jet area at efflux (per foot of width) 

 divided by wavelength, dimensionless 



Relative Jet Power, Pj 



Figure 157. Two-dimensional test on effect of relative jet power, P , and 

 incident wave steepness, ^±/^±> on attenuation of deepwater 

 waves for a hydraulic breakwater (after Rao, 1968). 



Rao's (1968) observations in studying the effect of relative depth of sub- 

 mergence, y /L. are shown in Figure 158. These data show that generally, as 

 the relative depth of submergence is increased, the attenuation decreases. 

 The local peculiarity observed in the trend of P = 2.52 was investigated. 

 The experimental results on the effect of manifold submergence on the current 

 for the same discharge indicated that as the depth of submergence, y , is 

 decreased, there is a range of y in which the momentum of the current at a 

 given section downstream may decrease with y . Since the interaction of the 

 incident deepwater wave and the current is strongly dependent on the free- 

 surface conditions, this could result in a decrease in the attenuation. The 

 experimental data were limited, and it was not possible to define all con- 

 ditions under which such an event would occur. 



The efficiency, e, of the entire hydraulic breakwater system was con- 

 sidered. Equation (79) defines the efficiency of the attenuation; Rao's 

 (1968) experimental data are shown in Figure 159. Two basic trends are 

 evident: (a) efficiency increases with wave steepness, H^/L^, and (b) effi- 

 ciency decreases as attenuation increases. The data indicate that the effi- 

 ciency of the hydraulic breakwater in attenuating deepwater waves does not 

 exceed about 12 percent for most of the experimental data in the range of 

 attenuation considered. This has serious implications because it raises the 

 question of economic feasibility in applying the hydraulic breakwater to 

 prototype conditions. 



217 



