4 Design Criteria 



Objective 



There are two primary indicators of the need for sand bypassing at an inlet: Navigational 

 problems caused by channel shoals and downdrift beach erosion caused by trapping of littoral 

 transport. Engineer Manual 1 1 10-2-1616 (HQUSACE 1991) cautions against trying to design a 

 dual-purpose bypassing system (one that tries to reduce navigation shoaling and alleviate beach 

 changes at the same time). Although channel shoaling and downdrift beach erosion are often 

 related at a particular site, attempting to solve both simultaneously can be difficult because: 



a. The interrelationship between the two problems is often more complex than it appears. 



b. The optimum approach to solving one of the problems can be very different from the 

 optimum approach to solving the other problem. These differences can result in a 

 compromise design that solves neither problem very well. 



At Shinnecock Inlet, navigation channel shoaling has been a concern, except during the mid- 

 1980s when both the navigation channel and ebb shoal experienced material losses. Recently, 

 greater emphasis has been placed on downdrift beach erosion because of the historically receding 

 shoreline, history of barrier breaches, and potential recurring damage to Dune Road. The two 

 primary areas of concern include the beach just west of the west jetty and farther downshore, 

 beyond 2,000 m. 



The erosion hotspot directly west of the west jetty appears to be a localized problem caused 

 primarily by a cutoff of easterly transport by the growth and attachment of the ebb shoal to the 

 downdrift beach. The regional net sediment transport is east to west such that the natural sand 

 bypassing mechanism is from the ebb shoal towards the west. Occasionally, transport reversals 

 occur due to seasonal changes in wave climate, currents, or winds, causing temporary longshore 

 transport to the east. However, as the ebb shoal has developed and attached to the downdrift 

 shore, this easterly directed transport has been unable to reach the areas nearest the west jetty, 

 thus exacerbating the local erosion problem (Moffatt & Nichol 1996). 



Chapter 4 Design Criteria 37 



