Ratio of Wavelength-to-Breakwater Width, L/W 

 J 3 A 5 6 



20 



30 40 50 60 70 



Wavelength, L (ft) 



Figure 205. Effect of relative breakwater width, L/W, 

 on transmission coefficient, C t , for twin 

 water chamber and pontoon floating break- 

 water (after Chen and Wiegel, 1969). 



of the rubber sheet. Hence, the motion of the gates was not confined to a 



simple rolling motion; there was also a parallel displacement. In addition, 



the simple flat platform provided a smaller moment of inertia than would be 

 the case of a structure equipped with a vertical barrier. 



The breakwater was modified by installing a fixed energy dissipator sim- 

 ilar to those used below large reservoir spillways. Since most of the energy 

 transmitted by deepwater waves occurs near the water surface, the horizontal 

 platform was redesigned to be submerged 1.75 feet below the water surface to 

 disrupt the orbital motion of the wave. Two vertical walls extending 11 feet 

 below the platform were attached to the platform (Fig. 206). The vertical 

 walls and the platform provided a large moment of inertia, because of added 

 mass, and prevented the transmission of wave energy under the platform to a 

 depth of 13 feet. The structure was 32 feet wide. Vertical sections, mounted 

 on the topside of the horizontal submerged platform, consisted of six rows of 

 2- by 2-foot stilling blocks. Each block was set in alternating patterns at 

 an angle of 30° to the direction of wave propagation. Energy dissipation 

 occurred as the waves passed through the maze of blocks and formed eddies and 

 a high degree of turbulence. 



Waves were observed colliding on the platform with great impact, appar- 

 ently caused by the combination of the motion of the platform and the wave 

 action; collision occurred at wavelengths of about 50, 70, and 90 feet 

 (related to the period of the waves and natural frequency of the structure). 



263 



