Channel Elevation 



Features 

 Wove generator 

 ® Seaward wave meter 

 ® Test breakwater 

 © Shoreward wave meter 

 (§) Permanent absorber 

 (§) Mooring line 







Dimensions 









w 



L 



D M/D N 



E 



F 



G 



6" 



24' 



10" 1:10 14' 



8' 



12' 



4' 



24" 



38' 



12" 1:10 21' 



14' 



14' 



10' 



9' 



210" 



45" 1:10 120' 



95' 



90" 



25' 



Channel Section 



Figure 172. Experimental facilities for investigating flexible water-filled 

 bags used as floating breakwaters (after Ripken, 1960b). 



(1) Wave Attenuation Effectiveness . A water-filled cylindrical bag 

 floating breakwater, moored with axes perpendicular to the wave crest and 

 floating just below the water surface, is capable of attenuating surface 

 gravity waves. The findings (Fig. 173) indicate that attenuation performance 

 is very dependent on the length of the wave relative to the length of the bag, 

 and to a lesser extent dependent on the relative depth of the bag and the 

 relative amount of water in the bag. Good wave attenuation (height reduction 

 more than 80 percent) can be achieved if the length- of the incident design 

 wave can be restricted to about 60 to 70 percent of the bag length. Wave 

 attenuation characteristics improve as the depth or diameter of the bag 

 increases (Ripken, 1960b) , but this influence is small relative to the 

 influence of the bag length. In view of the increasing structural problem 

 with increasing bag diameter, it was recommended that a bag diameter equal to 

 about 20 percent of the water depth be used with shallow-water installations, 

 and a diameter of about 75 percent of the design wave height be used for 

 deepwater conditions. 



For small values of the ratio of wavelength-to-bag width, L/W (0.5 or 

 less), wave steepness had little effect on the resulting attenuation of wave 

 height. For medium values of this ratio (approximately 1.0), steepness had 

 an appreciable influence (about 20 percent) on attenuation, with the higher 

 steepness experiencing the greatest attenuation. The attenuation action 

 of the water-filled bag on gravity waves appeared to be due to out-of-phase 



232 



