Wave transmission at detached breakwaters 



148. The design of detached breakwaters for shore protection requires 

 consideration of many factors, including structure length, distance offshore, 

 crest height, core composition, and gap between structures in the case of 

 segmented breakwaters. Several studies (Perlin 1979; Kraus 1983; Kraus , 

 Hanson, and Harikai 1984; Hanson 1989) have described numerical simulations of 

 the influence of detached breakwaters on the shoreline. However, an important 

 process absent in these works was wave transmission at the breakwaters. Wave 

 transmission, referring to the movement of waves over and through a structure, 

 is present in most practical applications, since it is economical and often 

 advantageous from the perspective of beach change control to build low or 

 porous structures to allow energy to penetrate behind them. 



149. One of the principal upgrades of Version 2 of GENESIS over the 

 previous version of the modeling system is the capability to simulate wave 

 transmission at detached breakwaters and its impact on shoreline change. This 

 capability was tested with excellent results for Holly Beach, Louisiana, a 

 site containing six breakwaters of different construction and transmission 

 characteristics (Hanson, Kraus, and Nakashima 1989). 



150. In order to describe wave transmission in the modeling system, a 

 value of a transmission coefficient K T must be provided for each detached 

 breakwater. The transmission coefficient, defined as the ratio of the height 

 of the incident waves directly shoreward of the breakwater to the height 

 directly seaward of the breakwater, has the range < K T < 1 , for which a 

 value of implies no transmission and 1 implies complete transmission. 



151. The derivation of the phenomenological wave transmission algorithm 

 in GENESIS was developed on the basis of three criteria: 



a. As K T approaches zero, the calculated wave diffraction 

 should equal that given by standard diffraction theory for an 

 impermeable, infinitely high breakwater. 



b. If two adjacent energy windows have the same K T , no diffrac- 

 tion should occur (wave height uniform at the boundary). 



c. On the boundary between energy windows with different K T , 

 wave energy should be conveyed from the window with higher 

 waves into the window with smaller waves. The wave energy 

 transferred should be proportional to the ratio between the 

 two transmission coefficients . 



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