jet area is increased, and the turbulent losses in the jet decrease as the jet 

 velocity decreases. The overall result of this combination is better effi- 

 ciency, e. However, as the jet area is increased, the required discharge is 

 accordingly increased; hence, the use of larger manifolds and supply pipes may 

 be necessitated. 



(4) Efficiency . More power was found to be required to attenuate 

 relatively steep waves than for the flatter waves; however, the efficiency of 

 the system, e, was found to be higher for the steeper waves. Defining the 

 efficiency, e, as 



(Pi " P t ) 



P i 



(79) 



where P. is the power of incident wave train, P t the power of transmitted 

 wave system, and P- the power of hydraulic jets. The experimental data are 

 illustrated in Figure 154. For a one-manifold system, the efficiency, e, 

 varied with incident wave steepness, H./L-, attenuation, and relative wave- 

 length, L/d, with the maximum efficiency about 12 percent. The attenuation 

 and total power requirements are presented in Figures 155 and 156. 



b. Deepwater Waves . Rao (1968) and Nece, Richey, and Rao (1968) con- 

 ducted two-dimensional studies of the effectiveness of hydraulic breakwaters 

 in attenuating deepwater waves. The studies covered mostly a range of 

 L/d < 1, since few detailed results were available in this range. The inci- 

 dent wave steepness, H-/Lj, varied from 0.01 to 0.11, which is the range of 

 typical representative deepwater conditions. The manifold was designed so 

 that the average efflux velocity would be the same for all jets, and the 

 current velocity was measured in the absence of waves. 



A major concern of this investigation was the effect of relative jet 

 power, P , on attenuation of deepwater waves of various steepness, and on 

 efficiency. The relative jet power is defined as 



p i 

 ? r = — (80) 



P,- 



where P- is the energy flux across the cross section of the jets (power 

 available from the jets) per unit width of tank, calculated at the plane of 

 orifice outlet, and P^ the rate at which energy is transmitted by the inci- 

 dent waves across the tank cross section per unit width of wave tank. The 

 effect of the relative jet power is shown in Figure 157 for L/d = 0.53; the 

 data extend over the complete range of attenuation. The attenuation increases 

 with increasing P , and the curves tend to flatten in the range of higher 

 attenuation. In other words, the amount of extra jet power required to 

 increase the attenuation from 80 to 100 percent, for example, is higher than 

 that needed to increase the attenuation from 20 to 40 percent. For the finite 

 observed attenuation at P = 0, an explanation is that the breakwater, when it 

 is not functioning, can be considered a fixed, submerged circular cylinder. 

 As the steepness increases, the relative jet power required to achieve a given 

 attenuation decreases. The breakwater is more efficient in damping steeper 

 waves, and this trend conforms with the fact that as steepness increases, the 

 crests tend to become less stable. Spilling rollers, although not completely 

 breaking, are formed, and energy is lost. 



214 



