VII. TETHERED- FLOAT BREAKWATER 



The tethered-float breakwater is constructed of a large number of very 

 buoyant floats, each 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 as a result of the wave pressure gradient, and the dominant 

 attenuation mechanism is drag resulting from the buoy motion. Seymour and 

 Isaacs (1974) developed a theoretical model which predicts the attenuation by 

 a particular array configuration of a given incident wave spectrum. This 

 model was verified in a two-dimensional laboratory flume, and its essential 

 features were confirmed in semiprotected bay field tests. Performance predic- 

 tions have been advanced for a wide range of design conditions. The system 

 can be constructed in any water depth greater than specified limiting minimum 

 depths. Potential applications include harbor and marina protection, beach 

 erosion control, and offshore terminal offloading facilities. 



The tethered-float breakwater design was conceived by Professor John D. 

 Isaacs of Scripps Institution of Oceanography, San Diego, California, in 1970 

 (Jones, 1978). Subsequently, a research project to evaluate the effectiveness 

 of this concept was initiated in 1972 by the California Department of Naviga- 

 tion and Ocean Development (DNOD) and the California Sea Grant College Program 

 of the National Oceanic and Atmospheric Administration. Engineering develop- 

 ment was undertaken with a joint project of the State of California and the 

 Naval Facilities Engineering Command, and continued under a consortium of 

 Federal and State agencies, including the COE and the Maritime Administration. 

 Moffatt and Nichol Engineers, Ogden Beeman, and International Maritime 

 Associates (1977) (a private engineering cooperative effort) conducted a 

 feasibility study to evaluate the commercial market for a tethered-float 

 breakwater system, and NCEL performed conceptual design studies (Jones, 1978). 



1. Operational Theory. 



Many floating breakwaters rely on reflection or turbulence generation to 

 disrupt the wave orbits and attenuate wave energy. The energy dissipated by 

 friction (drag) is normally a small component of the total wave energy, since 

 the relative velocities between the breakwater and the fluid particles are 

 low. However, the drag power is proportional to the cube of these relative 

 velocities. Hence, if a substantial increase in relative velocity can be 

 achieved, the energy dissipation due to drag can become the most important 

 mode. This constitutes, the fundamental mechanism that the tethered-float 

 breakwater serves as a wave energy dissipator. 



The tethered-float breakwater system is composed of a large number of 

 independently operating floats, each of which is quite small in comparison to 

 a wavelength. In the original design, the floats were spherical (other shapes 

 have since been investigated) with high buoyancy, and were tethered with their 

 crown just above the water surface with at least one-diameter clear space 

 between floats in all directions. The width of the array, or the number of 

 rows of floats past which the wave field must advance, is determined by 

 the desired level of wave energy attenuation. A schematic diagram of the 

 tethered-float breakwater is shown in Figure 114. 



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