2. Twin-Pontoon Floating Breakwater . 



Chen and Wiegel (1969) formulated a variation of the slope-floating beach 

 with pontoons which consisted of two pontoons separated by a perforated bottom 

 with a cross section of 7 feet by 9.5 feet prototype dimension (Fig. 202). 

 The width of the structure was 18.5 feet, with an additional 5 feet of sloping 

 beach extending seaward above the water surface, for a total width of 23.5 

 feet. A vertical barrier 9.5 feet high was attached below the lee side of the 

 perforated section. The pontoon on the lee side had a rectangular cross sec- 

 tion of 7 by 4.5 feet; the pontoon on the seaward side had unequal vertical 

 sides of 7 and 6 feet, with a sloping top extending about 5 feet outward. The 

 draft of the twin-pontoon floating breakwater was 4.5 feet, with a model-to- 

 prototype scale ratio of 1:12. 



w =23.5 



;lee i 



I PONTOON' 



, AIR ' 

 JCHAM8ERJ 



X 



PERFORATED 

 BOTTOM 



4.5" — 



VERTICAL- 

 WALL 



-18.5- 



as 



MODEL : PROTOTYPE 

 1 12 



Figure 202. Twin-pontoon floating breakwater for reservoir 

 applications (after Chen and Wiegel, 1969). 



The results of the wave transmission tests from the investigation of the 

 twin-pontoon floating breakwater are presented in Figure 203. For wavelengths 

 less than 60 feet (L/W < 2.7), the transmission coefficient, C t , was less 

 than 0.20. Considering the expected range of wavelengths in a reservoir, this 

 was believed to be a satisfactory attenuation. For wavelengths between 65 

 and 80 feet, the transmission coefficient varied between 0.30 and 0.50; wave- 

 lengths more than about 90 feet produced transmission coefficients in the 

 range of 0.60 to 0.65. 



Observations during the testing program indicated that more than half of 

 the energy of the incident wave was reflected by the breakwater as a wave 

 train with an apparent higher frequency than the frequency of the incident 

 waves. This breaking of the oncoming waves into a series of shorter reflec- 

 ting waves appeared to result in smaller forces acting on the mooring system. 

 Because of the vertical barrier and the overall geometric arrangements, this 

 floating breakwater had a rather large moment of inertia with respect to 

 rolling motion. The center perforated bottom had been designed to work as a 

 damping device, similar to the antirolling tanks on ships. Parts of the wave 

 energy were therefore dissipated by overtopping of the slope and by eddy 

 formation in the center section. 



260 



