d. Concrete-Caisson Breakwaters . Breakwaters of this type are built of 

 reinforced concrete shells that are floated into position, settled on a 

 prepared foundation, filled with stone or sand for stability, and then capped 

 with concrete or stones. These structures may be constructed with or without 

 parapet walls for protection against wave overtopping. In general, concrete 

 caissons have a reinforced concrete bottom, although open-bottom concrete 

 caissons have been used. The open-bottom type is closed with a temporary 

 wooden bottom that is removed after the caisson is placed on the foundation. 

 The stone used to fill the compartments combines with the foundation material 

 to provide additional resistance against horizontal movement. 



Caissons are generally suitable for depths from about 3 to 10 meters (10 

 to 35 feet). The foundation, which usually consists of a mat or mound of rub- 

 ble stone, must support the structure and withstand scour (see Ch. 7, Sec. 

 111,8). Where foundation conditions dictate, piles may be used to support the 

 structure. Heavy riprap is usually placed along the base of the caissons to 

 protect against scour, horizontal displacement, or weaving when the caisson is 

 supported on piles. 



IX. BREAKWATERS, OFFSHORE 



Offshore breakwaters are usually shore-parallel structures located in 

 water depths between 1.5 and 8 meters (5 and 25 feet). The main functions of 

 breakwaters are to provide harbor protection, act as a littoral barrier, pro- 

 vide shore protection, or provide a combination of the above features. Design 

 considerations and the effects that offshore breakwaters have on the shoreline 

 and on littoral processes are discussed in Chapter 5, Section IX. 



1. Types . 



Offshore breakwaters can usually be classified into one of two types: 

 the rubble-mound breakwater and the cellular-steel sheet-pile breakwater. The 

 most widely used type of offshore breakwater is of rubble-mound construction; 

 however, in some parts of the world breakwaters have been constructed with 

 timber, concrete caissons, and even sunken ships. 



A variation of offshore breakwater is the floating breakwater. These 

 structures are designed mainly to protect small-craft harbors in relatively 

 sheltered waters; they are not recommended for application on the open coast 

 because they have little energy-dissipating effect on the longer period ocean 

 waves. The most recent summary of the literature dealing with floating break- 

 waters is given by Hales (1981). Some aspects of floating breakwater design 

 are given by Western Canada Hydraulics Laboratories Ltd. (1981). 



Selection of the type of offshore breakwater for a given location first 

 depends on functional needs and then on the material and construction costs. 

 Determining factors are the depth of water, the wave action, and the avail- 

 ability of material. For open ocean exposure, rubble-mound structures are 

 usually required; for less severe exposure, as in the Great Lakes, the 

 cellular-steel sheet-pile structure may be a more economical choice. Figure 

 6-66 illustrates the use of a rubble-mound offshore breakwater to trap 

 littoral material, to protect a floating dredge, and to protect the harbor 

 entrance. 



Probably the most notable offshore breakwater complex in the United 



6-93 



