large salient behind each breakwater (EM 1110-2-1617) (Figure 19a). 

 Numerous more closely spaced segments will also result in a sinuous 

 shoreline, but with more closely spaced, smaller salients (Figure 19b). If 

 uniform shoreline advance is desired, a segmented system with small gaps or 

 a single long breakwater with adequate wave overtopping and transmission 

 should be considered. 



Gap width. Wide gaps in a segment system allow more wave energy to 

 enter the area behind the breakwaters. The ratio of gap width to wave length 

 can significantly affect the distribution of wave height in the lee (Dally and 

 Pope 1986). By increasing the gap-to-wave length ratio, the amount of wave 

 energy penetrating landward of the breakwaters is increased. 



Wave diffraction at a gap can be computed using the numerical shoreline 

 response model GENESIS (Hanson and Kraus 1989b, 1990; Gravens, Kraus, 

 and Hanson 1991). GENESIS calculates diffraction and refraction for random 

 waves and accounts for wave shoaling and breaking. The effect of diffraction 

 on a wave which passes through a gap can also be calculated using diffraction 

 diagrams found in the Shore Protection Manual (SPM) (1984); however, these 

 simple diagrams are for monochromatic waves and do not account for wave 

 shoaling or breaking. If the design wave breaks before passing the 

 breakwater, values estimated by the diagrams could be significantly higher 

 than may be expected. 



Dally and Pope (1986) suggest that gaps should be sized according to the 

 desired equilibrium shoreline position opposite each gap. Unless the gap-to- 

 incident wave length ratio is very small, there will be minimal reduction in 

 wave height at the shoreline directly opposite each gap. Without an adequate 

 sediment supply, the shoreline will probably not accrete and may even erode 

 in these areas. Generally, Dally and Pope recommend that gaps should be at 

 least two wave lengths wide relative to those waves that cause average 

 sediment transport. 



The "exposure ratio" is defined as the ratio of gap width to the sum of 

 breakwater length and gap width, or the fraction of the shoreline directly open 

 to waves through the gaps (EM 1 1 10-2-1617). Exposure ratio values for 

 various prototype projects are provided in Table 2 and range from 0.25 to 

 0.66. Projects that are designed to contain a beach fill within fixed 

 boundaries have larger ratios (such as Presque Isle, Pennsylvania). 

 Comparatively, the ratio at Winthrop Beach, Massachusetts, where wide gaps 

 were included to allow for small craft navigation, is 0.25. Comparison of 

 these prototype values provides insight to project design at other locations. 



Structure orientation. The size and shape of the resulting planform can 

 be affected by the breakwater's orientation relative to incident wave angle and 

 orientation of the pre-project shoreline. Shoreline configuration will change 

 relative to the wave diffraction patterns of the incident waves. If incident 

 wave energy is predominantly oblique to the shoreline, orientation of the 



Chapter 2 Functional Design Guidance 



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