HomeMy WebLinkAboutGoldsmith's Inlet Modeling StudySCOTT A. RUSSELL [~,~,~ JAMES A. RICHTER, R.A.
SUPERVISOR ~ ENGINEER
TOWN HALL - 53095 MAIN ROAD TOWN OF SOUTHOLD, NEW YORK 11971
Fax. (631)-765-g015 TeL (631)-~J~-CI~J~D
dAN 1 3 2006
OFFICE OF THE ENGINEER
TOWN OF $OUTHOLD
Soulhold Town Cleric
Scott A. Russell, Supervisor
Southold Town Board
Town Hall, 53095 Main Road
Southold, NewYork 11971
Janua~ 10,2006
Re: Goldsmith's Inlet Modeling Study
Dear Scott:
The Town's Consultant, Offshore and Coastal Technologies, has completed the
modeling study related to the potential impacts of reconfigudng the Goldsmith Inlet Jetty.
This study presents the results of hydrodynamic and shoreline response modeling for the
alternatives where the jetty is shortened 33% and 50%. In addition, a storm erosion and
inland inundation assessment is presented to estimate the impacts of shortening the jetty
with regard to the western side of the inlet.
A bdef synopsis or overview of the study has been indicated in Figure 8.10 (Page 52).
This photograph shows the potential beach reduction based on jetty shortening alternatives.
From this photo you can see the potential shoreline changed that would result from a 33%
and 50% reduction of the jetty. This figure also indicates that with a 50% reduction in the
jetty and a 50-year storm, the beach would still be seaward of the recorded 1955 shoreline
that existed pdor to the construction of the jetty. Please keep in mind that all of the existing
land or beach area that has accrued seaward of the 1955 shoreline in front of the homes to
the west is considered State Land and is not private property.
The Decision Matrix shown in the report on Page 53 goes on to indicate that a
reduction in jetty length of both 33% and 50% would allow for a 25% increase in the amount
of material that would bypass the structure on an annual basis, This material would no
longer be lost offshore and would improve conditions and reduce erosion rates on the down
drift side of the jetty. As you can see, this decision matrix indicates that there would be no
substantial increase of matedal bypassing the jetty after a 33% reduction is obtained.
I have reviewed this study with the Department of State and have discussed potential
options and alternates for construction. At this time, due to the results outlined in this recent
study and the existing conditions of the jetty, my off'ice would recommended a Jetty
Reconstruction Project that would include a proposed 33% reduction in jetty length.
Page 1 of 2
Scott A. Russell, Supervisor
Southold Town Board
Re: Goldsmith's Inlet Modeling Study
January 10, 2006
Page 2 of 2
Preliminary construction costs for a project of this magnitude have been discussed
with the Army Corp of Engineers. It has been estimated that a project which includes a 33%
reduction in jetty length, (approx. 125' +/-), and the reconstruction of a new double armor
stone end for the jetty (approx. 100' +/~), and a minimal amount of sand bypassing that is
necessary to accommodate the new work would be approximately five hundred thousand
($ 500,000) dollam.
Before I can begin to vedfy more detailed project costs and potential funding sources,
the Town Board will need to approve this modeling report and select a final alternate for
construction. I believe that this decision should be made in the form of a Town Board
Resolution. If the Board chooses to move forward with this project, I would recommended
that we begin discussions with the Dept of State and DEC to verify all additional project
parameters. This would allow for the creation of a work program that would detail a specific
project. (i.e. 33% or 50% Reduction of the Jetty and the overall amount of sand by-
passing)
If you have any questions or would like to offer more guidance concerning this matter,
please let me know.
CC:
James McMahon, DPW
Pat Finnegan, Town Attorney
Elizabeth Neville, Town Clerk
'nes A. Richter, R.A.
