can be placed directly on the bottom without special preparation of the 

 foundation material. Thus, the bottom material may be prepared by placing 

 riprap in layers until adequate bearing pressures are obtained. When the 

 water depths are appreciable, and when the economics and purpose of the 

 structure permit, a rubble-mound base for the vertical wall may be placed 

 on the bottom to an elevation that comprises a considerable part of the 

 water depth. This rubble mound with the vertical wall which surmounts 

 the rubble is called a composite breakwater. Breakwaters protect mooring 

 areas from wave action, whereas jetties are structures extending from 

 shore to prevent shoaling of channels by littoral material, or constructed 

 at the mouth of a river or tidal inlet to confine the flow and deepen and 

 stabilize the entrance channel. Jetties as breakwaters may be either a 

 rubble-mound, vertical-wall, or composite-type of structure. 



Seawalls are structures that separate land and water areas; in har- 

 bors they are frequently bulkheads on the landside and breakwaters on 

 the seaside. Seawalls may also be either vertical-wall, rubble-mound, 

 or composite-type structures. 



Floating breakwaters, as the name implies, are floating structures 

 (usually fixed in place by mooring cables) designed to prevent passage 

 of all but a minimum, allowable percentage of the wave energy. The 

 structures must extend deep enough into the water to be effective, and 

 must have a natural period of oscillation that is large relative to the 

 period of the selected design waves to prevent large oscillations and 

 the consequent generation of waves by the structure itself. Floating 

 breakwaters are not widely used because they are usually not efficient 

 wave reducers except for the conditions of relatively deepwater and 

 short -period waves. However, these breakwaters offer several advantages, 

 compared to fixed structures, such as mobility, ease of installation, 

 elimination of littoral drift and scour problems that are sometimes 

 caused by the construction of fixed impervious structures, freedom from 

 foundation problems, and relatively small initial and maintenance costs. 



The pneumatic breakwater consists of a bubble screen that is generated 

 by compressed air passing through a submerged perforated pipe, and rises 

 to the surface in the form of air bubbles. While rising to the surface, 

 the air bubbles induce a vertical current which, in turn, produces hori- 

 zontal currents away from the bubble screen on the surface in both the 

 upstream and downstream directions and toward the screen near the bottom. 

 The surface current moving against the direction of wave propagation pro- 

 duces some attenuation of the waves. 



The hydraulic breakwater is similar in principle to the pneumatic 

 breakwater, except that the surface currents are generated by forcing 

 water through a perforated pipe or nozzle system. Pneumatic and hydrau- 

 lic breakwaters, like the floating breakwater, are only effective for 

 relative deepwater and short -period waves. 



The erosive action due to waves and currents of beach material in 

 the toe areas of rubble-mound and vertical-wall breakwaters and jetties. 



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