water surface disturbances downstream were induced, and the effectiveness of 

 the breakwater as a function of frequency could be ascertained. The break- 

 water tended to operate as a low-band pass filter for the various components 

 of the spectrum. Figure 147 shows that the breakwater was ineffective below a 

 frequency of about 1.8 hertz, but was extremely effective for higher frequency 

 components. This observation was consistent with Taylor's (1955) conclusions, 

 and similar behavior may be expected on larger scale tests and prototype 

 installations. 



UPSTREAM RECORD 



V) 1200 -- 



> 600" 



POWER = 5 45 FT-LB/SEC-FT 



POWER = 25 FT-LB/SEC-FT 



10 2.0 



FREQUENCY 



Figure 147. Pneumatic breakwater laboratory investigation of 

 effect of wave frequency and applied power on cumu- 

 lative specrtral density at a 2-foot water depth 

 (after Colonell, Carver, and Lacouture, 1974). 



c. Large-Scale Experimental Studies . The need for a mobile breakwater 

 system that allows unrestricted passage over it and does not require the 

 costly construction points directly toward the adoption of pneumatic wave 

 attenuation systems. Although previous research indicated that the volume 

 of air required to produce any reasonable degree of wave height attenuation 

 prohibits the use of this system, recent research has indicated that good 



207 