An Assessment of Jetty Shortening Alternatives
Goldsmith Inlet, Bay, and Adjacent Shorelines
30 September 2005
2006
For
Town of Southold
53095 Main Road
Southold, New York 11971
By
Offshore & Coastal Technologies, Incorporated
P.O. Box 1368
Chadds Ford, Pennsylvania 19317
An Assessment of Jetty Shortening Alternatives
Goldsmith Inlet, Bay, and Adjacent Shorelines
Offshore & Coastal Technologies, Incorporated
30 September 2005
Table of Contents
1. Introduction ............................................................................................................................. 1
2. Field Measurements ................................................................................................................ 2
3. Regional Hydrodynamic Modeling ......................................................................................... 6
4. Goldsmith Inlet Modeling ..................................................................................................... 13
5. Goldsmith Inlet Vicinity Hydrodynamic Modeling .............................................................. 16
6. Shoreline Modeling ............................................................................................................... 23
7. Summary of Findings ............................................................................................................ 38
8. Potential Shoreline Flooding and Wave Impacts .................................................................. 40
9. Decision Matrix .................................................................................................................... 53
10. References ......................................................................................................................... 54
11. Appendix: Responses to Review Comments .................................................................... 55
An Assessment of Jetty Shortening Alternatives
Goldsmith Inlet, Bay, and Adjacent Shorelines
Offshore & Coastal Technologies, Incorporated
30 September 2005
Table of Figures
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Figure 3.5
Figure 3.6
Figure 3.7
Figure 3.8
Figure 3.9
Tidal Elevation Measurements at Goldsmith Inlet ....................................................... 3
Tidal Current Measurements at Goldsmith Inlet .......................................................... 4
Centerline Survey, Goldsmith Inlet Channel, 2004 ...................................................... 5
Current Measurements, Mouth of Goldsmith Inlet, April 9, 2004 ............................... 5
Regional ADCIRC Model Grid Coverage ................................................................... 6
Detail of ADCIRC Model Grid in Project Vicinity ...................................................... 7
Bathymctry in Vicinity of Goldsmith Inlet .................................................................. 8
Detail of Bathymetry in the Project Area ..................................................................... 8
Calibration of ADCIRC Model with Mattituck Inlet Tide Measurement Data ............ 9
Verification of ADCIRC Model with Goldsmith Inlet Measurements ...................... 10
Typical ADCIRC Ebb Tide Output ............................................................................ 11
Typical ADCIRC Flood Tide Output ......................................................................... 11
ADCIRC Output Showing Flood Tide Eddy in Project Vicinity ...............................12
Figure 4.1 DYNLET Calibration of Flow Velocity Inside Goldsmith Inlet (Model red,
Measurements blue) .............................................................................................................. 14
Figure 4.2 DYNLET Calibration of Water Level Inside Goldsmith Inlet (Model red,
Measurements blue) .............................................................................................................. 14
Figure 5.1 M2D Model Domain Overlain on ADCIRC Mesh .................................................... 16
Figure 5.2 M2D Model Grid Bathymetry .................................................................................... 17
Figure 5.3 M2D Model Cells and Bathymetry Near Goldsmith Inlet ......................................... 18
Figure 5.4 Typical M2D Results, Flood Tide Condition ............................................................. 18
Figure 5.5 Typical M2D Results, Ebb Tide Condition ................................................................ 19
Figure 5.6 Close-up of Goldsmith Inlet Area for Existing Jetty, Ebb Tide Condition ................ 20
Figure 5.7 Close-up of Goldsmith Inlet Area, Jetty Shortened by 33% (120 tt), Ebb Tide
Condition ............................................................................................................................... 20
Figure 5.8 M2D Results with Wave at Flood Tide Condition ..................................................... 21
Figure 5.9 M2D Results with Wave at Ebb Tide Condition ........................................................ 22
Figure 5.10 M2D Results with Wave (Close-up) at Ebb Tide Condition .................................... 22
