(14) 



A suggestion for a theoretical approach was made in 194-7 by Stoker ^ 

 who noted the similarily between broken Traves and hydraulic jumps 

 or shock waves. However this analogy has not been explored further, 



Notwithstanding the paucity of information on the problem of 

 broken waves and their characteristics at points landward of the 

 breaker zone, some logical criterion should be established to de- 

 termine how effective a seawall would be if so placed that the imping- 

 ing waves are already broken. 



At breaking, a wave reaches its maximum amplitude. Moving up 

 a beach from the point of breaking, this amplitude nust decrease, since 

 energy is dissipated in the turbulent flow, Hovirever, this decrease 

 has not been measured nor estimated and therefore no value may be placed 

 on it. In order to insure conservative results for seawall height, the 

 nBximum wave amplitude instead of some lesser value should be employed. 



The use of Figures 1 and 2 permits e stablishnent of the depth and 

 height of a breaking -wave, and from these the naximum crest «levation 

 may be determined, (ht + 0.7 H^ !JLW, see page 8). The criterion to 

 be adopted for total effectiveness of a seawall follows: 



If a horizontal line be proje cted from the point of the breaker 

 crest shoreward to a seawall's proposed location, the seavrall's crest 

 should be at least as high as this line. That is, the absolute height 

 of seawall crest should be equal to or greater than the absolute 

 height of a breaker crest. This is essentially an extension of the 

 analysis of a wall in the breaker zone. 



To determine the relative effectiveness of seawalls lower than 

 this height, the cui'ves of Figure 3 may be employed. Actually such 

 use is an extension of an approximate result, an an assumption of 

 accuracy would be unwarranted c 



VII. Summary 



On the preceding pages general criteria have been established for 

 absolute and relative effective heights of seawalls within, landward 

 of, and seaward of, the breaker zone. For structures ¥/ithin and Irmd- 

 ward of this zone, to be totally effective their height h^ above some 

 datum should be 



he = ht + 0,7 Hb 



where h-t, is the maximum expected tidal height above thn s datum and Hi^ 

 is the maximum expected breaker height at the vsll's pos:ition. A basis 

 for establishing relative effectiveness of lesser height walls is 

 presented on Figure 3. 



Similarly for seawalls located seaward of the breaker zone, for 

 total effectiveness 



15 



