which expresses the balance between the gross longshore sediment transport rate M
brought to the inlet annually and the tidal prism. Inlets with high ratios (r > 150)
bypass sand predominantly through tidal flushing, whereas inlets with low values (r
< 50) bypass sand predominantly through channel migration and bar complex
formation. Inlets with ratios ranging between 50 and 150 generally develop large ebb
shoals and bypass sediment across the throat of the inlet (tidal flushing) and along the
periphery of the ebb shoal (sand bridge transport). Application of Eq. 3 to
Shinnecock Inlet using the most recent estimate of the gross longshore sediment
transport rate (Williams et al. 1998) and tidal prism (Militello and Kraus 2001)
resulted in an r parameter ranging between 107 and 143, which suggests natural
sediment bypassing at Shinnecock Inlet should occur through a combination of wave-
induced transport and tidal bypassing.
Principal Component Analysis (PCA) was applied to the bathymetry data to
identify spatial patterns of morphology change (Buonaiuto 2003b). Bathymetric data
used in the PCA were collected by the USACE's water-penetrating LIDAR system
called SHOALS. The analysis indicated the locations of the bypassing bars, ebb
shoal, and attachment point, were largely controlled by the position of the navigation
channel. From 1994 through 2000, Shinnecock Inlet experienced a period of growth
accreting almost 2,000,000 m3 of sediment along the ebb shoal proper, bypass bar and
attachment point (Fig. 3). Between 1994 and 1997 the inlet accreted sediment along
the periphery of the ebb shoal, which included the eastern, up-drift flank of the main
channel (eastern ebb shoal lobe), the ebb shoal saddle, the western, down-drift
portion of the ebb shoal, the bypass bar and the attachment point (Fig. 4). Sediment
was also deposited as a linear shoal extending from the tip of the western jetty along
the west side of the channel. During this period the main channel appeared to scour
between 1 and 2 m.
From 1997 to 1998, the Shinnecock Inlet system showed a net accretion of
530,000 m3. Sediment was deposited along the periphery of the western lobe of the
ebb shoal and along the bypass bar. As the channel was naturally redirected toward
the west, the shoal that extended from the tip of the western jetty began to erode.
Additionally the landward face of the bypass bar eroded as the deflected ebb jet drove
sediment reserved in the bypass bar further offshore (Fig. 5). Dredging of the
channel and deposition basin in 1998 re-oriented the entrance toward the south,
promoting growth and seaward advancement of the ebb shoal proper.
As the channel naturally migrated, the westward movement of sediment deposited
on the eastern flank was accelerated by increased wave driven-currents from the east.
Analysis of ten years of directional wave data collected from National Data Buoy
Center (NDBC) station 44025 located 33 nautical miles off the coast of Long Island
(Buonaiuto 2003a) indicated an increase in wave activity from the east quadrant
between the 1997 and 1998 surveys associated with the ENSO (El Nino southern
Oscillation) cycle (Fig. 6).