Figure 6.1 Representative Sample of Historic Shorelines in GENESIS Grid ............................. 24
Figure 6.2 Goldsmith Inlet Jetty Fillet Infilling Calibration ........................................................ 25
Figure 6.3 GENESIS Beach Calibration, Goldsmith Inlet to McCabe's Beach ........................... 26
Figure 6.4 Comparison of Measured to Modeled Shoreline Position (Typical Position between
Goldsmith Inlet and Bittner Groin) ....................................................................................... 27
Figure 6.5 Comparison of Measured to Modeled Shoreline Position (Typical Position between
Bittner and Lochman Groins) ............................................................................................... 27
ii
Figure 6.6 Comparison of Measured to Modeled Shoreline Position (Typical Position East of
Lockman Gmin) .................................................................................................................... 28
Figure 6.7 Future Shoreline Conditions With Existing Geometry .............................................. 30
Figure 6.8 Difference in Shoreline Position, 2004 to 2014 ......................................................... 31
Figure 6.9 Difference in Shoreline Position With Beach Fill ...................................................... 32
Figure 6.10 Fillet Shoreline Positions After Shortening Jetty by 33% ........................................ 33
Figure 6.11 Change in Fillet Shoreline Positions After Shortening Goldsmith Jetty by 33% .... 33
Figure 6.12 Change in Shoreline Position due to Shortening Goldsmith Jetty by 33% (120 ft). 34
Figure 6.13 Fillet Shoreline Positions After Shortening Jetty by 50% ........................................ 35
Figure 6.14 Change in Fillet Shoreline Positions After Shortening Goldsmith Jetty by 50%.... 36
Figure 6.15 Change in Shoreline Position Due to Shortening Goldsmith Jetty by 50% (180 ft). 37
Figure 8.1 Historic Shorelines (photograph rectification and shoreline position estimates +/- 50
Figure 8.2 Approximate Shoreline Positions Due to Jetty Shortening ........................................ 42
Figure 8.3 Typical Storm Hydrograph ......................................................................................... 44
Figure 8.4 Nearshore Profile Data at Goldsmith Inlet, 1998-1999 .............................................. 45
Figure 8.5 Beach Profile with Offshore Extension ...................................................................... 46
Figure 8.6 Modified Inlet Fillet Profiles, Dune Area ................................................................... 48
Figure 8.7 Increase in Wave Height in Parking Lot Area Due to Jetty Shortening ..................... 49
Figure 8.8 Increase in Wave Height in Dune Area Due to Jetty Shortening ............................... 49
Figure 8.9 Increase in Storm-Induced Shoreline Erosion, Dune Areas ....................................... 50
Figure 8.10 Assumed Reduced Beach Width Shoreline Locations ............................................. 52
iii
An Assessment of Jetty Shortening Alternatives
Goldsmith Inlet, Bay, and Adjacent Shorelines
Offshore & Coastal Technologies, Incorporated
30 September 2005
Preface
This report presents the results of hydrodynamic and shoreline response modeling for the
alternatives where the jetty is shortened 33% and 50%. A storm erosion and inland inundation
assessment is then presented to estimate the impact of shortening the jetty on the updrifl
(western) side of the inlet. This document was prepared for the New York State Department of
State (DOS) with funds provided under Title 11 of the Environmental Protection Fund.
This study was performed by Mr. Daniel Behnke, P.E. and Mr. William Grosskopf, P.E.
of Offshore & Coastal Technologies, Inc. (OCTI). Mr. James Richter, Southold Town Engineer,
was the Town study manager and provided technical input. Mr. Barry Pendergrass, Coastal
Resources Specialist, New York State Department of State, provided technical and report
reviews. Dr. Nicholas Kraus, of the U.S. Army Corps of Engineers (ERDC, CHL, Senior
Scientist Group) provided valuable technical commentary, input and coordination with the
ongoing Federal Section 111 Study at Mattituck Inlet. Dr. Mark R. Byrnes, Applied Coastal
Research and Engineering, Inc., assisted in clarifying aspects of a past sediment budget analysis
of the area. The Southold Town Board, under the direction of Mr. Joshua Y. Horton, provided
meaningful commentary during meetings held to review the progress and results of the work.
An Assessment of Jetty Shortening Alternatives
Goldsmith Inlet, Bay, and Adjacent Shorelines
Offshore & Coastal Technologies, Incorporated
30 September 2005
1. Introduction
The Town of Southold has retained the services of Offshore & Coastal Technologies, Inc. - East
(OCTI) to examine the consequences of shortening the jetty at Goldsmith Inlet by as much as
50%. The evaluation includes an examination of shoreline response (updrift and downdrifi of
the jetty), hathymetric response in the inlet area, and hydrodynamic response in the inlet, pond,
and adjacent ocean areas. The evaluation is performed using a variety of numerical models that
are validated using recent field data.
