As a structure settles, wave transmission will increase. Wave transmission 

 improves water circulation, limits seaward growth of the salient, and reduces 

 wave forces on the structure, thereby increasing its longevity. 



A wave transmission coefficient K T can be defined as the ratio of the 

 height of the incident waves (prior to reflection) to the height of the wave 

 immediately behind the breakwater. The value of K T thus ranges between 

 and 1, with the value indicating an infinitely high impermeable breakwater 

 and the value 1 indicating complete transmission (no breakwater). At present, 

 the value of K T is specified empirically for use in the model. 



WAVE DIFFRACTION 



Waves incident to a detached breakwater diffract at the tips of the struc- 

 ture, and wave energy is transferred behind it (Fig. 2). The nearshore area 

 adjacent to the structure and directly reached by waves is called the illumi- 

 nated region, and the area sheltered from direct wave incidence and reached 

 solely by waves radiating from the tips of the structure is called the shadow 

 region. The change in wave direction at each tip and the decreasing wave 

 height with distance alongshore behind the structure both produce a longshore 

 current directed into the shadow region. Sand transported alongshore by the 

 current is deposited in the calmer wave shadow region behind the breakwater. 

 In Fig. 2, the reduced wave height is represented by decreasing values of the 

 diffraction coefficient K D . 



The longshore sand transport rate is normally expressed in terms of wave 

 conditions at breaking. To reproduce shoreline change behind detached break- 

 waters, therefore, both wave diffraction and transmission must be accurately 

 represented in the breaking wave calculation. A pragmatic procedure incorp- 

 orated in GENESIS to calculate breaking wave height and direction under 

 combined wave diffraction, refraction, and shoaling was developed by Kraus 

 (1983, 1984). The capability to calculate wave transmission together with 

 these transformations was recently added to the model (Hanson and Kraus, in 

 prep). 



Diffraction and transmission are interdependent. As waves propagate over 

 and through a detached breakwater and into the shadow region, the difference 

 in wave height between the illuminated and shadow regions will decrease, and 

 thus also the effect of diffraction. As a consequence, less sand will be 



43 



