transmission over the barrier . 



When the model was placed on a sloping beach at or near the surf 

 zone, with its top one wave height below still water level, the trans- 

 mitted wave height was approximately S0% of the incident height* Tilhen 

 the model crest was placed at the level of the trough of the incident 

 wave, this transmission coefficient was reduced to 55%, and when still 

 further raised to still water level the coefficient became U0%-, If we 

 assume that the relative energy transmitted is proportional to the 

 square of the relative transmitted wave height (this is not strictly 

 true since energy is also a function of wave length, but another study (27) 

 (See page 13 ) indicates that the relationship of heights squared is 

 sufficiently accurate^ the energy transmitted at these three barrier crest 

 heights is approximately 80^, 30%, and 15% of the incident wave energy. 

 Letting the energy transmitted over a wall be the measure of its 

 effectiveness, we have four points through which a curve of wall efficiency 

 versus its crest height relative to still water level may be drawn, 

 (See Figure 3)o 



V, Seawall Seaward of the Breaker Zone 



General - It is quite possible that a seawall must be placed on a 

 slope in such a position that the depth of water at the wall would not 

 be shallow enough to cause the maxim-um expected wave to break. That 

 this may come about may be seen by referring to section II in which 

 water depth variability is discussed. The wave attack at such a location 

 will differ from that on a structure in the breaker zone, therefore a 

 different approach must be used to find the maximum wave height expected 

 at the wall's depth <, 



Theoretically, many approaches have been made to determine the cliange 

 in wave parameters with decrease in depth. A few will be noted. For 

 waves of finite height, Stokes (^0:>11) and StruikC-^*^^) foiand to ? third 

 approximation that the velocity of oscillatory waves is given by 



(5) c2 =^f ^w--^^L, r^-^.^fi^...«^^^.< 



and the wave form by 



(6) 3,._„H:_ifs[-^-^:jJL^,--'-^j-'F 



w\ 



■fTTc/ , 





Since seawalls will always be located in relatively shallow water, 

 the solitary wave theory(6) may also apply. This gives for the velocity 



(7) C2 = g(d + H) 



and for the profile 



