provide desired percentage reductions in wave height, for various wave 

 height, wavelength, and water depth combinations as a function of the 

 Reynolds and Weber numbers, has not been performed. As a result, accurate 

 transference equations to determine the horsepower requirements for pro- 

 posed full-scale installations are not available, and there is a consider- 

 able divergence of opinion as to the effectiveness and feasibility of 

 full-scale pneumatic breakwaters. 



The nature of the compressed air-bubble screen phenomena and the types 

 of facilities required to assemble a pneumatic breakwater are such that it 

 is not necessary to perform model studies for other than the most complex 

 prototype installations. However, additional laboratory tests should be 

 conducted, using a wide range of test conditions and small, medium, and 

 large wave flumes, to determine transference equations and scale-effect 

 data that can be used by the design engineer for application to particu- 

 lar prototype situations. The test results would also indicate the lin- 

 ear scales to be used for model studies that may be deemed necessary for 

 particular, complex, prototype installations. For each series of tests 

 in the proposed investigation, values of the wave height reduction co- 

 efficient, H-(-/Hi^, should be obtained for values of d/X from about 

 0.05 to 0.5 and for values of %/X from about 0.01 to 0.10. The tests 

 should be conducted by the procedures outlined in Section VI, 2, f based 

 on equations (6-14) to (6-20). To prevent exaggeration of the effec- 

 tiveness of the pneumatic breakwaters, the transmitted wave height, H^, 

 should be measured a distance landward of the bubble screen where the 

 horizontal current generated by the bubble screen is essentially zero. 



(5) Hydraulic Breakwaters . Although the pneumatic breakwater 

 was invented in 190 7, the mechanism by which the reduction of wave heights 

 is achieved was apparently not understood until Thijsse performed a few 

 tests at Delft, Netherlands, in 1936 (Evans, 1955) and suggested that the 

 wave reduction was caused by water currents generated by the rising air 

 bubbles. In 1942, Taylor showed mathematically that a current directed 

 against oncoming waves should stop all waves shorter than those of a 

 certain critical length (Taylor, 1955) , and White investigated the prob- 

 lem experimentally in 1943, using both air bubbles and water jets to gen- 

 erate the horizontal water currents (Evans, 1955). Carr (1950) conducted 

 tests at the University of California in 1950 to investigate wave reduc- 

 tion by horizontal water currents generated by both the pneumatic and 

 water- jet methods. Since 1950 several hydraulic breakwater investiga- 

 tions have been conducted, mostly in the United States. These tests 

 were performed in v^ave flumes using water depths ranging from about 0.3 

 to 4.5 feet (Wetzel, 1955; Herbich, Ziegler, and Bowers, 1956; Williams, 

 1960; Williams and Wiegel, 1961; Nece, Richey, and Rao, 1968). However, 

 no known full-scale hydraulic breakwater tests have been made. Several 

 experimental investigations have been conducted to determine the ability 

 of the hydraulic breakwater to reduce wave heights, and a few tests have 

 been made to determine the effects of linear scale on the required dis- 

 charge from the jet manifold to obtain specified reductions in wave height. 

 However, more information is needed to obtain transference equations for 

 the design engineers to accurately determine the horsepower requirements 



338 



