they are modified into a composite structure by using a concrete cap for 

 stability. Precast concrete shapes, such as tetrapods or tribars, are 

 also used for armor stone when rock of sufficient size is not obtain- 

 able. In the Great Lakes area, timber, steel, or concrete caissons or 

 cribs have been used. In relatively sheltered areas breakwaters are 

 occasionally built of a single row of braced and tied Wakefield (triple 

 lap) timber piling or steel sheet piling. Several types of floating 

 breawaters have been designed and tested, but few are in use at this 

 time (1972). 



5„83 SITING 



Shore connected breakwaters provide a protected harbor for vessels. 

 The most important factor of siting a breakwater is to determine the best 

 location that will produce a harbor area with minimum wave and surge action 

 over the greatest period of time in the year. This determination is made 

 through the use of refraction and diffraction analyses. Other siting 

 factors are the direction and magnitude of longshore transport, the harbor 

 area required, the character and depth of the bottom material in the pro- 

 posed harbor, and available construction equipment and operating capability. 

 Shore-connected structures are usually built with shore-based equipment. 

 (See Section 5.7? - JETTIES - SITING.) 



5.84 EFFECT ON THE SHORELINE 



The effect of a shore-connected breakwater on the shoreline is 

 illustrated by Figure 5-16. As does a jetty, the shore arm of the break- 

 water interposes a total littoral barrier in the zone between the seaward 

 end of the shore arm and the limit of wave uprush until the impounding 

 capacity of the structure is reached and natural bypassing of the littoral 

 material is resumed. The same accretion and erosion patterns result from 

 the installation of this type of breakwater. The accretion, however, is 

 not limited to the shore arm, but eventually extends along the seaward 

 face of the sea arm, building a berm over which littoral material is trans- 

 ported to form a large accretion area at the end of the structure in the 

 less turbulent waters of the harbor. This type of shoal creates an ideal 

 condition for sand bypassing. A pipeline dredge can lie in the relatively 

 quiet waters behind the shoal, and transfer accumulated material to nourish 

 the downdrift shore. (See Section 5.5, SAND BYPASSING.) 



5.9 BREAKWATERS - OFFSHORE 



5.91 DEFINITION 



An offshore breakwater is a structure designed to protect an area 

 from wave action. Offshore breakwaters may serve as an aid to navigation, 

 a shore-protection structure, a trap for littoral drift, or may serve a 

 combined purpose. 



5.92 TYPE 



Almost without exception, offshore breakwaters in the United States 

 are of rubble-mound construction. 



5-50 



