92 



transmission at low-crested breakwaters, including Ahrens (1987) and recent 

 tests by Daemen (1991), Van der Meer assumed that a linear relationship 

 between the transmission coefficient K t and the relative crest height R^^q is 

 valid between minimum and maximum values of K t . Figure 54 shows the 

 basic graph for wave transmission. The linearly increasing curves are 

 presented by: 



K t = aR c /Drio + b (40) 



with: 



a = 0.031 H t /D^o - 0.24 (41) 



Equation 41 is applicable to both conventional and reef breakwaters. The 

 coefficient "b" for conventional breakwaters is given by: 



b = -5.42 s op + 0.0323 H^D^q -0.0017 (B/D^ 184 + 0.51 (42) 



and for reef breakwaters by: 



b = -2.6 s op - 0.05 ty/D^o + 0.85 (43) 



Based on the results of all tests analyzed (Van der Meer 1991), the 

 following minimum and maximum K t values were derived. The minimum and 

 maximum K, values for conventional breakwaters are 0.075 and 0.75, 

 respectively. For reef-type breakwaters, the minimum and maximum K t 

 values are 0.15 and 0.60, respectively. 



The validity of the wave transmission formula (Equation 40) corresponds 

 with the ranges of wave steepness and relative wave height tested. The 

 formula is valid for: 



1 < Hi/D^o < 6 and 0.01 < s op < 0.05 



Both upper boundaries are physical bounds. Values of Hf/D^Q > 6 will 

 cause instability of the structure and values of s > 0.05 will cause wave 

 breaking on steepness. The lower boundaries are given for too low wave 

 heights relative to rock diameter and for very low wave steepnesses. The 

 formula may be applicable outside the range, but the reliability is low. 



Reflection 



Low-crested rubble-mound breakwaters, because of their high porosity, 

 rough texture, and low profile, typically have low reflection coefficients. This 

 is an advantage because it reduces the potential for toe scour, navigation 

 problems, and erosion at nearby shorelines caused by reflected waves. The 



Chapter 4 Structural Design Guidance 



