c. Comparison of Prototype- and Model-Scale Wave Attenuation Effec- 

 tiveness. The model-scale tests of Harms and Bender (1978), using a wave 



steepness of 4 percent, were compared to the full-scale prototype-size waves 

 of Giles and Sorensen (1978) where only regular waves had been studied (Harms, 

 1979b). Harms' comparison (Fig. 85) indicates that it is principally the 

 influence of wave steepness that causes the data points to lie above or below 

 the transmission curve of the Harms and Bender (1978) investigation. The 

 inverse relationship between CL and H/L is apparent since waves of low 

 steepness are associated with relatively large values of C t (above the solid 

 curve), and waves of large steepness are associated with relatively low values 

 of C t . The floating tire breakwater acts as a wave energy filter that 

 discriminates according to both relative breakwater width, L/W, and wave 

 steepness, H/L (Harms, 1979b). The effectiveness of the structure increases 

 with H/L and decreases with L/W, all other things remaining constant. 

 Near breaking, the breakwater simply acts as a mechanism which induces wave 

 breaking and, therefore, large-scale turbulent dissipation of energy. Harms 

 (1979b) suggests that the data which incorporate large variations of H/L not 

 be indiscriminately combined to generate single best-fit curves. 



d. Prototype-Scale Mooring Line Forces. During Giles and Sorensen' s 

 (1978) testing of the prototype-scale floating tire breakwater at CERC, two 

 measures of the mooring force (peakload and average load) were obtained, each 

 per meter length of breakwater. In comparing the peak force to the average 

 force for each of the conditions, they noted that the peak force is the same 

 or only slightly higher for 8 modules in 4 meters of water and 12 modules in 2 

 meters of water. However, when comparing the 12-module structure in 4 meters 

 of water, the peak force is about 20 percent higher than the average force. 

 This indicates that for the same wavelength and wave height, additional 

 modules slightly increase the peak force. The peak forces shown in Figure 86 

 represent the maximum force measured during the test, and usually occurred 

 when the motionless breakwater was first subjected to wave motion. The 

 relative velocity between the water motion and the breakwater was largest at 

 that time. Average force values are shown in Figure 87. 



In all cases tested, Giles and Sorensen (1978) found the larger the wave 

 height and W/L ratio, the higher the peak and average forces, as shown in 

 Figures 86 and 87. However, no strong steepness or period effect was dis- 

 cerned in the data for either the peak or average force. Plotting all the 

 peak force data together and all the average force data together permitted a, 

 conservative prediction curve to be drawn through the upper boundary of the 

 data. Since the peak force represented the situation when the breakwater was 

 initially at rest and then subjected to monochromatic waves, the maximum force 

 that would be calculated using the peakload curve would probably be somewhat 

 larger than the peakload that would occur in a train of irregular waves. 

 Therefore, a conservative force prediction for the Goodyear Tire and Rubber 

 Company scrap-tire floating breakwater concept would be to obtain the mooring 

 force load based on the peakload curve in Figure 86. 



Harms (1979a) reanalyzed the CERC data, using the force parameter, F/yW 2 , 

 and relative breakwater width, L/W, and determined that the effect of vary- 

 ing the wave steepness, H/L, could be ascertained; these results are pre- 

 sented in Figure 88. 



132 