This report presents the results of hydrodynamic and shoreline response modeling for the
alternatives where the jetty is shortened 33% and 50%. A storm erosion and inland inundation
assessment is then presented to estimate the impact of shortening the jetty on the updrifi
(western) side of the inlet.
2. Field Measurements
Offshore & Coastal Technologies, Inc. has performed various surveys of the Southold coastal
area during the past several years. Field data collection included beach profile surveys and
hydrodynamic measurements in the project area.
Bathymetric data exist for Goldsmith Inlet, the back pond area, and adjacent beach profiles,
based on previous work in fall of 2002 by OCTI for the Coastal and Hydraulics Laboratory, U.S.
Army Corps of Engineers (Dr. Nicholas Kraus). Hydrodynamic measurements were also
collected for CHL including tides at the entrance to Mattituck Inlet (on Long Island Sound),
water levels inside of both Mattituck Inlet and Goldsmith Inlet, and current data using fixed
gauges.
Additional fieldwork was undertaken as part of the present study for use in model calibration and
verification, and to obtain information on the Goldsmith Inlet geometry following dredging of
the inlet in the spring of 2004. The measurements included tide gauges inside and outside of the
inlet, fixed current measurements in the inlet, and a cross-section and centefline survey of the
inlet. Measurements were taken on April 8 and 9, 2004, over two tidal cycles.
Figure 2.1 shows the results of the tidal elevation measurements. It can be seen that the outer
gauge measurement is a typical sinusoidal type tide signal. Inside Goldsmith Inlet, however,
while the peak of the tide is very similar to elevations outside the inlet, the tide does not drop
below about 0.5 feet NAVD (NAVD is the North American Vertical Datum of 1988, which is a
vertical datum for the United States, and is approximately equal to mean tide level in the
Goldsmith Inlet area). The tide does not drop within Goldsmith Inlet because of a flood tide
shoal, or bar, that blocks the mouth of the back pond area, preventing the water from dropping
below the 0.5-foot elevation. When the tide is above this elevation Goldsmith Inlet behaves as a
normal tidal inlet, with flows governed by the water level difference between the inside and
outside areas, as well as tidal flow friction. When the water level drops to the bar elevation, the
flow within the inlet is governed by the amount of water flowing over the bar, similar to weir
flow. In this case, the flow in the inlet channel is similar to stream flow, with flow velocities and
depths governed by the slope of the channel and bed friction.
2
Goldsmith Inlet Water Levels April 8-9, 2004
4
3
99.00
99,25 99.50 99.75 100.00 100.25 100.50 100,75 I01.00
Julian Day
_ Outer Ga~e Inner Gauge
Figure 2.1 Tidal Elevation Measurements at Goldsmith Inlet
Flow velocities within the Goldsmith Inlet channel were made with a fixed, sell:
recording acoustic Doppler current meter. This currant meter was fixed near the channel edge on
a stand, and measured current velocities at a number of locations across the channel. Because
the acoustic current meter could not take measurements when the water depth dropped below
about 0.5 meters (1.5 feet), additional current measurements were made with a handheld
electromagnetic current meter. These measurements were taken in several spots aloug the
channel near the location of the fixed current meter.
Typical mid-depth current measurements are shown in Figure 2.2, along with the outside tidal
elevation measurement for reference. It can be seen that as the tide is rising the currents flowing
into the inlet (negative currents in the plot) reach approximately one meter per second, or about
3.3 feet per second. As the tide drops, the currants exceed one meter per second (black dots in
the plot).
Goldsmith Current Speeds, April 8-9 2004
3
2
1
0
-1
-2
~4
99.00
99.25 99.50 99,75 100.00 100.25 100.50 100,75
Julian Day
~ADP e Sect, 3 a Sect 2 ® SecL 4 -----Water Level Outside
Figure 2,2 Tidal Current Measurements at Goldsmith Inlet
101,00
As noted previously, the inlet channel cross-section and centerline were also surveyed on April 8
and 9, 2004. The inlet chmmel had been partially dredged within the previous month, so it was
desirable to obtain channel geometry in order to calibrate and verify the inlet modeling for a
dredged condition.
