simulation. A simulation with groin lengths increased from 100 to 200 ft resulted 

 in a slight increase in benefits to the hot spot, offset by significant increases in 

 recession of the downdrift shoreline. 



Alternative 5 (with 1 00-ft-iong groins) offers less protection to the hot spot 

 than Alternative 3 at significantly greater cost. Therefore. Alternative 5 is not 

 recommended on the basis of the small return on investment compared with other 

 options. 



T-head groins. Although an alternative consisting of T-head groins could 

 not be modeled by the present form of GENESIS, qualitative assumptions can be 

 made about the functioning of T-head groins in the configurations of Alternatives 

 4 and 5. Because T-head structures have a greater capacity to retain sediments 

 within the groin compartment, substituting T-head structures into Alterna- 

 tives 4 and 5 is expected to reduce the impacts to the protective berm within the 

 hot spot. However, the enhanced trapping capacity of the T-head structures is 

 also expected to increase impacts to the 75-ft berm downdrift of Groin 44. 



T-head groins substituted into Alternatives 4 and 5 are expected to offer 

 additional protection to the hot spot as compared with straight groins. However, 

 T-head groins are not a recommended alternative because of increased con- 

 struction costs and detrimental impacts to the downdrift shoreline. 



Recommended alternatives 



Alternatives 1 and 3 are considered the preferred alternatives based on the 

 present analysis. Each alternative offers specific advantages over the other, and 

 each tends to meet the design objectives in different ways. Therefore, this study 

 will present both alternatives and define the advantages of each approach. Fig- 

 ures 30 and 31, respectively, show direct comparisons between Alternatives 1 

 and 3. 



Alternative 1 (no additional structures) minimizes erosion of the 75-ft 

 protective berm downdrift of Groin 44. while allowing erosion of the protective 

 berm there approximately 30-50 percent of the 6-year renourishment interval. 

 Erosion of the protective berm within the hot spot occurs approximately 

 65-80 percent of the 6-year simulation. Alternative 1 has economic and aesthetic 

 advantages because additional structures are not required. 



Alternative 3 (100-ft seaward extension of Groin 44) meets the design 

 objective by decreasing berm impacts within the hot spot to less than 20 percent 

 of the 6-year simulation. Erosion of the protective berm downdrift of Groin 44 

 increases over Alternative 1 and extends approximately 3,200 ft north of 

 Groin 44 because of the increased impoundment of material by the groin exten- 

 sion. The downdrift erosion for this alternative, although greater than Alterna- 

 tive 1 . is less than any other alternative involving structural modifications to the 

 existing conditions. Sand tightening of the existing portion of Groin 44 is not 

 necessary because sediments adjacent to the groin are sheltered from mobilizing 

 forces. 



32 Chapter 3 Functional Design 



