littoral drift of all su^h modifying structures now in use, since, being 

 usually in deeper water' than the seaward end of a jetty or groin, it 

 controls a wider portion of the band of littoral drift than do structure ^^ 

 attached to shore. Because littoral drift is the direct result of wave 

 action, the extent to which the breala-jater intercepts the littoral drift 

 is directly proportional to the extent of interception of wave action by 

 the brealrwatei', 



178. The effect on the shore line of an offshore breakvrater is typified 

 by the 2,000-foot structure at Santa Monica, California, This breala/ster 

 was constructed parallel to and 2,000 feet distant from shore in approx- 

 imately 27 feet of water. Figure 68 shovrs in dashed lines contours of 



the accretion that existed behind the brealcviater after 11 years. 



179. Operation of An Offshore Breala'iater . - An offshore breaki'.'ater 

 at first tends to cause sand to deposit in its lee by slowing the wave 

 generated littoral current in that area. Diffraction causes some wave 

 action within the geometric shadow, but such wave action is much less 

 than that which would exist in the area in the absence of the breakiriater. 

 The typical diffraction diagram drawn on Figure 68 shows that vp.ve heights 

 within the brealci'rater' s geometric shadow are less than one-half the wave 

 heights outside the brealniater, 



18c, As sand is deposited, a shore salient is formed in the still 

 water behind the breakwater. This salient itself acts as a groin, which 

 causes the updrift shore line to advance. Concomitant with the advance- 

 ment of the shore line which brings the zone of littoral drift closer to 

 the brealci-rater,is an increase of the efficiency of the breakrtjater in 

 acting as a sand trap. The salient is more rapidly fonaed and thus becomes 

 increasingly efficient as a groin. 



It'l. If the breakwater is of sufficient length in relation to its 

 distance from the shore to act as a complete littoral barrier, the sand 

 depositing action may continue until a tombolo is formed with the break- 

 water at its apex. Such a tombolo accretion is shown on Figure 69, an 

 aerial photograph of the offshore breaktv'ater at Venice, California, 



182, The precise shape of the deposit is difficult or impossible 

 to determine. In general there will be accretion updrift from the break- 

 water and erosion downdrift. The area immediately behind the , brealafater 

 customarily assumes a form convex seaward. It has been found for complete 

 barriers that a large percentage of the total accumulation collects in 

 the breakwater lee during the first year and that the ratio of material ' 

 in the lee of the structure to total material trapped decreases steadily' 

 until such time that the trap is filled and littoral drift begins mqvin^ 

 around the structure. A shore line profile for a complete barrier may 

 be roughly approximated by drawing the high water line so that the aice-% 

 in square feet betx^een this line and the original high water line is 

 equal to the anticipated accretion in cubic yards at the time for vihich the 

 profile is to be drawn. 



92 