The Goldsmith Inlet centerline survey is shown in Figure 2.3. In the plot, the seaward end of the
cham~el emptying into Long Island Sound is at the lelL The elevation of~l.7 feet NAVD is the
natural bottom of the offshore beach where the inlet ends. The channel steps up to 3.5 feet
NAVD on a small delta region fbrmed by inlet outflow. The inlet channel then increased in
elevation until it reaches the flood shoal bar at an elevation of about -0.5 feet NAVD, befbre
dropping into the Goldsmith Inlet pond with an elevation of about -4.5 feet NAVD. The flood
shoal bar is about 200 feet wide, and its elevation of-0.5 feet NAVD is consistent with the
minimum tide elevation measurements inside the inlet.
The inlet channel centerline survey data, taken after a recent dredging project, are not consistent
with the dredge plans provided f:br the project. The plans show a dredged channel bottom
elevation of 4 to-5 tket mean lower low water (MLLW), or 7 to -8 feet NAVD. It was
reported by local officials that a small storm from the east had greatly filled in the dredged
channel between the dredging and the survey. This is supported by the survey data, which more
closely resemble bottom slopes and cross-sections of a natural channel than a deep dredged
channel.
Figure 2.4 shows the mouth of the Goldsmith Inlet channel at the time of the measurements. In
the figure a current ~neasurement at the low tide condition is being made. The shallow flow
under this condition can be seen in the photo. Also in the photo is the small ebb delta formed at
the point where the channel flows into Long Island Sound.
Goldsmith Inlet Centerline Elevation, April 2004
0.0
-0,5
-1.0
-1,5
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
0.0
200.0 400.0 600.0 800.0 1000.0 1200.0 1400.0
Distance Along Channel, ft
Figure 2.3 Centerline Survey, Goldsmith Inlet Channel, 2004
1600.0
Figure 2.4 Current Measurements, Mouth of Goldsmith Inlet, April 9, 2004
Regional Hydrodynamic Modeling
The detailed numerical modeling of the Goldsmith Inlet area that is needed to develop and
understanding of the hydrodynamics and sedimentation in the area requires information on the
larger scale currents and water levels in Long Island Sound. The regional currents and water
levels provide input at the boundaries of more detailed models, and are known as boundary
conditions. The boundary conditions are required at closely spaced intervals in space and time,
and are therefore more detailed than measurements can provide. Therefore, regional numerical
modeling is used to provide the required hydrodynamic detail. The regional model is calibrated
and verified using measurements to insure that the model provides accurate results.
The regional hydrodynamic model ADCIRC is used to provide boundary conditions for more
detailed Goldsmith Inlet area modeling. A model finite element grid that encompasses all of
Long Island Sound and a large portion of the Atlantic Ocean was obtained from the Coastal and
Hydraulics Laboratory, as shown in Figure 3.1. Tidal constituents were applied to the boundary
of this model, and the tides propagated into Long Island Sound.
Figure 3.1 Regional ADCIRC Model Grid Coverage
Figure 3.2 shows more detail of the ADCIRC grid in the area of the project. The figure shows
the east end of Long Island, Long Island Sound, and the southern coast of Connecticut. The
dense black area towards the lefi end of the figure is an area of more detail in the immediate
vicinity of the project.
Figure 3.2 Detail of ADCIRC Model Grid in Project Vicinity
The bathymetry obtained from CHL was refined near the project area using beach profile surveys
conducted in 2002 and offshore bathymetric measurements collected near Goldsmith Inlet in
1999. Offshore contours between Duck Pond Point and Horton Point are shown in Figure 3.3
and 3.4.
The detailed bathymetric data demonstrate several features that influence conditions in the
Goldsmith Inlet area. The first of these is a deep trough just offshore of the beach. This trough
reaches over 6 meters (20 feet) below NAVD off the tip of the Goldsmith Inlet jetty and the
beach between Bittner's Groin and Lochman's Groin. Offshore of the trough are two spits less
than 3 meters (10 feet) NAVD deep. The spits, marked A and B in the figure, appear to result
from beach sediment being carried offshore at Duck Pond Point and the small point to the west
of Goldsmith Inlet by tidal and wave-driven currents. These spits probably reduce the wave
heights of the largest storm waves from the west, thereby partially protecting the beaches in the
project area.
The bathymetry can be seen in more detail in Figure 3~4. The deep trough and holes just seaward
of the beach are apparent, as are the steep slopes of the beach as it drops offinto the deep water.
