the maxiniuin water level expected at the -wall's location. It is necessary 

 however, to establish a criterion for a seatrall to be totally effective 

 when undergoing wave attack. The standard which will be followed is this; 



A seawall can be considered totally effective if its height is 

 sufficient to prevent any solid water from passing over the wall with 

 damaging horizontal momentum. This criterion will be considered satis- 

 fied if the height of the wall is equal to or greater than the height 

 of an impinging wave. 



It is ordinarily eoonomically infeasible to design a wall high 

 enough to prevent any overtopping under all wave conditions. However 

 • the primary purpose of a seawall is to prevent damage to the land or 

 structures behind, and this damage will be caused by that water which 

 overtops the wall with an appreciable horizontal momentum, 



A seawall whose crest height is equal to or greater than that of the 

 crest of an impinging wave -mill cause the wave to run up and overtop the 

 wall. The amount of this overtopping is dependent on the shape of the 

 wall, and on the characteristics of the waves at the wall. (e.g. the 

 ^'tfj^apotis" formed by a non-breaking wave at a vertical barrier). However 

 the momentum of this water thus thrown above the wall will have been 

 changed from a nearly horizontal one to one (depending on the wall shape) 

 nearly vertical, and since the horizontal momentum is reduced consider- . 

 ably, the damaging power of the wave is similarly reduced. 



It is true that the damaging effect of water falling in the 

 immediate vicinity of the wall must be considered in the structural design 

 of the wall itself, and of the embankment behind, which must be provided 

 with pavement and drainage. Damage to the wal^ will reduce its effective 

 protection of the land behind, but the prevention of this damage is a 

 problem of structural design. It is not a consideration in determining 

 the effective height of the wall, 



IV. Maximum Conditions (Seawall in the Breaker Zone) 



General - As the preceding discussion indicated, water depth at a 

 structure may be so highly variable, especially under storm conditions, 

 that it would be impossible to locate a structure outside the range of 

 damaging wave action. It would be well therefore to discuss the effective- 

 ness of a seawall under extreme wave conditions, that is when the wall is 

 so placed that the impinging wave will be of maximum size. J. Larras 

 has found that "When for a given swell, one set up the vertical wall at 

 various points of the terminal slope, the position of the wall for which 

 the breakers become most violent coincides with the position of the 

 rollers on the same slope in the absence of the vertical wall. In other 

 words, the waves break against walls in the same depths as they do upon 

 slopes, " (4.) (5). 



If we can determine the characteristics of a breaking wave and the 

 depth in which a wave may break in the absence of a wall, we can 



