143. The three unknowns, H b , Dj, , and 6 h , axe obtained at intervals 

 alongshore by iterative solution of Equation 18 together with Equations 14 and 

 16 as a function of wave height and angle at the breaking depth and period. 



x 2 X1 



DISTANCE ALONGSHORE 



Figure 11. Definition sketch for wave calculation 



144. Diagrams that give contours of the diffraction coefficient for 

 monochromatic waves (in uniform water depth) can be found, for example, in 

 Chapter 2 of the SPM (1984). In these diagrams, the value of the diffraction 

 coefficient along the line of wave incidence defining the shadow and illumi- 

 nated regions is about 0.5, indicating that the wave height is about 50 

 percent reduced along this line. However, for the field situation of sea 

 waves having a spread about the principal direction of incidence, the reduc- 

 tion in wave height is not expected to be as great as for monochromatic waves. 

 Goda, Takayama, and Suzuki (1978) developed methods for calculating diffrac- 

 tion of random waves as caused by large land masses based on the concept of 



68 