Figure 3.3 Bathymetry in Vicinity of Goldsmith Inlet
Figure 3.4 Detail of Bathymetry in the Project Area
The ADCIRC model was calibrated using the tidal data collected at Mattituck Inlet entrance in
the fall of 2002. The model was rrm with tidal constituent boundary conditions, and friction
within the model was adjusted to obtain a reasonable agreement with the Mattituck
measurements. The results are shown in Figure 3.5.
There is general agreement between the model results and the measurements in Figure 3.5. The
differences between the two am generally due to weather conditions, such as high and low
pressure systems and easterly and westerly winds, which cause fluctuations in water levels
within Long Island Sound, and which were not accounted for in the model.
Mattituck Tides, beginning 10/5/2002
0.5
0
-0.5
-1.5
0 12 24 36 48 60 72 84 96
Time, Hours GMT
Mattituck ADCIRC
Figure 3.5 Calibration of ADCIRC Model with Mattituck Inlet Tide Measurement Data
After the calibration was complete, the model was run with tidal constituent boundary conditions
for the period of the April 2004 measurements at Goldsmith Inlet. The results are shown in
Figure 3.6. A good agreement was obtained between the outer gage measurements and the
ADC1RC model. The Goldsmith Inlet inner gage measurements are shown in the figure for
comparison, although the ADCIRC model did not include Goldsmith Inlet itself.
The reduced heights of the ADCIRC output at the beginning of the plot are due to the gradual
"spin up" of the model used to prevent model instabilities caused by sudden accelerations within
the model. This spin up takes place over the first 24 hours of the model simulation, and doesn't
affect model results after that time.
Goldsmith Inlet Water Levels Apr 8-9, 2004
4
3
2
-3
98.00 98.50 99.00
99.50
100.00 100~50 101,00
Julian Day
-- 0uter Gauge ~'lnner Gauge --
Figure 3.6 Verification of ADCIRC Model with Goldsmith Inlet Measurements
The ADCIRC output was used primarily as boundary conditions t'or a more detailed local
hydrodynamic model. However, the results can also be used to examine general tidal circulation
patterns that might influence conditions in the Goldsmith Inlet area. Figures 3.7 and 3.8 show
typical ebb tide and flood tide flow patterns in the project vicinity. The vectors show the
direction of tidal flow at a particular time within the simulation. In these figures, the flows are
smooth and consistent over the area.
Figure 3.9 shows a similar figure late in the flood tide cycle, just prior to the ebb tide reversal. In
this figure, it can be seen that there is a large eddy offHorton Point. This eddy has caused a
reversal in the tidal current flow offthe beach in the project area. While the currents are flowing
to the west further out in Long Island Sound, the currents are to the east along the beach. This
effect may contribute to increased sediment transport to the east (and also much higher
variability) above and beyond simply wave-driven transport, along the project beaches.
10
0.00 ftts
Figure 3.7 Typical ADCIRC Ebb Tide Output
3 53 ~s
Figure 3.8 Typical ADCIRC Flood Tide Output
11
Depth. m NAVD
Figure 3.9 ADCIRC Output Showing Flood Tide Eddy in Project Vicinity
12
4. Goldsmith Inlet Modeling
As discussed previously in the Field Measurements section, Goldsmith Inlet behaves like a tidal
inlet at high water levels, with flows governed by water level differences between Long Island
Sound and the inner pond area, while at low tide levels it behaves like a stream, with flows
governed by the flood shoal bar and local channel slopes. Therefore, typical two-dimensional
hydraulic models used for tidal inlets are not applicable for this situation, and a more robust
model must be used.
Based on analysis and modeling by CHL, the inlet hydraulics model DYNLET (CEDAS Version
2.01G, copyright Veri-Tech, Inc.) was chosen for simulating tidal flows in Goldsmith Inlet. The
tidal elevation, current velocity and channel survey data obtained for this project in April 2004
was used to set up and calibrate DYNLET for Goldsmith Inlet.
