the effect of such a wall on the processes 



of erosion cannot be determined (in certain instances these processes 

 seem to have been halted or reversed), for safety in design they must be 

 considered to continue. A rigorous determination of the beach profile 

 that will obtain after the construction of the wall being impossible, 

 approximate methods must be relied upon. 



126, One such method is to assume that a wall would have no effect 



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