a. Tenakee Springs, Alaska, Installation . The Alaska-type floating 

 breakwater at Tenakee Springs has 3- by 5- by 15-foot reinforced precast 

 concrete pontoons with a minimum 4-inch wall thickness and an interior of 

 solid polystyrene foam. Design strength of the concrete is more than 5,000 

 pounds per square inch, and the unit weight is between 90 and 100 pounds per 

 square foot. Four 15-foot units were assembled on barges into 60-foot-long 

 modules and posttensioned with four 60-foot-long, 1. 75-inch-diameter galva- 

 nized bars of 160 kips per square inch tensile-strength steel. The joints 

 between the 15-foot precast segments were fabricated with an epoxy mortar to 

 ensure even bearing. On the site, two 60-foot-long modules were connected 

 together on a grid at low tide with three 15-foot-long units crosswise and 

 posttensioned with four 21-foot-long, 1-inch-diameter , high-stress bars 

 through each of the cross units. A 70-durometer Neoprene bearing pad was used 

 between the abutting concrete surfaces, and the result was a 5-foot-deep by 

 21-foot-wide by 60-foot-long rigid frame of prestressed concrete. The 21- by 

 60-foot modules were then coupled with chain links and rubber compression 

 bumpers to form an Alaska-type floating breakwater 308 feet long. The break- 

 water was anchored at each 60-foot junction with two 26-ton concrete anchors 

 and 270 feet of chain (diameter 1-3/8 inches) to the seawardside and one 26- 

 ton anchor on 180 feet of chain on the harborside (Miller, 1974b). 



There are two main directions for waves incident to the harbor at Tenakee 

 Springs — one from the southeast with a fetch of approximately 3.5 miles, and 

 the other from the southwest (out of Crab Bay) with a fetch of about 5 miles, 

 although the effective fetch length is uncertain due to the extension of the 

 bay. No wind records of the site exist, but local residents have reported 

 winds in the 60- to 70-mile-per-hour range. The harbor is shielded partially 

 by landforms for waves which move up Tenakee Arm and strike the breakwater at 

 an appreciable angle. The tidal range is about 20 feet, and the water depth 

 is about 30 feet at low tide. 



Prototype field measurements of the performance characteristics of the 

 Alaska-type floating breakwater at Tenakee Springs, Alaska, were made during 

 the 1973-74 winter period by Christensen and Richey (1974, 1976). They eval- 

 uated the Alaska-type floating breakwater, under different wave exposures, 

 and obtained basic data for the verification and development of a theoretical 

 predictive model for breakwater performance. Wave characteristics were 

 recorded at three locations. Two spar buoys, instrumented to measure wave 

 amplitudes, were placed outboard of the breakwater and positioned so that one 

 measured the incident wave field and the other measured the incident plus 

 reflected wave field. A third gage (stationary) was positioned behind the 

 breakwater to measure the transmitted wave height. Anchor cable forces were 

 measured using bonded strain gage-type load cells that were placed in the 

 anchor chains beneath the water surface. These cells calibrated to an accu- 

 racy of 0.75 percent of the rated total load-cell capacity, over a temperature 

 range of 10° Celcius (design load 12,500 pounds). 



The transmitted wave spectra showed very distinct peaks at 5.5, 11, and 

 about 100 seconds with an appreciable amount of energy spread over the lower 

 end of the spectra from the 100- to the 11-second peak. This energy could be 

 from many sources. A preliminary investigation of the across-channel oscilla- 

 tion, which could easily be excited by the extreme tides and inlet geometry, 

 showed a natural period of this mode of about 2 minutes. Also, oscillations 

 within the harbor, between the breakwater and the shore, give a natural period 



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