Figure 27. Definition of terms for wave transmission for permeable breakwaters, 



The relative water depth, d/gT^ , is one of the most important parameters 

 controlling the reflection coefficient, K^ (Fig. 28), with the reflection 

 coefficient increasing as d/gT^ decreases. The wave steepness, H/gT^ , and 

 the ratio of water depth to structure height, dg/h, have less influence. In 

 general, the reflection coefficients for rough permeable breakwaters are much 

 less than for similar smooth impermeable breakwaters (Fig. 10). Since no 

 comprehensive model is currently available for predicting reflection coeffi- 

 cients, laboratory model results should be used to estimate K^. A rough 

 estimate of the reflection coefficient for permeable subaerial breakwaters 

 may be obtained using the method of Madsen and White (1976) (computer program 

 MADSEN in App. G) . Typical comparisons between predictions and laboratory 

 measurements are shown in Figure 29. 



The wave transmission coefficient, Ky, is primarily a function of wave 

 steepness for a given permeable breakwater design and hydraulic conditions 

 where there is no transmission by overtopping (Fig. 28). Since the wave steep- 

 ness increases the amount of energy dissipated on the face and inside the 

 breakwater increases (Madsen and White, 1976), the transmission coefficient 

 decreases. However, as soon as the wave runup level exceeds the breakwater 

 crest, wave transmission by overtopping occurs and the transmission coefficient 

 increases with increasing steepness. Figure 30 (lower part) shows the case 

 where no overtopping occurs and Ky decreases (low steepness waves) , then Kj^ 

 increases with increasing steepness where transmission by overtopping and 

 transmission through a breakwater occur simultaneously. In the case of a 

 submerged breakwater the wave transmission coefficient decreases as the wave 

 steepness increases (upper part of Fig. 30). 



b. Estimation of the Coefficient of Wave Transmission Through Permeable 

 Breakwaters Using the Madsen and White Model . The advantages of the Madsen and 



White (1976) model 

 is completely self 

 wide range of cond 

 height, breakwater 

 various layers in 

 in the breakwater 



for predicting transmission coefficients are that the model 

 -contained and it can be used to predict coefficients over a 

 itions. Parameters that can be varied include the breakwater 



width, breakwater slope, the size and relative location of 



the breakwater, and the size and porosity of materials used 



Another advantage of the model is that it can be used to 



40 



