width that is 30 to 40 percent of the design wavelength. Unfortunately, this 

 ratio leads to very wide units for long wavelengths, and it has been hypothe- 

 sized that deeper sections may be more appropriate for the deepwater locations 

 such as in the Pacific Northwest. Adee (1977) concluded that the overall 

 section profile (width and depth) of the Alaska-type and the single-pontoon 

 structures has essentially the same degree of effectiveness. Some researchers 

 feel the single-pontoon structure is more economical than the Alaska-type 

 because of construction and forming aspects, even though it may require 

 slightly more material. 



The "averaging" effect of wave forces on breakwaters has been found to 

 exist where the continuous sections are four to five times as long as the 

 wavelength. The result of this phenomenon is that anchor cable stresses are 

 much less than ordinary two-dimensional wave flume investigations indicate. 

 Where deepwater conditions exist, a continuous breakwater with a relatively 

 deep section (perhaps 12 to 15 feet square) will probably be highly effective. 

 Polystyrene foam as an interior void filler is probably desirable for sections 

 up to 5 feet wide by 6 feet deep. Larger cavities would be best designed as a 

 hollow interior. For larger box units, concrete construction recommendations 

 are to precast the sidewalls and top deck, and use a cast-in-place bottom; 

 good construction in a prestressed structure will result in a leak-free unit. 



The connection unit between modules requires much continued research and 

 field experience evaluation. The rubber-type connectors appear to be success- 

 ful (even though they are quite costly); however, more experience under severe 

 loading conditions is required to ascertain their longevity under repeated 

 cyclic stresses. 



4. Anchor Systems . 



The magnitude of anchor forces under field prototype conditions is a 

 parameter which should be determined by direct measurement methods. As noted 

 by Richey and Adee (1975), anchor forces scaled from two-dimensional labora- 

 tory experiments are unrealistically large because (a) the elasticity and 

 restraint conditions of the anchor system are not simulated and are inordi- 

 nately stiff in the model; (b) the model is short, with respect to the crest 

 length of the incident wave, and receives the wave force over its entire 

 length; and (c) monochromatic laboratory waves force the breakwater to trans- 

 late to the end of the tether (taking up all slack in the anchor system) to 

 develop large forces. In a wind-generated random wave environment, the proba- 

 bility is very low that a series of waves would strike the full length of a 

 floating breakwater (except possibly for boat-generated waves). 



Field data are becoming available on anchor forces of floating breakwaters 

 (e.g., Tenakee Springs, Alaska; Sitka, Alaska; Friday Harbor, Washington), and 

 it is becoming apparent that the breakwater must move appreciably and stretch 

 the anchor system before large forces will develop. The actual displacement 

 of a floating breakwater in a real wind-wave exposure appears to be small, due 

 in part to the fact that the waves do not approach the structure with crests 

 simultaneously along the breakwater. The largest anchor force measured at 

 Tenakee Springs, Alaska, from September 1973 to August 1974, was 7,146 pounds 

 for a 60-foot module, about 6 percent of the weight of the structure (Richey 

 and Adee, 1975). Similar experience with an A-frame structure at Lund, 

 British Columbia, Canada, in 1964 (Harris, 1974) indicated mooring forces were 

 relatively low, at less than 2 percent of the deadweight of the structure. 



268 



