If a dimensionless quantity yL is introduced, the solution may be displayed 

 efficiently in dimensionless form (Figure 44) . For large values of y » Equa- 

 tion 106 approaches Equation 64, which is valid for a jetty and constant 

 oblique breaking wave angle. 



alongshore: distance (*/b) 



Figure 44. Shoreline evolution behind a jetty with 



exponential variation in breaking wave angle 



(a = 0.4 rad , vL = 1) 

 m 



91. The solution obtained for a variable breaking wave angle over- 

 estimates the rate of erosion behind the jetty since it is assumed that the 

 amplitude of the longshore sand transport rate is everywhere the same (and 

 thus that the wave height, in principle, is constant). In reality, diffrac- 

 tion reduces the wave height in the shadow region and, accordingly, the ampli- 

 tude of the longshore sand transport rate there. Despite this reduction, 

 Equations 103 and 104 provide a better description of the actual situation 

 than the commonly used solution (Equation 64) for which maximum erosion will 

 always appear immediately adjacent to the jetty or long groin. 



70 



