for the occurrence of total or partial breaking of the wave train, with the 

 resulting attenuation depending on the degree of breaking accomplished by the 

 system. The active breakwaters evaluated to date involve the release of low- 

 pressure air at a preselected water depth, or the ejection of high-velocity 

 water jets near the water surface. The underlying basis of the air-release 

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

 water which rises to the surface and spreads horizontally in both the upstream 

 and downstream directions from the breakwater. In the water- jet release sys- 

 tem (hydraulic breakwater) , high-velocity water is released in a horizontal 

 layer near the surface of the water. In either case, entrainraent of the 

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

 breaking results. A definitive sketch outlining the operation of each system 

 is shown in Figure 136. 



'//////////A 



PneLirv 



Brec« 



■■///////////7/V77777/V//////////y///V/y/y//////////////777//////////// 

 Hydrate Brea<>::?er 



Figure 136. Conceptual model of operation, pneumatic and 

 hydraulic breakwater systems. 



Parts of the pneumatic breakwater system require placement on the sea 

 floor or harbor bottom to allow for the rise of air bubbles to entrain 

 surrounding fluid particles. The compressors necessary for generating 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 critical 

 for successful attenuation by the hydraulic breakwater. From these considera- 

 tions, the hydraulic breakwater more nearly satisfies the criterion of a 

 floating breakwater system than does the pneumatic breakwater. However, 

 because of the mobility of both systems, they are considered closely 

 related. Both concepts should be evaluated further. 



197 



