may be found since the ratio of transmitted to incident wave energy may 

 be used as an indication of the relative wave competence to move sand, 

 (The energy E, per unit length along the crest, in a wave of height H, 

 length L, is given approximately by E = wH^L where w is the unit vreight 

 of the water) , 



188, Exposed Breakwaters . - Exposed breakvraters include those un- 

 covered only at low tide stages, as well as those never completely sub- 

 merged except perhaps by overtopping waves. Data are lacking on the 

 effects of such structures on transmitted wave characteristics, however 

 it has been standard practice to assume insignificant overtopping if the 

 breakwater crest is l|- times the height of the design wave over the design 

 water level , Under this assumption^ it would seem conservative to ex- 

 tend the curves of Figure 71 and 72 by a linear relationship to %,/Hg 



or E^/Ej, = at the point of zero overtopping. For example, a structure 

 founded' in 20 feet of water at high tide, with a design wave of 10 feet 

 should have its crest l5 feet above the high water line* In this case, 

 d =. 20 feet, a = d + l5 = 35 feet and Hj/Hg or Ej^/E « at d/a * 20/3^ = 

 0.57, and by extending the curves of Figures 71 and 72 to this point, 

 effectiveness of lesser height structures may be found, 



189, Length and Distance F rom Shore „ - No definite relationships 

 have been established between the length of break&jater, its distance 

 from shore, and the depth of water at the site. It is known that such 

 relationships do exist. When the length of a breakwater is only a fraction 

 of its distance offshore, diffracted and refracted wave crests have enough- 

 distance to turn through at leavSt one quadrant, thereby propagating 



wave energy sufficient to maintain sand in suspension and transit behind 

 the breakwater (Handin and Ludwick, 1950). At Santa Monica, a 2,000 foot 

 breakwater, 2,000 feet offshore in 27 feet of water, acted as a complete 

 sand barrier. About two miles avjay at Venice, a breakt-rater about 300 

 feet long and 1,200 feet from shore allowed sand to pass along the shore 

 behind it with only a slight accretion upcoast and a corresponding erosion 

 downcoast. When the beach was artificially widened by about one-half the 

 distance, or 6OO feet, a tombolo promptly formed between the breafa-Jater 

 and the new shore line, completely intercepting littoral drift as may be 

 seen on Figure 70, 



190, The principal lesson to be learned is that extreme care should 

 be used in planning any offshore structure designed for the sole purpose 

 of reducing the rate of littoral drift. It is far better to defjign too 

 short a structure requiring later extension than one too long which creates 

 a serious erosion problem, 



191, Spacing of Offshore Breakwaters . - Again no definite relation- 

 ships have been established to determine criteria for the spacing of off — 

 shore breakwaters where it is desired to protect a reach of coast line 

 yet not form a complete littoral barrier. Accordingly, extx'eme care must 

 be used to avoid creating more damage than would have occurred without 

 the structure. The following general rules raay be observed; 



97 



