In the evaluation of floating breakwater concepts for prototype installa- 

 tion, Jones (1974) pointed out that an important consideration is the upper 

 limit of the range of wavelengths over which a given level of effectiveness is 

 achieved. The performance of a given structure may be compared to the given 

 performance requirement. However, this criterion does not address the rela- 

 tive merit of two types of wave barriers; the one with second-best performance 

 can be made to yield the best performance merely through an increase in size 

 (e.g., draft or width). Numerous other factors, which are not apparent in 

 Figures 168 to 171, must enter into an evaluation of breakwaters for physical 

 applications. 



2. Floating Fluid-Filled Bags . 



Frederiksen and Wetzel (1959) studied a variety of mobile floating break- 

 water configurations fabricated from flexible membranes. They recommended 

 that additional studies be made of a fluid-filled flexible bag. Ripken 

 (1960b) and Frederiksen (1971) reported on two-dimensional laboratory inves- 

 tigations of the configuration. The University of California, Berkeley, on 

 contract by the U.S. Navy, performed three-dimensional experimental studies 

 and conducted field tests of a surface floating fluid-filled bag configura- 

 tion known as a "hovering" breakwater; the studies were discussed by Wiegel 

 (1959b), Wiegel, Shen, and Wright (1960), Shen (1961), and Wiegel, Shen, and 

 Cumming (1962). 



A typical installation consists of a blanket composed of several individ- 

 ual fluid-filled bags joined together, with the length and thickness of each 

 bag depending on the wave characteristics to be encountered and the attenua- 

 tion desired. The horizontal position of such a floating breakwater must be 

 stabilized for effective use. Stabilization requires the application of 

 substantial forces and design considerations of load handling both within and 

 upon the breakwater structure, and an external mooring system that must be 

 comprehensive. Since this concept was a new innovation, there were no estab- 

 lished procedures to be followed during the two- and three-dimensional 

 investigations. 



a. Two-Dimensional Experimental Studies . Ripken (1960b) and Frederiksen 

 (1971) performed laboratory studies in flumes which varied in width from 6 

 inches to 9 feet. The significant dimensions used in these tests are shown in 

 Figure 172. Wave conditions during the tests were varied over the maximum 

 practical range permitted by the generator. In the larger channel, the wave- 

 length varied from 5 to 40 feet, and the wave height varied from 0.1 to 1.5 

 feet. The values of wave steepness varied from 0.02 to 0.10, which is repre- 

 sentative of the range found in prototype situations. Preliminary tests 

 indicated that effective wave attenuation could be achieved, and that increas- 

 ing the viscosity of the bag fluid substantially increased the attenuation. 

 It was presumed that the bag motion initiated a wave or relative motion of the 

 fluid within the bag and that a high fluid viscosity damped this internal 

 movement in a continuous action, which, in turn, dissipated the energy of the 

 exciting external water wave. Because of the economic implications associated 

 with prototype use of a viscous fluid, attention was directed toward the bulk 

 dimensions of water-filled bags. 



231 



