b Small-Scale Experimental Studies. 



Several extensive, although 



relatively small-scale, experimental studies have been conducted on the mech- 

 anism and efficiency of wave attenuation by pneumatic breakwaters (Wetzel, 

 1955; Straub, Bowers, and Tarapore, 1959; Colonell, Carver, and Lacouture, 

 1974). The studies of Wetzel (1955) and Straub, Bowers, and Tarapore (1959) 

 were performed in two wave channels which were geometrically similar but of 

 two different sizes (Fig. 138). The smaller of the two channels was 2 feet 

 wide, 1 foot deep, and 50 feet long; the larger was 9 feet wide, 4.5 feet 

 deep, and 253 feet long. The test procedures consisted essentially of measur- 

 ing the incident and transmitted wave heights, and correlating these measure- 

 ments with the amount of air discharge during the specific test. Figure 139 

 shows the variation in velocity at three points in each channel, as a function 

 of air discharge, and Taylor's (1955) theoretical development. If the point 

 of velocity measurement is other than at a maximum, then the constant 0.00454 

 would not apply. It was found in these laboratory studies that the maximum 

 velocity measured one water depth distance from the breakwater was in good 

 agreement with the Taylor theory. 



Differential 



Measuring 



Probe 



14' 



Laboratory Floor 



7777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777771777777777777777. 



a. Water depth = 1 ft 



u/s 



Probe 



■+-M — 25' — I 



D/S 

 Probe 



b. Water depth = 4.5 ft 



Differential 

 Manometer 



Static Pres. 

 Manometer 



Figure 138. Two sizes of small-scale laboratory flumes used to investigate 

 pneumatic breakwater effectiveness (after Straub, Bowers, and 

 Tarapore, 1959). 



200 



