An additional set of data is provided by Keulegan (1973, Table 20). 

 Again the reference porosity is taken as n^ = 0.46 and the diameter, d, 

 as the median diameter of the gravel tested. These data are plotted in 

 Figure 5 and exhibit considerably more scatter than that of Sollitt and 

 Cross (1972). The data are fairly well represented by the curve given 

 by equation (59) corresponding to Bg = 2.2 and R^, = 70 whereas the 

 curve corresponding to 6q = 2.7 gives values of Cf slightly on the 

 high side. It should be noted that the data used in Figure 5 are the 

 uncorrected data as obtained by Keulegan (1973). The scatter exhibited 

 in Figure 5 may therefore partly be attributed to the effect of shape 

 of the granular material. 



All in all the comparison of the empirical formulas with the 



experimental data is quite good when considering that the formulas 



originally were derived from experiments with sand whereas here they 



are compared with experiments performed with gravel, i.e., of diameters 



an order of magnitude larger. Whether or not the same formulas may be 



extended further to prototype scales (rubble) with complete confidence 



is a question which remains to be answered. However, at present it 



does seem that a value of 6 - 2.7 may be used as a reasonable first 



o 

 approximation. 



b. Comparison between Predicted and Observed Reflection and 

 Transmission Coefficients of Rectangular Breakwaters . The 

 empirical formulas for the hydraulic properties of a porous medium 

 were shown to be reasonably satisfactory in reproducing observed 

 characteristics of porous media in steady flow. The ultimate test of 

 these formulas is, however, their use as part of the entire procedure 

 developed in Section II. 2 for the prediction of transmission and 

 reflection coefficients of crib-style breakwaters. Two sets of experi- 

 mental data on reflection and transmission characteristics of porous 

 rectangular breakwaters are available for this purpose (Wilson, 1971; 

 Keulegan, 1973). '^ 



The experiments by Wilson (1971, Tables 5,6, and 7) were performed 

 on three different scales, and for the present purpose only, the experi- 

 mental data corresponding to relatively long waves, k^hg^^ 0.5, are 

 utilized. Wilson's experimental data for R and T are plotted in 

 Figures 6,7, and 8 as functions of the incident wave steepness, Hj^/L. 

 The predicted variation of R and T with H^/L following the procedure 

 developed in Section 1 1. 2 is shown based on the assumption of 

 Bq = 2.7, R(, - 170, and R^ = 70. In view of the results presented in 

 Figure 4 it is hardly surprising that the experimental data are 

 represented better by the curves corresponding to R^, = 170 than by the 

 choice Rj. = 70. The predicted values of the transmission coefficient, 

 T, are seen to be in excellent agreement with experimental values 

 whereas the agreement between reflection coefficients leaves something 

 to be desired. 



33 



