d. A-Frame Arrangement Floating Breakwaters . Because of the availability 

 of timber in many parts of the United States and Canada, log structures have 

 been utilized to provide protection for harbors and boat anchorages. The 

 Canadian Department of Public Works has developed and evaluated a circular 

 cylinder floating breakwater which incorporates a vertical wall for supple- 

 mental attenuation purposes. Of particular interest in this development has 

 been the determination of the method and extent to which the requirement of 

 large mass may be usefully replaced by a large moment of inertia. It appears 

 the effectiveness range of this concept (the A-frame) can be significantly 

 increased by enlarging its radius of gyration, which involves only a slight 

 increase in the mass of the structure. 



e. Tethered-Float Breakwaters . This unique breakwater concept consists 

 of a large number of buoyant floats with a characteristic dimension about 

 equal to the wave height. The floats are independently tethered at or below 

 the water surface. Initially, the concept was developed for a water depth 

 many times the float diameter; later a bottom-resting concept was developed 

 for shallow water. The floats move because of the periodic fluctuation of the 

 pressure gradient, and the dominant attenuation mechanism is drag resulting 

 from the buoy motion. The tethered floats respond periodically with a defi- 

 nite phase relationship to the periodic driving force. Because of their 

 dynamic response, it is possible to cause the buoys to pendulate out of phase 

 with the wave orbital motions. This buoy motion transforms wave energy into 

 turbulence. 



f. Porous-Walled Floating Breakwaters . A solid vertical or sloping-face 

 floating breakwater causes nonbreaking waves which strike it to be partially 

 reflected. To be an effective reflector (good attenuator) , a floating break- 

 water should remain relatively motionless. Such a breakwater requires great 

 structural strength, and large forces are imposed on the mooring system. Any 

 part of the wave energy dissipated by the floating breakwater system is no 

 longer available to be either transmitted or reflected; hence, forces in the 

 mooring system are accordingly reduced. From the standpoint of maximum wave 

 energy dissipation internally and a resulting minimum reflection from the 

 structure, the perforated (porous-walled) breakwater and open-tube wave atten- 

 uation systems have been investigated experimentally. 



g. Pneumatic and Hydraulic Breakwaters. The most effective natural mech- 

 anism of wave energy dissipation is the phenomenon of wave breaking. Active 

 wave attenuation systems are those devices which inject kinetic energy into 

 the wave so that total or partial breaking of the wave train occurs. The 

 resulting attenuation depends on the degree of breaking accomplished by the 

 active breakwater. The underlying basis of operation for the air-release 

 breakwater concept (pneumatic system) is the development of a vertical current 

 of water which rises to the surface and spreads horizontally. In the water- 

 jet release system (hydraulic breakwater), high velocity water is released in 

 a horizontal layer near the surface. In either case, entrainment of the sur- 

 rounding water results from momentum exchange, and partial or total wave 

 breaking results. Compressors necessary for the generation of the low- 

 pressure air may be situated on harbor docks, floating platforms, or ships. 

 The hydraulic breakwater systems must be positioned at or slightly below the 

 water surface; hence, the proper and effective flotation device is critial for 

 successful attenuation by the hydraulic breakwater system. 



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