Flooting Dock 



Figure 38. Flank drop-skirt on channel side of floating dock for short-period 

 wave attenuation. 



In 1972, a floating breakwater of modular polyolefin pontoon units secured by timber 

 stringers, was installed at Friday Harbor, Washington. The shape of the units when 

 assembled, leave vertical openings occupying 30 percent of the surface area covered between 

 the units. The breakwater is 24 feet wide, 400 feet long, and 5 feet high. It is water -ballasted 

 to float half submerged and is anchored with fore-and-aft chains to submerged piling on the 

 bottom. Scale model tests at the University of Washington have indicated that the 

 coefficient of attenuation should be about 0.2 for the relatively short -period waves that 

 approach the site from the San Juan Channel. Alongside -docking is permitted in calm 

 weather, increasing the harbor capacity by about 60 boats. The structure cost about $320 a 

 linear foot and has performed as expected. The pontoon units have a life expectancy of 25 

 years; however, it is too soon to indicate how well the assembly wiU hold up over an 

 extended period of time. 



Another approach required a maze of wornout rubber tires secured together to form a 

 continuous porous mass that floats just above the surface (Fig. 39). Test results (Kamel and 

 Davidson, 1968) on various sizes and shapes of tires show that to be effective, the depth of 

 the maze below the surface should be about half the water depth, and the crest width 

 (length in direction of wave propagation) should be about one wave length. Also, the 

 effectiveness of a tire maze is very sensitive to wave length. For example, a maze 10 feet 

 deep and 100 feet wide in 20 feet of water would dissipate about 80 percent of the energy 



77 



