erosion of the 75-ft protective berm downdrift of Groin 44 is minimized (40- 

 50 percent of the 6-year renourishment interval). In addition, this akernative has 

 economic and aesthetic advantages in that no additional structures must be placed 

 within the hot spot. 



Alternative 3 (seaward extension of Groin 44 by 100 ft) impounds additional 

 material within the hot spot and significantly reduces impacts to the protective 

 berm. By extending Groin 44 by 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. A 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 experiences more episodes of landward 

 encroachment than for Alternative I (Figure 21). Simulations indicated that the 

 protective berm directly north of Groin 44 will be less than 75 ft wide approxi- 

 mately 40-65 percent of the 6-year renourishment interval. Cumulative berm 

 impacts decrease with distance north of Groin 44. 



The New York District may wish to consider other alternatives than 

 Alternatives 1 and 3 as described above. For example. Alternative 2 (350-ft 

 seaward extension of Groin 44) reduces frequency of landward encroachment of 

 the 75-ft-wide berm to less than 5 percent, but at the expense of increasing 

 recession frequency downdrift. 



Wave conditions more energetic than the 3-year measured wave record are 

 expected to produce impacts greater than those expressed in the design evalua- 

 tions. To estimate potential variability in the recommended alternatives, a vari- 

 ability analysis was performed by incorporating estimated annual extremes in 

 longshore sand transport as obtained from the 3-year wave record. The vari- 

 ability analysis revealed that the protective berm impacts at the hot spot are more 

 severe for years in which the percentage of transport to the south is greater than 

 average. Conversely, greater erosion downdrift of Groin 44 is anticipated for 

 years in which the percentage of transport to the north is greater than average. 

 Alternative 3 is the recommended alternative for protecting the hot spot, although 

 Alternative 1 provides marginally fewer detrimental impacts to the area 

 downdrift of Groin 44. 



Groin Design 



The structural design for the 100-ft extension of Groin 44 (Alternative 3) was 

 developed based on consideration of the forces exerted by depth-limited breaking 

 waves for three storm water levels. For the design. Groin 44 was extended 

 seaward 1 00 ft perpendicular to shore on an eroded beach profile to produce a 

 conservative estimate for maximum breaking wave height for a given water level. 

 The depth-limited breaking wave height for a range of periods typical for 

 Monmouth Beach was calculated from the depth at the toe of the structure on the 

 beach profile and from the nearshore slope. By applying procedures outlined in 

 SPM (1984), stone size was calculated and potential damage estimated for stone 

 smaller than specified. 



Chapter 5 Conclusions 57 



