HARBORSIDE 



Figure 6-3. Example of composite-type breakwater. 



reflects a part of the incident wave energy and reduces the transmitted 

 wave heights further by out-of-phase damping. Some of the incident wave 

 energy is converted into heat by turbulence, primarily by waves breaking 

 onto the top of the structure. For the structures to be effective, cross 

 sections must be designed so as to combine a large mass, with as large a 

 mass moment of inertia as possible, and a relatively small restoring force, 

 to obtain large periods of roll, pitch, and heave relative to the periods 

 of the incident waves. These factors, and the necessity of reducing 

 mooring-line forces to a minimum, make it difficult to design the struc- 

 tures to meet local conditions. Three floating breakwaters of the out- 

 of-phase damping type (shown schematically in Fig. 6-4) are: the rectan- 

 gular box, the cross or Bombardon (used during the invasion of France in 

 World Vi?ar II), and the A-frame tested by Brebner and Ofuya (1968). The 

 A-frame floating breakwater is an example of the type of cross section 

 designed to obtain a relatively large mass moment of inertia with a mini- 

 miMi of mass. 



Although gravity, inertia, elastic, surface tension, and viscous 

 shear forces exist in a moored floating breakwater assembly, it is not 

 possible or necessary to design the model to be dynamically similar for 

 all of these forces. The waves and breakwater structure are modeled 

 according to Froude's law. The structure is constructed geometrically 

 similar to its prototype, and with the correct weight and mass distribu- 

 tion so that the correct buoyancy and mass moments of inertia are modeled 

 correctly. The linear scale is selected so that the waves and model break- 

 water (thus Reynolds number) are as large as possible, and the scale effects 

 due to nonsimulation of the surface tension and viscous forces are assumed 

 negligible. Thus, the time and velocity scales are 



1/2 



(6-9) 



327 



