h. Flexible-Membrane Floating Breakwaters . Several varieties of surface 

 floating membranes have been investigated, and among these concepts the blan- ' 

 ket layer types appear preferable to bag types for operational and logistic 

 reasons. For short waves, deep draft is not needed by a floating breakwater; 

 for long waves, deep draft may be desirable but it is difficult to contend 

 with the resulting large mooring forces. Hence, an optimization is required 

 which balances wave attenuation aspects and mooring loads, and the breakwater 

 usually floats with a draft much less than the depth of the water. Soft 

 floating systems have certain operational advantages over a rigid structure; 

 damage, and possible loss through collisions in rough water, should not be a 

 problem for flexible floating breakwaters. The mooring arrangement, hence, is 

 less critical. While strength of the material may be a limiting factor, a 

 properly designed flexible breakwater is not subject to the resonances which 

 increase the peak mooring loads of rigid breakwater systems. Because of the 

 collapsible nature of a flexible breakwater, a transport vessel could carry 

 more linear feet of this type than any other. The major disadvantage of all 

 thin surface membrane types is that a rather great width relative to the 

 wavelength is required to obtain satisfactory wave attenuation. 



i. Turbulence-Generator Floating Breakwaters . A floating breakwater con- 

 cept consisting of relatively thin, horizontal barriers has been developed 

 which causes dissipation of wave energy without creating major stresses in the 

 structure and moorings. The dissipative mechanism for this design arises as 

 the wave breaks over the upper surface of the system with great turbulence. 

 Major eddy formation develops as the fluid moves between the breakwater mem- 

 bers with supplementary loss of energy. The advantages of this design include 

 shallow draft, relatively lightweight, and modest mooring loads even in fairly 

 strong currents. Each unit is a long, rigid pontoon of specialized design 

 which enables a string of units to be joined together. 



j. Peak Energy Dispersion Floating Breakwaters . The opportunity often 

 exists for a floating breakwater to alter the peak energy density occurring at 

 narrow frequency bands to a broader spectrum of frequencies with much lower 

 energy intensity. A method of wave interference to accomplish this desired 

 objective has been developed. An offset breakwater configuration of vertical 

 reflecting surfaces oriented normal to the direction of wave propagation and 

 displaced from each other by one-half wavelength reduces the anchoring forces 

 required to hold the floating breakwater in place, as the net pressure distri- 

 butions on the various sections are out of phase by 180°. A slightly differ- 

 ent version utilizes an array of wave-excited modules which act as sources of 

 elliptical wave fronts radiating outward and interfering with the incident 

 wave field. The radiated waves have characteristics which trigger instabil- 

 ities in the incoming wave field and result in premature breaking. 



k. Reservoir Application Floating Breakwaters. The development of 

 multiple-purpose reservoir marinas requires breakwaters that can function 

 over a wide range of water levels, as the water surface elevation of a flood 

 control reservoir may fluctuate 50 feet or more. An adequately designed 

 breakwater must be able to follow the water surface without undue stress in 

 the moorings, and must have the capability to function at all elevations. 

 Raichlen (1968) found that some smaller boats have larger natural periods of 

 response than some larger boats. This is because the larger boats may have 

 stiff er mooring lines in comparison to their weight than do the smaller boats. 

 Extrapolated, this emphasizes that the moorings are extremely important to the 

 problem of designing a floating breakwater to reservoir requirements. 



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