up-to-date techniques. To begin, MNE divided the shore near Shinnecock Inlet into three littoral 

 cells-east of inlet, inlet, and west of inlet. They also split their analysis into three time periods 

 based on the different stages of inlet evolution (Table 1 1). MNE examined historical shoreline 

 positions to estimate beach/dune erosion and deposition volumes for each of the littoral cells. 



Table 11 



Sediment Budget Epochs (after Moffatt & Nichol (1996)) 



Period 



Years 



Description 



Epoch I 



1830-1933 



Preinlet 



Epoch HA 



1938-1956 



Postinlet & prejetties 



Epoch MB 



1956-1984 



Postjetties 



Epoch III 



1979-1995 



Increased erosion 



Using wave energy flux calculations, MNE made a thorough analysis of longshore transport 

 rates into and out of each cell balanced by ebb and flood shoal volume changes, offshore/onshore 

 volume changes, and channel shoaling and dredging volume changes. These results showed that 

 the previous estimate of 229,000 mVyear (300,000 ydVyear) overestimated transport by over 

 100 percent. MNE revised sediment transport rates are summarized in Table 12. 



The design bypass rate should consider both longshore transport and ebb and flood shoal and 

 channel volume changes. Ideally, one should identify the shoal and channel volume balances 

 (that material that is deposited and eroded) and compare these with the longshore transport rate 

 to determine the target mechanical bypass rate. To do this, accurate hydrographic surveys of the 

 ebb shoal, flood shoal, and channel over time must be available for analysis, along with dredging 

 records. At Shinnecock Inlet, the only complete ebb shoal surveys were conducted recently 

 (hydrographic and SHOALS surveys, 1996), so only a long-term average ebb shoal volume 

 change rate can be calculated. However, this volume change rate is misleading since it cannot 

 account for probable nonlinear volume changes as the ebb shoal has grown to its current size. 

 Additionally, little, if any data exist for flood shoal growth since the inlet was created, which 

 severely clouds the entire inlet sediment budget picture. Based on the volume change rates 

 shown in Table 12 from MNE, one can see that the rate of ebb shoal growth has decreased 

 between the three epochs presented. Initial ebb shoal growth was relatively high (1 17,000 

 mVyear from 1938 to 1949; see "Volumetric analysis, 1933 to 1949" on page 15), but the overall 

 average ebb shoal growth from 1933 to 1996 (1 10,000 mVyear, see "Volumetric Analysis, 1933- 

 1996" on page 16) suggests that the rate of growth decreased in later years. Probable factors 

 contributing to the decrease in growth in later years include updrift sediment impoundment 

 resulting after jetty construction in 1953 and 1954, natural relocation of the main ebb shoal 

 channel in the mid-1980s, and dredging of the deposition basin in 1990 and 1993. This limited 

 information leaves too much unknown to comfortably estimate the rate of deposition (or erosion) 

 of the ebb and flood shoals and channel. Therefore, the design bypass rate will be based 

 primarily on the longshore transport rate as developed by MNE with some flexibility for 

 uncertainty. 



42 



Chapter 4 Design Criteria 



