Summary and Conclusions 



The effectiveness of five design alternatives for mitigating tiie hot spot at 

 Monmouth Beach. New Jersey, was evaluated with the numerical shoreline 

 change model GENESIS. The objective of the designs was to maintain a 75-ft 

 protective berm fronting the seawall within and adjacent to the hot spot at 

 Monmouth Beach during a 6-year renourishment interval. A 3-year record of 

 waves measured at Long Branch, New Jersey (near the hot spot), served as the 

 environmental forcing for the 6-year simulations. Performance of each design 

 alternative is summarized in Table 3. 



Two design alternatives are recommended for consideration by the 

 New York District. Alternative 1 (no new structures) allows beach-fill material 

 placed to the south to nourish the hot spot. Simulations using the 3-year mea- 

 sured wave record suggest that the protective berm width within the hot spot will 

 be less than 75 ft approximately for 60-80 percent of the 6-year renourishment 

 interval. The strength of this alternative is that erosion of the 75-ft protective 

 berm downdrift of Groin 44 is minimized (40-50 percent of the 6-year renourish- 

 ment interval). In addition, this alternative has economic and aesthetic advan- 

 tages in that no additional structures must be placed within the hot spot. 



Alternative 3 (extending Groin 44 seaward by 1 00 ft) impounds additional 

 material within the hot spot and reduces impacts to the protective berm. By 

 extending Groin 44 100 ft. erosion of the 75-ft protective berm within the hot 

 spot is reduced to approximately 20 percent of the 6-year renourishment interval. 

 One disadvantage of Alternative 3 is the increased erosion downdrift of Groin 44. 

 For a distance of approximately 3,600 ft north of Groin 44. the 75-ft protective 

 berm is impacted by shoreline erosion. Simulations with the 3-year measured 

 wave record indicate that the protective berni immediately north of Groin 44 will 

 be less than 75 ft in width approximately 40-60 percent of the 6-year 

 renourishment interval. Cumulative berm impacts decrease with distance north 

 of Groin 44. 



Table 3 



Design Alternatives Considered and Associated Impacts 



Alternative 



Description 



Hot-Spot 

 Impacts, % 



Downdrift 

 Impacts, % 



1 



No added structures. Allow beach fill south of 

 hot spot to supply sand. 



60-80 



30-45 



2 



Extend Groin 44 350 ft 



<5 



60-80 



3 



Extend Groin 44 100 ft, 



20 



40-60 



4 



Add four 85-m groins between Groins 44 and 

 45. 



15-70 



30-70 



5 



Add four 30-m groins between Groins 44 and 

 45. 



15-75 



30-55 



Note: Impacts are defined as a percentage of 6-year simulation in wtiich the 75-ft protective berm 

 would be eroded. 



A variability analysis incorporating annual extremes in longshore sand 

 transport as obtained from the 3-year wave record reveals that protective berm 

 impacts at the hot spot are more severe for years in which the percentage of 



36 



Chapter 3 Functional Design 



