of the first mode of oscillation of about 10 seconds. The low-frequency part 

 of the load-cell output was exactly 180° out of phase, indicating that sway 

 and possibly roll are the primary motions involved in this frequency range. 

 The maximum force variation measured due to local storm conditions was less 

 than 700 pounds; the variation due to tides alone was more than 1,000 pounds. 

 Regretfully, this record of incident waves had a significant wave height of 

 about 1 foot. Data from much larger storms are required before definitive 

 statements can be made about the anchor forces, especially for design pur- 

 poses. A sample record of the incident and transmitted wave spectra, along 

 with the corresponding transmission coefficients at Tenakee Springs, Alaska 

 (Christensen and Richey, 1974), is shown in Figure 40. 



0.2B 0.43 0-B7 0.71 



FREQUENCY (C/M) 



Figure 40. Incident wave spectra, S(ru), transmitted wave spectra, 

 S(n t ), and coefficient of transmission, CL, from prototype 

 field investigation of Alaska-type floating breakwater at 

 Tenakee Springs, Alaska (after Christensen and Richey, 1974). 



b. Sitka, Alaska, Installation . The Alaska-type floating breakwater 

 installation at Sitka, Alaska, was completed in autumn 1973. This site is 

 exposed to a tidal range of 17 feet, and the bottom elevation under the 

 breakwater is about 35 feet below low tide elevation. The total length of the 

 breakwater was 960 feet. The design of the Sitka structure was basically the 

 same as that for Tenakee Springs, but with improvements in design and 

 construction details learned from the previous installation. The fabrication 

 was simplified by using a single cage of rebars, and cast-in-place bearing 

 plates and connection assemblies. Posttensioning the concrete in the 

 transverse direction was eliminated to facilitate assembly while the 

 breakwater is floating. Two 1-inch transverse rods of 83 kips per square inch 

 tensile-strength steel were used instead of the four 160 kips per square inch 

 rods for each transverse unit used at Tenakee Springs. The rods were 

 tensioned to 36,000 pounds each with equipment from atop the floating units. 

 The connections at the junctions for Sitka incorporated 1-foot-square by 3- 

 1/2-f oot-long rubber compression bumpers and an added lower chain-link 

 connection. The chain and timber bumpers were sized to provide a slight 

 compression of the rubber. 



75 



