IX. BREAKWATERS, OFFSHORE 



1. Definition. 



An offshore breakwater is a structure that is designed to provide protec- 

 tion from wave action to an area or shoreline located on the leeward side of 

 the structure. Offshore breakwaters are usually oriented approximately paral- 

 lel to shore. They may also provide protection for harbors or erodible shore- 

 lines, serve as a littoral barrier-sediment trap (Fig. 5-26), or provide a 

 combined function. Table 5-3 is a partial list of offshore breakwaters that 

 have been constructed in the United States. These are generally of rubble- 

 mound construction, although some cellular sheet-pile, rock-filled concrete 

 caisson, timber crib, and floating concrete cellular designs have been used. 

 Offshore breakwaters overseas have been constructed with timber, quarrystone, 

 concrete armor units, concrete caissons, and even sunken ships. 



2 . Functional Operatio n. 



An offshore breakwater provides protection by reducing the amount of wave 

 energy reaching the water and shore area in its lee. The breakwater structure 

 reflects or dissipates the incident wave impacting directly on the structure 

 and transmits wave energy by means of diffraction into the barrier's geometric 

 shadow (see Ch. 2, Sec. IV). This reduction of wave energy in the 

 breakwater's shadow reduces the entrainment and transport of sediment by wave 

 action in this region. Thus, sand transported from nearby regions by a 

 predominant longshore current or circulation will tend to be deposited in the 

 lee of the structure. This deposition causes the growth of a cuspate spit 

 from the shoreline (see Fig. 5-27). If the structure's length is great enough 

 in relation to its distance offshore, the cuspate spit may connect to the 



Channel Islands, California 

 Figure 5-26. Offshore breakwater as a littoral barrier-sediment trap. 



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