For a 50-percent wave height attenuation, results of Straub, Bowers, and 

 Tarapore (1959) and other investigators are compared in Figure 140. The 

 horsepower ratio for the data of the other investigators was about 50 percent 

 less than that of Straub, Bowers, and Tarapore, and may be due in part to 

 different methods of measuring the attenuated wave. 



1 1 ! 



Symbol Source 

 O SAF 9ft Channel 



1 



Depth of 

 Water in feet 



4.5 



| 



r 

 i 

 < 



] SA 

 ^ SA 

 > Jap 

 7 Jap 



- 2ft C 



- 6 in C 

 anese 1 

 onese F 



lannel 

 lannel 

 dodel Te 

 >roto. Te 



sts 

 sts 



1.0 



1.0 



2.3 



28.0 





\ 







































O 

 n 



O 





















I 



V 



k O 



vo 









3.0 



2.5 



2.0 



1.5 



Z 1.0 



T= 0.5 



e 



O u 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 



Wove Length L 

 Depth of Water d 



Figure 140. Effect of relative water depth, L/d, and comparison 

 of dimensionless horsepower requirements, $, for 

 pneumatic breakwater at a 50-percent wave height atten- 

 uation (after Straub, Bowers, and Tarapore, 1959). 



If scale effects were not present (i.e., the Froude law relating the 

 model to the prototype was completely valid) , the value of $ would remain 

 unchanged for a given value of L/d. Kurihara (1958) reported that his proto- 

 type tests required much less power than that predicted from the model tests. 

 A factor which may affect data comparisons of various sources is the location 

 of the measuring element with respect to the breakwater. If the wave sensor 

 is less than twice the depth from the breakwater, the attenuation will be 

 higher; for a given attenuation, the power requirement will be lower. Table 7 

 shows the $ values for 50-percent attenuation for Kurihara' s (1958) 

 model and prototype tests, and comparative values from Straub, Bowers, and 

 Tarapore's (1959) laboratory tests. 



(1) Effect of Wave Steepness . The steepness in the laboratory exper- 

 iments varied from 0.02 to 0.08. It was found that the air requirement for a 

 given attenuation was essentially independent of the wave steepness (Fig. 

 141). The power requirements for hydraulic breakwaters to achieve the same 

 degree of wave attenuation increase with wave steepness by as much as a factor 

 of about 3; wave steepness increases from approximately 0.02 to 0.08. This 

 indicates that the pneumatic breakwater is more efficient for waves of higher 

 steepness than the hydraulic breakwater. 



202 



