designing a bypass system, because the breakdown between major and minor transport directions 

 may be seasonally related. For example, if during a certain period of the year the predominant 

 direction of transport is opposite to the net yearly direction, it may be counterproductive to 

 operate a bypass plant during that time. 



Seasonal effects 



The seasonal sediment transport direction was analyzed at Shinnecock Inlet to determine if 

 certain seasons or months of the year were associated with a predominant transport direction that 

 differed from the annual net west-directed transport. The procedure used the monthly statistics 

 of wave height, period, and direction from the 1976-1993 WIS (Brooks and Brandon 1995) data 

 for Station 78 off the coast of Long Island, NY (this is the same data set used by MNE for the 

 sediment budget calculations). Following a related procedure developed by Bodge, Creed, and 

 Raichle (1996), the offshore wave data were easily transformed from offshore to incipient 

 breaking. From incipient breaking, an idealized potential longshore transport rate was calculated 

 for each month using methodology outlined in the Shore Protection Manual (1984) with 

 arbitrary constants. Because of some calculation simplifications and assumptions, the actual 

 transport quantities are less meaningful than relative monthly transport differences and 

 directions. The analysis indicates that during June and July, the predominant direction of 

 transport is to the east. In May and August, transport is to the west, but the relative rate is so 

 small that the total period from May through August could be considered as months where 

 easterly (or nonwesterly) transport predominates. Conversely, October and September generated 

 the largest transport rates (westerly) for either direction for the entire year. December also 

 produced easterly transport, but the rate was small compared to the west-directed transport 

 occurring in November and January. 



These results suggest that any bypass system (especially a fixed system) probably should not 

 be operated from May through August in any year. This period also coincides with part of the 

 summer tourist season during which beach nourishment may not be desired. If a fixed or semi- 

 fixed plant is used, the period from November through April/May (to avoid tourists and 

 eastward transport) would be optimal for a continuous operation during acceptable weather. 

 Operational constraints (winter waves, etc.) may limit using a floating plant system during the 

 August through May time period. However, if a floating plant system were used on a periodic 

 basis between August and May, downdrift placement from March to May may be preferred over 

 placement during September/October. Material placed in the spring would be initially exposed 

 to an easterly transport during June and July, which would help to slow the westerly movement 

 out of the system. If material were placed in September/October, it would be initially exposed to 

 the largest westerly transport of the year and thus would not remain where placed as long. 

 Moffatt & Nichol (1996) hypothesize that the area where the ebb shoal attaches to the beach on 

 the downdrift side may serve to limit this localized eastward transport (between the western 

 limits of the ebb shoal and the west jetty), thereby essentially storing it (west of the ebb shoal or 

 in the ebb shoal itself) for westward transport after July. If periodic placement were conducted 

 during September and October, the physical processes that cause the largest monthly transport 



44 Chapter 4 Design Criteria 



