SoMAS logo

SBU logo 

 NYSDOS logo

The Great South Bay Project

Breach at Old Inlet

Gridding -
   Historically, the Smith Point Channel area and extending west into Great South Bay have been locations for barrier island breaches.  Sometimes these have closed quickly on their own while at other times the breaches have lasted for years.  So one of the obvious places to place a breach was in this area and the Old Inlet was a logical place since it already experiences periodic washovers.  The figures to the right show the modifications made to the base-case grid to simulate a developed and approximately stable inlet at this location.  The breach measures 1.3 km across at its narrowest point with a cross-section of about 3600 m2 (39,000 ft2).  The current conditions in the backbay area of Old Inlet are quite shallow, unrealistically so if a breach were to be established in this area.  So as the bathymetry figure shows, we excavated a broad channel northward into the deep part of the Belport Bay to represent a more plausible backbay condition.  For this scenario there were 60465 nodes and 116482 triangular elements.
    As in the base-case we ramped-up the Old Inlet breach simulation from an at-rest condition for 200,000 internal time steps (see discussion for the base-case).


Tidal Range -
The opening at Old Inlet had a substantial effect on the tidal range (the difference between high and low tide) from Seaford (at -8 km) all the way through Moriches Bay.  The upper panel shows the tidal range with the breach opening , Grid12b, while the lower panel shows the change in tidal range relative to current conditions, Grid12a.  Currently, the tidal range for this same area is about 0.3 m but with Old Inlet open the tidal range for the central Bay increased to about 0.5 m throughout the central portion of the Bay, somewhat less immediately west of Howells Point while in Bellport Bay the tidal range increased to about 0.7 m.  The saddle point in relative response just west of Howells Point (at 40 km) perhaps indicates some sort of interaction between the tidal forcing originating at Old Inlet and that from Fire Island Inlet.  Between Seaford and Johns Inlet (-8 to -16 km) there is very little change in tidal range but there is a relative increase again in the channel north of Atlantic Beach and Long Beach. 

Temperature and Salinity -
As for the base-case, the simulation was started from rest with climatolgical temperature and salinity distributions. Temperatures are not forced in the present version of the model through air/sea interaction and so temperatures slowly equilibrate due to river and oceanic effects.  Of course, the climatological salinities are grossly inappropriate for the eastern portion of the Bay so there was an immediate, tidally modulated increase in salinity in Bellport Bay (node 18173).  The salinity increase in the Bay seems to have been largely confined to Bellport Bay although the node south of Sayville (node 37235, yellow) representing the large central portion of the Bay continues to show a gradual although small salinity increase (~0.2/day).  Similarly, the salinities in Moriches Bay did not change too much.  Given the proximity of Moriches Bay to Old Inlet one might have expected more of an impact by a breach at Old Inlet.  A probable explanation is that these is a small westward mean flow through the Smith Point Channel due to the relative setup of Moriches Bay compared to the now open Bellport Bay area.  Under current conditions there is a small westward flow through the Channel driven by the differential sea level set-up between the Bays and this does not seem to have been changed much by the presence of a new Old Inlet opening.  This westward mean flow is slightly larger with the breach than in the base case but less than for the case with an opening at Atlantique.  It is probable that the response would be very sensitive to slightly different inlet morphologies in and around an Old Inlet breach.

  Tidal-Mean Conditions-
      The panels below show tidal-mean conditions from the last 12.4 hours of the ramp-up to stable conditions.  With the breach opening at Old Inlet the mean elevation remains higher than on the shelf but the mean set-up is generally less than in the base except, oddly enough, for the area of the state boat channel south of Robert Moses Bridge and Cedar Island.  In Bellport Bay the mean elevation is lower than before by a couple of centimeters because of the increased access to the ocean.
    In terms of salinity, the breach has increased the mean salinity in Bellport Bay and westward into Patchogue Bay.  Given the evident westward propogation of the salinity, it's possible that we haven't quite reached an equilbrium state but it does seem as though the response in the larger central Bay will be rather muted.
    The overall tidal-mean, or residual currents are similar to the base-case except in the immediate vicinity of Bellport Bay where the eastward mean flow south of Howells Point seems to produce a anti-cyclonic eddy.  The mean flow is still outward in Fire Island Inlet and in at the original three smaller inlets.  The mean flow is westward but small through Smith Point Channel and out of the Old Inlet Breach.  The last panel shows the tidal-mean transport streamlines which makes visualizing the flow patterns easier.  The transport streamline figure again shows the interior residual motions with several but fewer eddies within the Bay as well as a dominance of the eastward flow out of Hempstead Bay.  Compared to the current condition simulation, the presence of the Old Inlet breach has change the flowpattern to the east of Connectquot River.  There are fewer eddies in the central and eastern portion of the Bay with a more organized flow pattern toward the breach which appears to originate in a clockwise eddy south of Connectquot. 
Tidal Mean Elevations



Vertical Sections
A series of sections have been generated to show the vertical and horizontal structure of the salintity and velocities under the Old Inlet breach scenario.  The chart below shows the locations of the vertical sections while the lower panels show vertical sections of the tidally-averaged salinity and nominally, east and north velocity components.  (The velocity components have been rotated 15o  to be aligned along and across the Bay so that the "east" component is actually toward 75oT while the "north" component is toward 345oT.) 






    There are some interesting similarities and differences in the salinity and velocity distributions compared to current conditions in the Bay that arise from the breach at Old Inlet.  In the western section (1) south of Hempstead there has been almost no difference in either the mean salinity or eastward current as a result of the greater flushing and increased access to the ocean represented by the Old Inlet breach.  Within Fire Island Inlet (section 2) the opening at Old Inlet has caused a reduced mean outflow.  In the base case the bulk of the south channel was completely dominated by westward velocities of 8 to 10 cm/sec.  With the presence of the Old Inlet breach these velocities were confined to the upper half of the water column and farther south.  Farther east in section 3 south of Islip and extending southward into the approaches to Fire Island Inlet and across the deep east-west channel, the salinity has decreased along the north shore while that in the deep channel along Fire Island has increased.  This has been accompanied in the southern half of the section by increased eastward flow in the deep channel and along the bottom, and a decreased westward flow in the upper water column.  In the northern half of section 3 the eastward flow of relatively saline water along the bottom has shifted south while the westward surface flow has decreased.  South of Sayville in section 4 the salinity has increased throughout the section by about 0.5 and there is a very clear north to south uppward slant of the isohalines.  Overall, there has been an increase in eastward transport through section 4 although the magnitudes have not and the westward flow is now confined to the surface layer along the north shore.  Despite this increased near surface westward flow in the northern half of the section, possibly associated with a residual eddy (see the streamlines above) there is a more coherent eastward transport through the deeper portion of the section.   Of course the biggest differences relative to the base case occurred in Bellport Bay where salinities increase from ~24 psu to as much as 30 psu.  The presence of the breach also gives rise to greatly increased velocities  and a more complicated current structure.  The section cuts across the Bay just east of the new inlet and this give rise to the band of strong southerly flow leading to the breach.  As the current vectors show in the tidal-mean vector and transport streamline plots above, the breach causes a substantial eastward mean transport past Howells Point that sweeps first northward along the north shore and then southward around the Bay and out the breach.  In the western Moriches section (6) the presence of the Old Inlet breach has caused relatively little change in the mean salinity in the western basin although there is some eddy motion along the north shore just east of the Forge River causing a localized and shallow salinity decrease.  The change in estuarine balance  in Moriches Bay as a result of the the breach has increased vertical shear within the western portion with both stronger eastward near surface and westward near bottom cmean currents.