1. Twin-Log Floating Breakwater . 



Jackson (1964) conducted experimental studies on 1:6 scale models of twin- 

 log floating breakwaters to determine the feasibility of this structure to 

 protect Small-Boat Basin No. 2, Juneau, Alaska, from wave action. Specifi- 

 cally, it was desired to determine the most economical floating log breakwater 

 that would reduce a 2-foot-high wave to approximately 0.5 foot in the mooring 

 area. The basin site is exposed to waves from the southeast through the 

 northwest direction, with maximum wave height due to storms from these criti- 

 cal directions about 2 feet. Water depths at the breakwater site range from 

 about 2 to 25 feet; the maximum tide is 21 feet. Because of the large water 

 depths, the large range of tides, and poor foundation conditions prevent the 

 construction of a rubble-mound breakwater, and since logs of considerable 

 diameter and reasonable cost were available at the project site, a twin-log 

 floating breakwater was proposed. The structure would consist of two logs 

 floating parallel, spaced about 1.5-log diameters on centers, connected by a 

 timber superstructure, and restrained in a horizontal plane by vertical piles 

 but allowed to float with the rise and fall of the tide. 



Tests were conducted on l:6-scale section models of the twin-log floating 

 breakwater in a concrete flume 119 feet long, 5 feet wide, and 4 feet deep. 

 The various floating log breakwater designs were installed in one end of the 

 wave flume and subjected to waves generated by a plunger-type wave generator 

 located at the other end of the flume. The effectiveness of the breakwaters 

 was determined by measuring the heights of the incident and transmitted waves. 

 The incident waves were measured at the breakwater site before the structure 

 was installed; the transmitted waves were measured at a distance of about two 

 wavelengths toward the harbor of the breakwater. 



Jackson (1964) conducted a series of tests to determine the effect of 

 angle of wave attack, water depth, and wave period on wave attenuation. The 

 model structure simulated a twin-log floating breakwater constructed of 4- 

 f oot-diameter logs spaced on 5.7-foot centers. Based on the specific weight 

 of prototype logs, the flotation depth of the model structure was 3.45 feet. 

 Wave attenuation tests were conducted using (a) 45° and 90° angles of wave 

 attack; (b) water depths of 10 and 20 feet, referred to Stillwater level; and 

 (c) 2.0-second waves of 2.0-foot height, and 2.5-second waves of 2.5- and 2.6- 

 foot height. Test results are given in Table 4. 



Table 4. Twin-log floating breakwater tests No. 1 (after Jackson, 1964). 



Wave period 

 (s) 



d/L 



^ (ft) 



ll t 



(ft) 



H t 



/H ± 



3, 45° 



e, 90° 



3, 45° 



3, 90° 



3, 45° 



3, 90° 



3, 45° 



3, 90° 





d 



= 10 ft; y/d = 0.35 









2.0 

 2.5 



0.40 

 0.32 



0.49 

 0.32 



2.0 

 2.5 



2.0 

 2.5 



0.4 

 1.2 



0.4 

 0.8 



0.20 

 0.48 



0.20 

 0.32 





d 



= 20 ft, y/d = 0.17 









2.0 

 2.5 



0.98 

 0.63 



0.98 

 0.63 



2.0 

 2.6 



2.0 

 2.6 



0.4 

 1.3 



0.4 

 0.6 



0.20 

 0.50 



0.20 

 0.23 



150 



