Breakwaters have both beneficial and detrimental effects on the shore. All 

 breakwaters reduce or eliminate wave action in their lee (shadow). However, 

 whether they are offshore, detached, or shore-connected structures, the 

 reduction or elimination of wave action also reduces the longshore transport 

 in the shadow. For offshore breakwaters this leads to a sand accretion in the 

 lee of the breakwater in the form of a sandbar (called a tombolo) which grows 

 from the shore toward the structure, as well as the associated downdrift beach 

 erosion. 



Shore-connected breakwaters provide protection to harbors from wave action 

 and have the advantage of a shore arm to facilitate construction and mainte- 

 nance of the structure. In recent years, shore-parallel breakwaters built of 

 short detached groupings have provided adequate large storm protection without 

 adversely affecting the longshore transport. 



At a harbor breakwater, the longshore movement of sand generally can be 

 restored by pumping sand from the side where sand accumulates through a pipe- 

 line to the eroded downdrift side. This type of operation has been in use for 

 many years at such places as Santa Barbara, California, and Channel Islands 

 Harbor, California. 



Offshore breakwaters have also been used in conjunction with navigation 

 structures to control channel silting. If the offshore breakwater is placed 

 immediately updrift from a navigation opening, the structure impounds sand in 

 its lee, prevents it from entering the navigation channel, and affords shelter 

 for a floating dredge plant to pump out the impounded material across the 

 channel to the downdrift beach. This method has been successfully used at 

 Channel Islands Harbor near Port Hueneme, California. 



While breakwaters have been built of everything from sunken ships to large 

 fabric bags filled with concrete, the primary material in the United States is 

 a rubble-mound section with armor stone encasing underlayers and core mate- 

 rial. Some European and Japanese breakwaters use a submerged mound foundation 

 in deeper water topped with a concrete superstructure, thereby reducing the 

 width and overall quantity of fill material necessary for harbor protection. 



Groins are barrier-type structures that extend from the backshore into the 

 littoral zone. Groins are generally constructed in series, referred to as a 

 groin field or system, along the entire length of beach to be protected. The 

 basic purposes of a groin are to modify the longshore movement of sand and to 

 either accumulate sand on the shore or retard sand losses. Trapping of sand 

 by a groin is done at the expense of the adjacent downdrift shore unless the 

 groin or groin system is artificially filled with sand to its entrapment 

 capacity from other sources. To reduce the potential for damage to property 

 downdrift of a groin, some limitation must be imposed on the amount of sand 

 permitted to be impounded on the updrift side. Since more and more shores are 

 being protected, and less and less sand is available as natural supply, it is 

 now desirable, and frequently necessary, to place sand artificially to fill 

 the area between the groins, thereby ensuring an uninterrupted passage of the 

 sand to the downdrift beaches. 



Groins that have been constructed in various configurations using timber, 

 steel, concrete, or quarrystone are classified as high or low, long or short, 



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