(c) Laboratory . St. Anthony Falls. 



(d) Test Period . April 1955 to April 1956. 



(e) Problem . Previoios theoretical and experimental in- 

 vestigations have shown that a rising curtain of air bubbles can provide 

 adequate protection to boats from waves with relatively small dimensions 

 in deep water, and that the wave attenuation is accomplished by the hori- 

 zontal water currents generated by the rising air bubbles. This problem 

 concerns the practicality of generating the horizontal currents by water 

 jets directed horizontally toward the oncoming waves. 



(f) Purpose of Study . This investigation was conducted to 

 determine the relationship between wave attenuation and the incident wave 

 characteristics as a function of water depth, jet submergence, jet diam- 

 eter, horizontal and vertical jet spacing, angle between wave direction 

 and jet direction, and jet discharge. The tests used two wave flumes, 

 one considerably larger than the other to obtain some information on 

 scale effects. 



(g) Test Facilities . The smaller of the two wave flumes 

 was 50 feet long, 2 feet wide, and 1.25 feet deep, and was equipped with 

 a pendulum-type wave generator. This generator is essentially a form of 

 the piston generator, in which the piston is suspended by two movable arms 

 connected above to a fixed beam. The movable part (piston and arms) is 

 oscillated by a connecting rod attached to a drive wheel. By shifting the 

 upper pivot points of the arms, the piston path is deviated from the hori- 

 zontal during each cycle of the drive wheel. The result is a motion of 

 the piston similar to the orbital motion of water particles in generated 

 waves. The generator, powered by an alternating current, 0.5-horsepower 

 (1,724 revolutions per minute), electric motor, could produce waves with 



a maximum height of 0.25 and 10 feet in length. The larger wave flume 

 was 253 feet long, 9 feet wide, and 6 feet deep, and was equipped with 

 a flap-type wave generator capable of producing waves with a maximum 

 height of 1.5 and 18 feet in length. Both channels were equipped with 

 wave absorbers on the test structure end of the facility. Wave heights 

 were measured with a capacitive-type gage and recorded on an oscillograph. 

 Water was supplied to the manifolds by a 3-horsepower pump in the small 

 channel and a 25-horsepower punqj in the larger channel. Suction lines 

 for the pumps were located near the wave absorbers; thus, the currents 

 in the flumes formed a closed system. Discharges into the manifolds 

 were measured by Venturi meters (Fig. 6-66) . 



(h) Test Procedures . The test procedure consisted essen- 

 tially of measuring incident and transmitted wave heights for various 

 water discharges, jet spacings, jet diameters, jet directions relative 

 to wave direction, depths of water, wave heights, and wavelengths. The 

 tests were in two wave flumes; the larger flume had linear dimensions 

 4.5 times those in the smaller flume. 



440 



