217 « Wa%'"es Breaking Seaward of a Structure . - Certain protective 

 structures may be so located that even under severe storm and tide con- 

 ditions waves will brealc before striking the structure o For example, 

 bulkheads built landward of the high water shore line may have water against 

 them only under extreme tidal and storm conditions and then only to a depth 

 of one or two feet. Und.er these conditions, approximate design forces may 

 be computed i-ri.th the following assurptions j 



(1) That after bre .-ilang a wave will run up a slope to an elevation 

 no higher than the elevation of the crest of the i-rave on 

 breaking o 



(2) Immediately after bi"eaJdLng, the water mass moves forward with 

 the velocity of wave propagation just before breaking] that is 

 upon breaking the water particle motion changes from orbital 

 to translatory. 



Under these assumptions, the wave pressure or force on an obstruction raay 

 be calculated by assiuning a uniform velocity decrease to zero from the 

 point of breaking to the point of maximum uprush. It l"ias been found from 

 model tests, that upon breaking, approximately 70 percent of the full break- 

 ing wave height K is above <. the still water level. Then in Figure "]$ , the 

 veloc ity v of wave uprush at the wall for a breaking wave velocity of 

 C = v^is: 



V = ( ~~ ) C = G(l - ^ ) 03) 



^2 ^2 



where x, = distance from the brealdng wave to the structure 



x_ = distance from the breaking xirave to the limit of wave uprush 

 '^ = (d^ + h^) cota = (d^ + 0,7 H^) cc 



;ota 



a ~ the angle of the beach slope with the horizontal 



d^ = brealving depth 



h = the height of the breaking wave abo'/e still water level 



if = the breaking wave height. 



218 o The pressure on the wall id.ll be partly dynaiiiic and partly 

 static; the dynamic par-t being 



2 , X., 2 



r 



'm 2g" 



p = ^•''^ wd .^ _ 1 ^ (3h) 



and the static part varying from zero at a height h above still water 



level to a maximum P = w(h - h ) at the wall base (depth d, ) h being the 

 SOW/ ^ -^ b w 



110 



