transport. The typical diffraction diagram drawn on Figure 5-19 shows 

 that wave heights within the breakwater geometric shadow are less than 

 one-half the wave heights outside the breakwater. As sand is deposited, 

 a seaward projection of the shore is formed in the still water behind 

 the breakwatero This projecting shore alignment in turn acts as a groin, 

 which causes the updrift shoreline to advance. As the projection enlarges 

 and the zone of longshore transport moves closer to the breakwater, the 

 salient becomes increasingly efficient as a littoral barrier. If the 

 breakwater is long enough relative 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 breakwater at its apex. (See Figure 

 5-20.) 



The precise shape of the salient deposit is difficult to predict. 

 In general, there is accretion updrift from the breakwater and erosion 

 downdrift. The area immediately behind the breakwater assumes a form 

 convex seaward. At complete barriers, a large percentage of the total 

 accumulation collects in the breakwater lee during the first year, and 

 the ratio of material in the lee of the structure t cotal material 

 trapped decreases steadily until the trap is fille- and littoral material 

 begins moving seaward around the structure. 



5.96 OFFSHORE BREAKWATERS IN SERIES 



It is not necessary to build offshore breakwaters as a single unit. 

 A series of short structures will have the same general effect as one long 

 structure, but the efficiency of the series as a sand trap will be decreased, 

 a condition which is sometimes desirable. The tendency for a tombolo to 

 form will be decreased. Figure 5-18 is an aerial photograph taken in 1949 

 of the breakwater series off Winthrop Beach, Massachusetts, constructed in 

 1931-33. The characteristic convex accretion in the lee of the breakwater 

 is evident, as is also the erosion zone downdrift. (See Figure 5-19.) 

 The shoals extending landward from the breakwater in Figure 5-18 indicate 

 that this breakwater series is in the littoral transport zone, 



5.97 HEIGHT OF AN OFFSHORE BREAKWATER 



One of the factors determining the effectiveness of an offshore 

 breakwater as a sand trap and in providing a protected area is the height 

 in relation to the wave action and variation in water levels at the site. 

 A structure which can completely eliminate wave action in its lee will 

 provide a protected harbor and function as a complete littoral barrier. 

 The most efficient type of breakwater then is one whose crest permits no 

 significant overtopping by waves. Model studies (Hudson, 1959) indicate 

 that the required crest elevation of a rubble type of breakwater depends 

 on wave height, wave period, wave length, face slope of the structure, 

 and the permeability of the structure. Data given in Figure 7-20 indicate 

 values for wave runup on rubble-mound structures. Until further verifica- 

 tion can be made, runup values at least as great as those indicated by 

 these curves should be accepted as design criteria. It might be desirable 

 to build an offshore breakwater that is not completely effective as a 

 littoral barrier. This may be accomplished by constructing the breakwater 



5-55 



