wave will be less than the specified upper limit of 1 foot. Under reasonable 

 operating conditions, vessel wake is likely to be noticeable within the 

 marina, but should not cause any problems (Richey and Adee, 1975). 



(2) Auke Nu Cove, Alaska. Stormer (1979) prepared a detailed in- 

 depth design of an Alaska-type floating structure to protect Auke Nu Cove, 

 Alaska. The design wave for Auke Nu Cove was determined to be 3 feet high 

 with a 2.7-second period. As general guidance, the beam width of a floating 

 breakwater should be at least 0.5 times the incident wavelength to provide 

 adequate attenuation. This also permits parts of the wave to exert forces on 

 the structure in opposite directions, which tends to reduce the net force on 

 anchoring systems. Based entirely on the design wavelength, the width of the 

 proposed floating breakwater should be at least 10 feet. The criterion for 

 harbor design under consideration specifies the transmitted wave should not 

 exceed 1 foot in height. With a coefficient of transmission of 0.33 from 

 laboratory tests of similar structures, the design width was approximated to 

 be about 20 feet. The same type of analysis of prototype data from the 

 Tenakee Springs site provides a breakwater width of about 20 feet. Based on 

 these similar situations, it appeared reasonable to Stormer to design a struc- 

 ture 21 feet wide, assuming all other dimensions are similar. Construction 

 cost for the Alaska-type structures is estimated to be about $425 per linear 

 foot of breakwater (1979 dollars), including shipment and installation. 



5. Hybrid Pontoon Structures . 



a. Friday Harbor, Washington, Installation . The floating breakwater at 

 Friday Harbor, Washington (Fig. 45), was installed in late 1972; 904 feet of 

 the structure was placed in an L-shaped configuration. Some features of this 

 breakwater identify with various other configurations, but the breakwater is 

 unique of all currently existing structures. The operational experience has 

 been documented by Adee (1975b, 1977). This breakwater has a continuous 

 structure with a wooden deck across the beam and is supported by heavy wooden 

 stringers which run the length of the breakwater. Buoyancy is provided by 356 

 polyolefin, irregular-shaped pontoons (10 by 5 by 5 feet), arranged four- 

 across the 25-foot beam, which leaves a gap in the middle and gives the 

 structure a catamaran appearance (Fig. 46). 



Occasional severe winter storms with winds from the northeast are the most 

 troublesome at Friday Harbor. The site is well protected from very long 

 waves, but the short-period winter waves strike almost directly on the beam of 

 the long leg of the structure. The performance of the breakwater in protect- 

 ing the moorage has been satisfactory, although boat wakes have caused some 

 concern. The Washington State Ferry, which stops at Friday Harbor, creates 

 waves which seem to pass through the breakwater, but this is of a short-term 

 nature. 



The mooring lines of this breakwater are made up of a section of chain 

 (7/8 inch) which extends about 45 feet below the breakwater. Beyond this 

 point, a three-strand, double-braided nylon line is used. At the bottom, 

 another length of the same size chain is connected to a piling. Mooring lines 

 are spaced at 50-foot intervals. The major problem encountered at Friday 

 Harbor has been with the plastic pontoons. During a severe storm shortly 

 after the structure was built in 1972, 34 of the 356 pontoons were destroyed. 

 More pontoons have failed since that time, with failure occurring just below 



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