Variability 



Deterministic evaluation of design alternatives does not account for the 

 intrinsic variability of the environmental forcing, in this case variability in the 

 wave climate. The 3-year wave record is insufficient to define the long-term 

 wave and longshore transport climate, but considered adequate for characterizing 

 responses of the recommended alternatives to measured variation in wave 

 conditions. 



Wave measurements at the Long Branch, New Jersey, DWG show distinct 

 annual and seasonal variability. Annual potential longshore sand transport rates 

 were calculated for the 3-year wave record. Northward-directed transport 

 accounted for 93 percent of the gross transport volume for the 3-year record. 

 From the 1996 wave record, potential transport to the north was slightly greater 

 than average, and potential transport to the south was slightly less than average. 

 Northward-directed transport from the 1996 record accounted for 95 percent of 

 gross transport volume. 



Northward-directed transport during the summer 1997 through spring 1998 

 wave record was of similar magnitude as the 1996 record, but southward-directed 

 transport was greater than average by a factor of 1 .6 (indicating more transport 

 reversals). Northward-directed transport accounts for 89 percent of the gross 

 transport volume. Differences evident in the 1996 and 1997/98 wave records are 

 useful in evaluating the influence of wave climate variations on the recom- 

 mended solutions. Therefore, the 1996 and 1997/98 wave records were repeated 

 through 6-year simulations to estimate variability for Alternatives 1 and 3. 



Figures 32 and 33 present the cumulative impacts to the 75-ft berm for the 

 3-year record and repeated 1996 and 1997/98 wave records for Alternatives 1 and 

 3, respectively. Note that the dominant longshore transport (to the north) during 

 1996 increases impacts to the protective berm downdrift of Groin 44. Con- 

 versely, transport reversals (to the south) during 1997/98 cause increased impacts 

 to the protective berm within the hot spot. When wave conditions cause signifi- 

 cant sediment transport reversals (1997/98), the recommended alternatives differ 

 little in regard to protection of the hot spot. When wave conditions generate 

 persistent transport to the north, erosion downdrift of Groin 44 is less severe for 

 Alternative 1 . Alternative 3 provides more protection to the hot spot, whereas 

 Alternative 1 causes less detrimental impacts to the area downdrift of Groin 44. 



The variability analysis presents responses of the recommended alternatives 

 to plausible extremes of the 3-year wave record. The 3-year record is insufficient 

 to quantify the full variability of the wave climate at Monmouth Beach. How- 

 ever, the extremes in the transport rate from the available wave data provide 

 guidance on the potential variation and qualitative response of the candidate 

 alternatives to characteristics of the transport rate. The extremes in annual trans- 

 port rate occurring in the 3-year record were cycled for 6 years to define the 

 variable shoreline responses, ft is unlikely that biases in the longshore sand 

 transport rate selected for the variability analysis would reoccur for 6 consecutive 

 years. 



34 Chapter 3 Functional Design 



