(5) The case of porosity of the breakwater. Porosity of each of 

 the materials must be known in order to use the computer program 

 MADSEN. However, in many design situations the value of porosity 

 may be poorly known. Typical values of porosity, P, are given in 

 Table 5. The recommended method of determining the influence of 

 porosity on the predicted transmission coefficient is to run the 

 program ^4ADSEN at various values of porosity keeping all other param- 

 eters fixed. Figure 38 shows predicted transmission coefficients over 

 a range of wave steepnesses for three different values of porosity. 

 For this example, the absolute change in Ky^ produced by a given 

 change in P is largest for waves of small steepness. The largest 

 percent change in Ky^ for a given change in P occurs for the 

 steepest waves tested. In general, the same trend will be observed 

 for any breakwater; the value of Ky^ will increase as porosity 

 increases for a given set of conditions. However, the magnitude of 

 change of Ky^^ is a complex function of all of the parameters in a 

 design (breakwater geometry, water depth, wave height and period, 

 etc.)- A sensitivity analysis with the use of the program MADSEN, 

 similar to the analysis shown in Figure 38, is recommended if the 

 porosity of proposed materials is poorly known. 



Table 5. Porosity of various armor units (from 

 U.S. Army, Corps of engineers. Coastal 

 Engineering Research Center, 1977) . 



Armor unit 



No. of 



Placement 



Porosity 





layers 





(P) 



Quarrystonc (smooth) 



2 



Random 



0.38 



Quarrystonc (rough) 



2 



Random 



0.37 



Quarrystone (rough) 



>3 



■Random 



0.40 



Cube (modified) 



2 



Random 



0.47 



Tetrapod 



2 



Random 



0.50 



Quadripod 



2 



Random 



0.49 



llexapod 



2 



Random 



0.47 



Tribar 



2 



Random 



0.54 



Dolos 



2 



Random 



0.63 



Tribar 



1 



Un i f rm 



0.47 



Quarrystone 



Graded 



Random 



0.37 



c. Wave Transmission for Submerged Permeable Breakwaters . The coefficient 

 of wave transmission over a submerged permeable breakwater, ^To' ^^Y ^^ esti- 

 mated by the methods given in Section IV, 2. However, no generalized model is 

 currently available for determining the coefficient of wave transmission through 

 the structure, Ky^ . Saville's (1963) data for similar permeable and impermeable 

 structures show that the total coefficient, Ky, approaches the transmission by 

 overtopping coefficient, ^To > ^"d transmission through the breakwater becomes 

 less important as the structure becomes more submerged and the incident wave 

 height increases (Fig. 39). At dg/h > 1.2, the data from breakwaters BW3, BW3W, 

 BW4, and BW4W show that the coefficients of transmission through the structure 

 are approximately zero, so that Ky^^/Ky = 1.0. An upper estimate of the coeffi- 

 cient of transmission through the structure, Ky^, for a submerged breakwater 



50 



