PART IV: DESIGN CONSIDERATIONS 



27. Discussed in this part are the parameters that affect shoreline 

 response to detached breakwaters and so guide their design. The primary 

 accretionary features associated with breakwaters, tombolos (structure- 

 connected), and salients (nonconnected) are compared. Breakwater positioning 

 and the significance of single versus segmented design are discussed, as well 

 as available techniques used to control shoreline response. 



Significant Parameters 



28. Shoreline response to detached breakwaters is primarily controlled 

 by wave diffraction (SPM 1984, Chapter 2, Section IV). Wave length, height, 

 and angle and (in the case of segmented structures) the ratio of gap to wave- 

 length affect the diffraction pattern and the wave height behind the break- 

 water (Figure 2) . The shoreline tends to align itself parallel to the dif- 

 fracted wave crests. The rate of shoreline response is governed predominantly 

 by the wave energy and the incident angle of the diffracted waves as they ap- 

 proach the shore. Other important parameters are (a) the local water level 

 range, (b) natural beach slope, (c) available supply of sediment, and (d) sed- 

 iment grain size. 



Wavelength 



29. In general, the amount of wave energy diffracted into the region 

 behind the breakwater increases with increasing wavelength. If diffraction 

 theory using linear waves and a flat bottom are assumed, wavelength will not 

 affect the pattern made by the crests, but will affect wave height at each 

 location. Longer waves will provide more energy to the shadow zone behind the 

 breakwater and might tend to prevent tombolo formation. The amount of energy 

 that penetrates behind a detached breakwater can be computed by using the 

 diffraction diagrams presented in Figures 2-28 to 2-39 of the SPM. An example 

 of the use of diffraction analysis for designing a segmented detached break- 

 water is presented in Appendix A. 



Breakwater gap size versus wavelength 



30. The ratio of gap size to wavelength greatly affects the distribu- 

 tion of wave height behind segmented detached breakwaters. Stated simply, 

 increasing the gap-to-wavelength ratio increases the amount of energy 



30 



