On rivers or tidal channels that freeze over in winter, ice breakers can be installed 

 upstream from the harbor site to break large ice formations. These are usually timber 

 dolphin-type structures with vertical railroad-rail cutting edges near the water surface, alined 

 in the direction of current flow and bent to form smooth sloping tops. Ice sheets are either 

 broken by the cutting edge or forced up the rail slope until the overhand causes breakage 

 into smaller pieces. Chaney (1961) presents a more detailed discussion of icebreakers. 



/. Floating Wave Attenuators. No floating breakwater has yet been devised that will 

 economically and sufficiently attenuate large ocean waves. However, many harbor sites in 

 lakes and partially protected coastal waters (e.g., roadsteads and sounds) meet all the site 

 selection criteria except for lack of an appropriate fixed-breakwater foundation. One reason 

 for this may be that the bottom slopes too steeply from the shoreline so that at the 

 breakwater site the water is too deep. Another reason may be that restrictions in the interest 

 of ecological preservation prohibit fixed-breakwater construction of any kind. In such cases, 

 some type of floating wave attenuator may prove to be the best solution. Although no 

 floating attenuator will eliminate all wave energy, several types are now capable of reducing 

 moderate waves to acceptable proportions. 



The simplest type of floating protection is the chained-log boom, used mainly on rivers 

 and small lakes. The log boom is effective only against very short-period waves or "wind 

 chop" and is better for protecting swimming areas or berths for canoes, rowboats, and small 

 sailboats from waves that would not bother most powered small craft. The log boom can be 

 made twice as effective by binding the logs together in units of three; the boom then floats 

 with one log down and two on the surface (Fig. 37). 



The next step above the log boom is a stiff membrane skirt extending into the water 

 from a float or barge (Fig. 38). The skirted units must be anchored end-to-end to form a 

 continuous wave attenuator. This principle is sometimes used in skirting the fairway sides of 

 finger floats at the outer ends of floating-slip piers and is partially effective in reducing small 

 boat-wake waves entering the berthing area from the fairway. However, any skirted-float 

 system lias little effect on wind waves generated in fetches of a mile or more because of the 

 longer periods of such waves and consequently the greater depths of the water particle 

 orbits that propagate them. 



Any soUd floating device having small width and depth dimensions in comparison to the 

 wave length is intended to attenuate, and will only "ride" the wave rather than break it. 

 Research has been directed toward the development of devices that break up the orbital 

 continuity of the water particles within a wave or that pit the energ)^ in one part of the wave 

 against another part. Orbital continuity can be destroyed by the controlled release of air 

 bubbles, but the cost increases roughly with the square of the wave height; it would be 

 prohibitive when used in conjunction with a small-craft harbor. This also applies to water 

 jets. 



75 



