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d. Concrete-Caisson $reakwaters.~ 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 uaave overtopping. In general, concrete zaissons have reinforced concrete bottom, although open-bottom concrete caissons have been used. The open-bottom type is closed with a temporary wooden bot-tom 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 (ZO n.o 35 feet). fibs foundation, which usually consists of a mat or mound of rub- ?51e stone, must support the structure and withstand scour (see Ch. 7,• Sec. III,8). ~bhere 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 mater 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- ~ide 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. i;. 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; :..owever, in some parts of the world breakwaters have been constructed with *imber, 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 recause they have little energy--dissipating effect on the longer period ocean Wives. `I'tie most recent summary of the literature dealing with floating break- «-aters is given: by Hales (1981). Some aspects of floating breakwater design .are given by Western Canada Hydraulics Laboratories Ltd. (1981). Selection Qf the type of offshore breakwater for a given location first depends on functional needs and then on the material and construction costs. termining factors are the depth of water, the wave action, and the avail- ~bility of material. For open ocean exposure, rubble-mound structures are usually required; for less severe exposure, as in the Great Lakes, the eeLlular-steel sheet-pile structure may be a more economical choice. Figure ~~E~ illustrates the use of a rubble-mound offshore breakwater to trap littoral material, to protect a floating dredge, and to protect the harbor ~w!-rance . Probably the most notable offshore breakwater complex in the United 6-93