to about 6.5 feet. Wave heights were recorded by step-resistance gages 

 in conjunction with a Brush oscillograph. The gage consisted of a ver- 

 tical plastic staff with equally spaced plugs interconnected by appro- 

 priate resistances providing a linear relationship between plug distance 

 and wave height. Both incident and attenuated wave heights, were meas- 

 ured at distances between 187 and 396 feet from the wave generator posi- 

 tion. Air temperature was measured by a commercial, single well-type 

 thermometer with a range of 30° to 240°. A submerged rubble-mound wave 

 absorber was constructed in the wave flume 465 feet from the wave gener- 

 ator piston. When waves impinged and broke over the absorber, a rise in 

 water level or setup occurred that extended back toward the wave measur- 

 ing area. Since it was necessary to monitor the water level in the area 

 of the wave height measurement, a water level gage was installed near the 

 wave absorber. A rotary meter of the cone anemometer-type was used for 

 the water current measurements, A signal was transmitted to the Brush 

 recorder tape for each revolution of the current meter shaft. Four air 

 compressors were used, each with a capacity of 500 cubic feet per minute 

 (the piping arrangement is shown in Figure 6-64a) . The test rig consisted 

 of that part of the equipment from the outlet of the flow-metering appara- 

 tus up to and including the discharge tubes (Fig. 6-64b) . Three discharge 

 manifolds were used, each 3 inches in diameter and 15 feet in length with 

 26 discharge holes (evenly spaced) approximately 1/4 inch (6 millimeters) 

 in diameter. The most important and accurate measurement needed to de- 

 termine the effectiveness of pneumatic wave attenuation is the volume of 

 air discharged per foot length of breakwater. The pipe arrangement, pres- 

 sure gages, flow-meter, and thermometer are shown in Figure 6-65. Flow 

 calibration charts were prepared for the flowmeter from data of pressure 

 and percentage flow, correcting for temperature and pressure. By knowing 

 the pressure used for a particular test, the charts can be used to deter- 

 mine the air discharge volume. 



(h) Test Procedures . Tests were conducted using water 

 depths from 13.4 to 16.75 feet; manifold depths from 7.0 to 16.75 feet; 

 single, double, and triple manifolds; wave periods from 2.6 to 16.0 sec- 

 onds; wave heights from 3.06 to 6.40 feet; wavelengths from 35 to 347 

 feet; and air discharges from about 9 to 105 cubic feet per minute at 

 21° Celsius (70° Fahrenheit) and 14.7 pounds per square inch absolute 

 pressure. The procedure for a typical test was as follows: 



(1) The four compressors were started and allowed 

 to reach normal rimning temperature; 



(2) the Brush recorders were adjusted and the wave 

 gages were positioned according to the water depth while the com- 

 pressors were warming up; 



(3) the wave generator was then started; 



(4) the recorders were started when the waves 

 reached a steady-state condition; 



436 



