$,.°* 



/-^ 



J* 



& 



>u — 



f/ / De c -0.25 in 



a $y / ° ; 50m 



• =1.00 in 

 A = 1.50 in 



• % t =2.00 in 





0.0 



0.5 10 1.5 2.0 2.5 



Dimensionless Wave Frequency, jk/g = 2uh/L 



3.0 



3.5 



Figure 128. Effect of bottom removal, incident wave steepness, H./L, and 

 dimensionless wave frequency, a 2 h/g, on coefficient of reflec- 

 tion, C r , for the porous-walled floating breakwater (after 

 Richey and Sollitt, 1969a); various symbols denote different wave 

 generator eccentricities, e . 



b. Mooring Line Force Investigation. Marks (1966) conducted mooring line 

 force studies of the porous floating breakwater in Figure 125, and the results 

 were compared to those forces existing in mooring lines attached to a nonpor- 

 ous structure of similar dimensions. The perforated breakwater experienced 

 less force in all mooring lines; at the design wave, the mooring lines in the 

 perforated breakwater experienced less force by about a factor of 2. Wave 

 reduction in the lee of the structure varied from approximately 0.2 to 0.8. 

 However, the motion of the breakwaters as measured by horizontal and vertical 

 accelerations in these two-dimensional tests showed no clear superiority, and 

 this was reflected in wave reduction behind the breakwaters. It was evident 

 that the mooring arrangement influences the rolling motion and, hence, the 

 waves generated by the breakwater. An optimum mooring arrangement must be 

 devised whereby motion is minimized without sacrifice of minimum mooring line 

 force. A comparison of the seaward mooring line force for the perforated and 

 the caisson-type floating breakwaters is shown in Figure 129. 



2. Open-Tube Floating Breakwater System . 



The search for breakwaters that are less expensive than the full-depth 

 penetrating rubble-mound variety has led to many proposals for instituting 

 interactions with waves, primarily in the upper layers of the water column 

 where most of the wave energy is concentrated. The requirements for such a 

 breakwater, which extends over only part of the depth, include: (a) maximum 

 wave attenuation, (b) maximum energy dissipation with minimum reflection, 



189 



