IV. ANALYSIS OF TEST RESULTS 



This section provides an analysis of the wave transmission and reflection 

 results of the model tests. Impermeable and permeable breakwaters were 

 investigated, and a separate discussion is devoted to each type breakwater. 

 The first part of this section describes observed trends in the values of the 

 transmission and reflection coefficients as a function of the parameters var- 

 ied in this study. The second part includes development, description, and 

 evaluation of methods for predicting wave transmission coefficients. The third 

 part discusses the effect of a breakwater on other wave characteristics, such 

 as the wave height distribution and shape of the transmitted wave spectra. 

 Since good models are not available for predicting wave reflection coefficients 

 for breakwaters, it is recommended that the model tests be used directly to 

 estimate breakwater wave reflection coefficients. 



1 . Wave Transmission and Reflection for Impermeable Breakwaters . 



a. Observed Trends in Transmission and Reflection Coefficients . As a wave 

 approaches an impermeable breakwater some of the wave energy is supplied to 

 wave runup, some of the energy is dissipated, and the remaining wave energy 

 moves seaward in the form of a reflected wave. If the runup exceeds the crest 

 elevation of the breakwater, waves will be regenerated on the landward side of 

 the structure. Figure 7 shows aspects of this process and defines some of the 

 terms used in wave transmission by overtopping. 



Figure 7, 



Kto " Ht /Hi 

 Definition of terms for wave transmission by overtopping. 



Madsen and White (1976) found that low reflection coefficients and corre- 

 spondingly large amounts of wave energy are dissipated on smooth nonovertopping 

 structures. This observation has been verified using the data of Ahrens (1979) 

 for breaking and nonbreaking waves. The data show that for the case of no 

 overtopping the reflection coefficient decreases and a larger fraction of the 

 wave energy is dissipated as the wave steepness increases (Fig. 8). More than 

 80 percent of the wave energy is dissipated by the smooth slope of 1 on 1.5 for 

 the steepest waves tested. Note that the magnitude of the wave reflection 

 coefficient is approximately the same for monochromatic and irregular waves, for 

 a given value of wave steepness. 



As the height of the breakwater is reduced the magnitude of the wave reflec- 

 tion coefficient decreases because much of the wave energy is transmitted by 

 overtopping. For example, with a freeboard of zero (water level at the break- 

 water crest) BWl has reflection coefficients that are less than 20 percent of 

 the reflection coefficient for a structure that is not overtopped for the 

 steeper waves tested (Fig. 9). At values of small wave steepness the size of 



22 



