directional spreading of waves and penetration of energy to the lee of a land 

 mass or long structure. Their results show that the value of the diffraction 

 coefficient along the separation line is about 0.7. 



145. Because GENESIS was developed to simulate waves and shoreline 

 change in the field, the procedure of Goda, Takayama, and Suzuki (1978) (see 

 also, Goda (1984)) was adapted. Details of application of the method to 

 calculate wave breaking produced by combined diffraction, refraction, and 

 shoaling as used in GENESIS are given by Kraus (1981, 1982, 1984, 1988a). In 

 GENESIS it is assumed that the method is valid for relatively short structures 

 such as detached breakwaters. 



Contour modification 



146. The beach plan shape changes as a result of spatial differences in 

 longshore sand transport. The change in the beach shape, in turn, alters the 

 refraction of the waves. Within the framework of the wave model internal to 

 GENESIS, the interaction between the beach and waves is accounted for in two 

 ways. First, with change in position of the shoreline, the distance to the 

 source of refraction (P x in Figure 12) will change, and hence the ray start- 

 ing angle 8 X will also change. Second, the shape of the shoreline will 

 distort in the vicinity of a structure, and the offshore contours will tend to 

 align with this shape. This effect is accounted for by assuming that the 

 orientation of the shoreline at a particular point extends to the depth where 

 the diffraction source or reference depth is located. Thus, although plane 

 and parallel contours are assumed, their orientation is allowed to change as a 

 function of position alongshore to conform with the local beach plan shape. 



147. Such a local coordinate system aligned with the local contours is 

 defined by the (x' , y') axes in Figure 12. This coordinate system is rotated 

 by the angle of orientation of the local shoreline 8 S = tan _1 (3y/3x) eval- 

 uated at point P 3 . In the rotated coordinate system, an angle 8' is 

 related to the angle 8 in the fixed (original) system by 8 ' — 8 + 6 a . 

 Equation 16 can be used to calculate wave refraction in the primed coordinate 

 system but with angles on both sides replaced by corresponding primed wave 

 angles. Similarly, the refraction coefficient (Equation 10) can be calculated 



69 



