results could be obtained, with refinements in the system, for smaller volumes 

 of air. Sherk (1960) considered that new evidence showed such good possibil- 

 ities that the concept should not be ignored, and was worth testing to verify 

 or negate the claims. A full-scale experimental tank test program was exe- 

 cuted in as near-actual field conditions as possible. 



Sherk's experimental study was conducted in CERC's wave tank, 635 feet 

 long, 20 feet deep, and 15 feet wide. A reciprocating wave generator was 

 located at one end of the flume and a submerged rock wave absorber was located 

 at the other end. The normal operating water depth in the tank was 16 feet, 

 and various wave heights, lengths, and periods were generated. The most 

 important, and accurate, measurement needed to regulate and evaluate the 

 effectiveness of a pneumatic breakwater system is the volume of air dis- 

 charged. Commercial flowmeters were used for this critical airflow measure- 

 ment. The wave periods tested ranged from 2.61 to 16.01 seconds, sufficiently 

 covering the range of wave periods most often found in the open ocean. A cone 

 anemometer-type rotary meter was used for the water velocity measurements. 



The experimental results recorded during the pneumatic wave attenuation 

 tests provided both a quantitative and a qualitative nature, and showed close 

 agreement with past theory. Much of of the analysis is qualitative, and such 

 factors as prototype concepts, construction cost, and ease of operation should 

 be subjectively evaluated. The fact that these tests were large-scale tank 

 tests ensures that the results more closely approach the magnitudes required 

 for a prototype installation than the data from small-scale laboratory tests. 

 Typical examples of the current profiles are presented in Figure 148 for 

 distances of 3 and 16 feet from the pneumatic breakwater. The velocities 

 remained fairly constant for about 36 feet from the system. The attenuation 

 produced by various air discharges is shown in Figure 149 for periods of 3.75 

 and 5.3 seconds. Figure 149(b) indicates a multiple manifold system may be 

 more effective than a single manifold under prototype conditions. This con- 

 clusion could not be ascertained from the results of the previous smaller 

 scale tests, which indicates the necessity for large-scale (near-prototype) 

 investigations when phenomena cannot be extrapolated directly from laboratory 

 tests. 



As a result of these large-scale pneumatic breakwater tests, Sherk (1960) 

 concluded that the use of this system in limited problem areas appears to be 

 feasible, although the air requirement would still be high. However, since 

 the need is for large volumes of relatively low-pressure air, it should be 

 within the capability of gas turbine compressors. Pneumatic wave attenuation 

 installations are easily traversed by ship or offloading craft. The large- 

 scale tank tests indicated that approximately one-sixth less air horsepower 

 than was predicted from previous small-scale tests is needed to produce a like 

 attenuation. Open-ocean applications may require even less air than the full- 

 scale tank tests because reflections in the model are not possible under 

 prototype conditions; however, this is not conclusive. 



2. Hydraulic Breakwater System. 



A hydraulic breakwater is formed by discharging water under pressure 

 through a manifold in a direction opposed to a train of surface gravity 

 waves. The water jets diffuse, a horizontal current is formed, and a high 

 degree of turbulence and mixing occurs. When waves propagate into a current, 



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