investigated. The cost of the currently designed system for use under long- 

 fetch, deepwater conditions is too great for practical applications at many 

 sites. Dissemination of the advances in the state-of-the-art of tethered- 

 float breakwater design and possible practical applications may stimulate 

 Interest by potential users. Agerton, Savage, and Stotz (1976) independently 

 developed and field-tested a dynamic floating breakwater of the tethered 

 concept, using funds from several government agencies. The system is not 

 patented and is available to commercial developers without license. 



VIII. POROUS-WALLED FLOATING BREAKWATERS 



Reflection is one of the mechanisms by which a floating breakwater reduces 

 incident wave energy. To be an effective reflector to particle movement, a 

 breakwater should remain relatively aotionless. Such a breakwater requires 

 great structural strength under wave exposure conditions, as large forces are 

 imposed on the mooring system. Any part of the incident wave energy which can 

 be dissipated by turbulence is no longer available to be either reflected or 

 transmitted. Hence, forces in the mooring system are accordingly reduced. 

 Perforated breakwaters have been specifically designed for such a mission. A 

 variation of this concept which is constructed of a horizontal array of open 

 tubes has also been evaluated. 



1. Perf orated Portable Floating Breakwater Units. 



O'Brien, Kuchenceuther, and Jones (1961) considered i framework through 

 which proposed concepts of floating breakwaters could also serve as piers or 

 docks for offloading purposes. Marks (1966), Marks and Jarlan (1968), and 

 Terrett, Osorio, and Lean (1968) conducted experimental studies on the per- 

 forated breakwater developed by Jarlan (1960, 1965) to compare the behavior of 

 the perforated breakwater with the caisson type from the standpoint of wave- 

 dam effectiveness and forces on mooring lines. Such a system is potentially 

 applicable not only for military applications, but also for many civil works 

 areas; for example, to dissipate energy at floating bridges on large bodies of 

 water. Garrison (1968) showed analytically that a rigid plate of zero draft 

 fixed at the Stillwater surface in deep water (a floating bridge analogy) has 

 a reflection coefficient, C , of 90 percent for wavelength-to-structure 

 width of 2.4:1. If internal energy dissipation at the structure could be 

 increased, then a reduction in loading on the structure and its mooring 

 system, along with a decreased amount of reflected wave energy, would result. 



From the standpoint of maximum wave energy dissipation internally and a 

 resulting minimum reflection from the structure, Rlchey and Sollltt (1969a, 

 1969b, 1970) experimentally investigated Jarlan's (1960) concept of the 

 perforated portable floating breakwater unit (Fig. 124). The dynamic 

 processes resulting from the incidence of waves on this structure can be 

 interpreted by considering the pressure differential across the porous wall 

 (Fig. 125). As the waves impinge on the front wall, part of the energy is 

 reflected and the remainder passes through the perforations. For n > a, the 

 potential energy In the wave Is converted to kinetic energy, in the form of a 

 jet, which then tends to be partially dissipated by viscosity in the channel 

 and by turbulence In the chamber. Kinetic energy is lost as the flow expands 

 and diffuses in the chamber; fluid influx causes the water surface, a, to 

 rise. Then, as a becomes greater than n, the flow reverses and the cham- 

 ber empties. The water flowing back out of the holes then encounters the next 



184 



