3, Computation of Transmission and Reflection Coefficients for 

 Trapezoidal, Multilayered Breakwaters . 



Sollitt and Cross (1972, App. G) presented the results of a 

 laboratory investigation of reflection and transmission characteristics 

 of their model breakwater (Fig. 25). For the present purpose of 

 comparison with predicted reflection and transmission coefficients 

 only the tests performed by Sollitt and Cross (1972) with relatively 

 long waves will be used. 



Thus, the wave conditions to be used in the following for the 

 purpose of demonstrating the computational aspects of the approximate 

 method described in Section IV. 1 are 



h =1.167 feet : T = 2.5 seconds ; L = 14.56 feet , (164) 







and the breakwater configuration is that shown in Figure 25. 



a. Determination of the External Energy Dissipation . As discussed 

 in Section IV. 1 the first step in the approocimate procedure for 

 evaluating the reflection and transmission coefficients of a 

 trapezoidal, multilayered breakwater is to estimate the external energy 

 dissipation on the seaward slope using the procedure developed in 

 Section III. 



From the breakwater characteristics shown in Figure 25 it is seen 

 that the seaward slope consists of various stone sizes. Since the 

 main part of the front face consists of stones of diameter 

 djj = dj = 0.125 foot (3.8 centimeters) it is reasonable to adopt this 

 stone size as the roughness of the slope. This is further justified 

 by the fact that this stone size is the size in the cover layer near 

 the Stillwater level, where one would expect the major part of the 

 external energy dissipation to take place. Hence, for the purpose of 

 estimating the external energy dissipation the slope characteristics 

 are taken as : 



Roughness = d^^ = 0.125 foot ; tan$^ = 1/1.5 = 0.667 . (165) 



This information in addition to the incident wave characteristics 

 specified by equation (164) is sufficient to use the procedure developed 

 in Section III for the prediction of the reflection coefficient, Rttj of 

 rough slopes when the incident wave amplitude, a j , is specified. 



Comparison of the incident wave characteristics (eq. 164) and the 

 slope characteristics (eq. 165) with the corresponding values for the 

 Froude model characteristics given in the numerical example presented 

 in Table 4 show that the calculations presented there correspond to the 

 conditions being considered here. The numerical example presented in 



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