a part of the energy is dissipated by partial or complete breaking. Histori- 

 cally, hydraulic breakwaters were preceded by pneumatic breakwaters. It was 

 a simple advancement to the initial creation of the surface current by the 

 injection of high-velocity water near the surface in a horizontal layer. This 

 development occurred when it was noted that the pneumatic breakwater produces 

 two horizontal currents, one opposing and the other following the incident 

 wave. Nearly half the energy input of the pneumatic breakwater is wasted in 

 generating the unnecessary current. This would not be true for a hydraulic 

 breakwater. 



Herbich, Ziegler, and Bowers (1956), Straub, Herbich, and Bowers (1958), 

 and Williams (1960) investigated the characteristics of two-dimensional 

 hydraulic breakwaters for intermediate depth waves (where 2 < L/d < 20) 

 because it appeared that this type of breakwater would be useful mainly in 

 attenuating waves near the coast. Rao (1968) and Nece, Richey, and Rao (1968) 

 believed that the hydraulic breakwater would have important applications in 

 deepwater waves; e.g., preventing deepwater waves from reaching structures 

 such as floating bridges or offshore drilling operations. The hydraulic 

 breakwater should be most effective for deepwater waves because most of the 

 kinetic energy of the waves is concentrated in the upper layers of the water 

 column. 



a. Intermediate Water Waves . The primary objectives of the two- 

 dimensional hydraulic breakwater studies by Herbich, Ziegler, and Bowers 

 (1956) and Straub, Herbich, and Bowers (1958) were to obtain information con- 

 cerning the effect of various parameters on wave attenuation, discharge, and 

 horsepower requirements. Major parts of the studies were conducted in the 

 larger wave flume (4.5 feet deep) with the use of only one manifold (Fig. 

 150). The experimental data indicated that the power requirements of the 

 hydraulic breakwater primarily depend on wavelength, water depth, wave steep- 

 ness, and submergence, spacing, and size of nozzles. The dimensionless horse- 

 power ratio, $, was again expressed as equation (76), but a dimensionless 

 discharge ratio, ft, was also found to be convenient for data display 

 purposes. 



ft = 



[L(gd) 1/2 ] 

 where q is the discharge per linear foot of breakwater. 



(78) 



Hydraulic Measuring 

 Je> Probe 





Figure 150. Experimental facility used to evaluate hydraulic breakwater 

 effectiveness (after Herbich, Ziegler, and Bowers, 1956). 



211 



