oversimplification to neglect the process of wave diffraction. Consequently, 

 although Equation 64 (with reversed sign) may give a satisfactory description 

 of shoreline evolution at some distance downdrift of a jetty, in the vicinity 

 of the jetty this solution does not represent what is commonly observed. Ero- 

 sion just behind the jetty will be overestimated if diffraction is neglected 

 since the wave height is assumed to be constant alongshore. Accordingly, by 

 allowing a variation in wave height (and thus in the amplitude of the sand 

 transport rate) in the shadow zone, a more realistic description of shoreline 

 change will be obtained. 



84. There are a number of ways to account for a varying amplitude in 

 the longshore sand transport rate (resulting from varying wave height) . One 

 way is to assume that, outside the shadow zone, the incident breaking wave 

 angle and the amplitude of the sand transport rate are not influenced by the 

 jetty. In the vicinity of the jetty, Equation 11 may be used to account for a 

 variation in the amplitude of the sand transport rate. An alternative way is 

 to divide the shadow region into distinct solution areas, each having a con- 

 stant amplitude of the sand transport rate. The incident breaking wave angle 

 may also be varied from one solution area to another. With this procedure, a 

 coupled system of equations is obtained which involves intensive calculations 

 for even a small number of solution areas. If the simple case of two solution 

 areas (one inside the shadow zone and one outside) is considered, the mathe- 

 matical formulation is the same as for a detached breakwater. However, the 

 incident breaking wave angle outside the shadow region is not zero (in which 

 case no sand transport would occur) but has a finite value. Therefore, the 

 boundary condition on continuity in sand transport across the border between 

 the two solution areas takes the following form: 



8y l 1 1 8y 2 



-r- L =<x- i --a+± T -— ^ (92) 



9x ol r 2 o2 .2 3x 







2 

 where 6 is the ratio between the amplitudes of the sand transport rate in- 

 side and outside the shadow region. The analytical solution to this problem 

 is formally identical to Equations 81 and 82, except that certain constants 

 are different. The following substitutions should be made in order to apply 

 Equation 81 and Equation 82 to the diffracting jetty case: 



64 



