Table 4 shows that the transmission coefficient, B. t /E.^, varied primarily 

 with wave period and angle of wave attack. When the angle of wave attack was 

 45°, an increase in the wave period from 2.0 to 2.5 seconds resulted in a max- 

 imum increase of 150 percent in the value of Hj./h^. When the angle of wave 

 attack was 90°, an increase in wave period from 2.0 to 2.5 seconds caused a 

 maximum increase of 60 percent in the value of H t /H.. Assuming that 0.5- 

 foot-high waves are the maximum size that can be tolerated on the harborside 

 of the structure, a twin-log floating breakwater constructed of 4-foot- 

 diameter logs spaced on 5.7-foot centers would not be satisfactory for atten- 

 uation of waves with heights greater than 2.0 feet and periods greater than 

 2.0 seconds. Figure 101 shows waves attacking the model breakwater. The most 

 severe movement of the breakwater structure occurred when the angle of wave 

 attack was 90° and the wave period 2.5 seconds. The structure rolled and 

 oscillated about its longitudinal centerline, and resisted violent pounding by 

 the fixed pile mooring system. 



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

 of log diameter, log spacing, water depth, and wave period on wave attenua- 

 tion, repeating the previously used angles of attack and water depths, and 

 adding (a) center-to-center log spacings of 4.0, 5.7, and 8.2 feet; and 

 (b) 1.7-second waves 0.9 foot high, 2.0-second waves 2.0 feet high, and 2.5- 

 second waves 2.5 and 2.6 feet high. The test results are given in Table 5. 



Table 5 indicates that the ratio \/R± varied primarily with wave 

 period. Varying the center-to-center spacing between logs from 4.0 to 8.2 

 feet did not improve the wave attenuation characteristics of the structure. 

 On the basis of the criterion that waves in the mooring area should not be 

 greater than 0.5 foot high, a twin-log floating breakwater constructed of 3- 

 foot-diameter logs would not be satisfactory for this degree of attenuation of 

 2.0-second waves greater than approximately 1.5 feet high. Decreasing the 

 diameter of the logs from 4 to 3 feet resulted in a maximum increase of 75 

 percent in the value of the ratio H(-/ H i for the 2.0-second waves, and a 

 maximum increase of 33 percent for the 2. 5-second waves. 



Based on the experimental results, it was concluded that for incident 

 waves not greater than 2.0 feet in height and 2.0 seconds in period (wave 

 attention criterion: h\ < 0.5 foot), a twin-log floating breakwater con- 

 structed of either 4- or 3-foot-diameter logs with vertical barrier plates 

 attached, extending to a 4-foot depth below Stillwater level, would provide 

 sufficient protection for Small-Boat Basin No. 2 at Juneau, Alaska. Logs 

 spaced 5.5 to 6.0 feet apart would be satisfactory. Increasing the center-to- 

 center log spacing from 4.0 to 8.2 feet had a negligible effect on the wave 

 attenuation characteristics of the structure tested. Wave attenuation charac- 

 teristics of twin-log floating breakwaters vary primarily with flotation depth 

 and wave period. An increase in the flotation depth results in a decrease in 

 the value of H /H. ; an increase in the wave period results in an increase 

 in Hj./H^. Varying the angle of attack from 45° to 90° resulted in a slight 

 and somewhat erratic variation in the wave attenuation characteristics; how- 

 ever, the test data indicate that such a structure would be slightly more 

 effective when the angle of wave attack is 90° than when it is 45°. For the 

 conditions tested, the most economical and effective breakwater configuration 

 was that which utilized 3-foot-diameter logs with vertical barrier plates 

 attached to the channel-side face of the channel-side log. The addition of 

 barrier plates to the harborside face of the harborside log, in conjunction 



151 



