predicted salient. Small deviations in wave direction increase the longshore 

 component of wave energy flux, which results in more sand being moved 

 alongshore. Because of the sheltering provided by the impermeable structure, 

 this transported material tends to collect in the protected region to the lee of 

 the structure, advancing the salient seaward. 



The final sensitivity test used a 200-m-long breakwater located 250 m 

 offshore to evaluate the effect of varying structure transmission on predicted 

 beach response. Normally incident waves with a 1.5-m significant wave 

 height and 6-sec wave period were used in the 180-hr simulation. In 

 GENESIS, a structure transmission K T value of indicates an impermeable 

 structure, whereas a value of 1 indicates a structure that is transparent to 

 incoming waves. This sensitivity test used four K T values ranging from to 

 0.8 (Figure 33). As expected, an impermeable structure (K T = 0) results in 

 greater salient growth, while the more permeable tests show less salient 

 progradation. For example, K T = 0.2 decreases the maximum shoreline 

 advance 36 percent from the impermeable structure simulation, and reduces 

 the accumulated volume by 25 percent. 



Hanson, Kraus, and Nakashima (1989) also present example calculations 

 illustrating GENESIS'S breakwater transmission capability. A three-segment 

 system, each segment with a different transmission coefficient, is used to 

 simulate beach response as a function of varying wave approach. A second 

 test series uses a continuous structure with varying transmission properties 

 alongshore, which might occur in nature due to differential settling of the 

 structure, or uneven loss of armor stone. 



Shoreline Position (m) 







Hi 



0.0 



rsss; vt ; rj >?>>>>))> J ;\ 











Wave 



Crests 









20 



\ 



/ 







0.2 







15 





- - 0.4 



0.8 







10 



- 









5 







r — " ~" ~" — - __^^ 









i I I .J 



100 200 300 



Distance Alongshore (m) 



400 



500 



Figure 33. Shoreline change as a function of transmission (Hanson, Kraus, and Nakashima 

 1989) 



Chapter 3 Tools for Prediction of Morphologic Response 



57 



