When submergence of a crest is small (h/dg >_ 0.8 for shallow water) 

 or when transmission is by overtopping, the results of hydraulic model 

 studies must be used to evaluate transmission coefficients. The range 

 of dg/gT^ values included in numerous laboratory investigations are 

 summarized in Figure 7-32. Figures 7-33 through 7-35 show some experi- 

 mental results obtained by Saville (1963) for an impermeable rubble-mound 

 breakwater. Interpolation between curves permits an estimate of wave 

 heights on the leeward side of similar prototype structures. If experi- 

 mental results in Figure 7-32 are used to determine wave transmission, 

 transmission coefficients obtained by several investigators should be 

 computed and compared for the appropriate value of dg/gT^ whenever 

 possible. 



Figures 7-36 and 7-37 show experimental values of the transmission 

 coefficient for the permeable rubble-mound breakwater sections investi- 

 gated by Saville. Higher transmission coefficients result for permeable 

 structures than for similar impermeable structures, since part of any 

 incident wave energy is transmitted through a permeable structure in 

 addition to the energy transmitted over it. Because of the difficulty of 

 modeling permeability in laboratory studies, transmission coefficients 

 obtained by interpolation between the curves of Figures 7-36 and 7-37 

 should be considered as estimates of the true transmission coefficient. 

 The data shown in Figures 7-36 and 7-37 can be supplemented by the 

 experimental results of Jackson (1966), Hudson and Jackson (1966), Dai 

 and Jackson (1966), and Davidson (1969) for wave transmission through 

 typical rubble breakwater sections. 



The use of Equations 7-9 and 7-10 and Figures 7-33 through 7-37 to 

 obtain transmitted wave heights is illustrated by the following example 

 problems. 



************** 



EXAMPLE PROBLEM ************** 



GIVEN : A design wave with H=6ft,T=8 sec and a submerged impermeable 

 breakwater with a crest width of b = 30 ft. Depth in front of the 

 structure is dg = 15 ft, the height of the crest above bottom is 10 ft, 

 and seaward and landward slopes are 1 on 2. 



FIND: Wave height on the leeward side of the breakwater assuming no 

 energy dissipation at the structure. 



SOLUTION: Calculate; 



15 



gT2 (32.2) (8)^ 



= 0.0073 



which is greater than dg/gT^ = 0.00155, hence the wave is not a 

 shallow-water wave and Equation 7-9 cannot be used. Assuming that 



7-54 