Figures 4.1 and 4.2 show the model output compared to the field measurements. In Figure 4.1, it
can be seen that the modeled peak flood velocities compare well to the measurements. The ebb
currents do not match well because, as noted previously, the ebb currents at low tide elevations
are governed by flow over the bar at the head of the channel and behave more like river (or
stream flow) with higher velocities and shallower depths than assumed in the model. Because
the water levels are low during this condition, the total contribution to the inlet flow is small, so
the overall influence on channel hydraulics is relatively minor.
Figure 4.2 shows the water surface elevations in the inner pond ama compared to the
measurements. In the figure, the measured data have been repeated over additional tidal cycles
for comparison to the model results. It can be seen that the model successfully reproduces the
non-sinusoidal tide signal within the Goldsmith Pond area.
As noted earlier, it appears that the inlet flow during the low water portions of the ebb tide is
more like river or stream flow than typical inlet hydraulic flow. In this case, the velocity of the
flow is governed by the slope of the channel, and it can be seen in Figure 4.1 that the resulting
velocities are considerably greater than predicted by the hydraulic model. This could have a
direct impact on the Goldsmith Inlet Pond if further dredging of the channel is carried out as
planned.
During the field measurement program it was observed that a portion of the channel near the
flood tide shoal bar had been eroded below its long term position. This was apparent because
beds of mature clams had been eroded, and the edge of the newly eroded channel was still
vertical where the channel had been deepened. It is likely that this erosion had been caused by
the partial channel dredging conducted in the previous month. Dredging the channel would
effectively cause the average slope of the undredged portion of the channel to steepen. If half the
channel were dredged, the channel slope would approximately double, causing the ebb velocities
to increase by about forty percent. This could account for the observed erosion of the channel.
13
2.0
1.0
Figure 4.1 DYNLET Calibration of Flow Velocity Inside Goldsmith Inlet (Model red,
Measurements blue).
1,0
0.8
0.7
0,3
0.2
0.1
-0.1
0 10 20 30 40 50 60 70 80 gO
L Time (hr)
'Figure 4.2 DYNLET Calibration of Water Level Inside Goldsmith Inlet (Model red, Measurements
blue).
14
If the inlet channel were to be dredged back near the flood shoal bar that currently governs the
elevation in the pond area, the channel slopes could be increased significantly over the existing
condition or the previous partial dredged channel condition. It is conceivable that velocities in
the channel during the low water ebb period could double, leading to significant erosion of the
bar. If the increased velocities were to erode a deeper channel through the bar, the pond would
empty to a lower elevation, leading to more extensive exposure of the pond bottom and possible
negative environmental and aesthetic consequences.
15
5. Goldsmith Inlet Vicinity Hydrodynamic Modeling
Detailed hydrodynamics and sedimentation effects were simulated with the model M2D
(Militello et. al., December 2003). This model has the capability of interfacing with the
ADCIRC model to obtain hydraulic boundary conditions, as well as the option of including wave
driven currents, flow inputs, and sedimentation. A grid was set up for the model in the
immediate area of Goldsmith Inlet. The model extends about 2000 meters (1.25 miles) to the
east of Goldsmith Inlet, 1200 meters (0.75 miles) to the west of the inlet, and 2200 meters (1.4
miles) offshore. The outline of the model domain is shown overlain on the ADCIRC grid in
Figure 5.1. In the figure the denser black in the ADCIRC grid shows the smaller ADCIRC mesh
triangles used in the vicinity of the M2D grid to provide greater detail at the boundaries of the
M2D model.
The bathymetry for the M2D Mesh was identical to the refined area of off'shore bathymetry used
in the ADCIRC grid near the project. ']['he bathymetry for the M2D model can be seen in Figure
5.2.
Figure 5.1 M2D Model Domain Overlain on ADCIRC Mesh
16
Figure 5.2 M2D Model Grid Bathymetry
The model was gridded with a cell size of 20 meters (66 feet) for the main portion of the domain.
Near Goldsmith Inlet Jetty and the mouth of Goldsmith Inlet the cell size was decreased to 10
meters (33 feet) lo provide greater resolution in this area of special interest. The model cells near
Goldsmith Inlet are shown in Figure 5.3. In the figure, the jetty and the inlet mouth can be seen
as they are represented in the M2D model.
The M2D model was run with current and water level boundary conditions interpolated from the
regional ADCIRC model for typical tidal conditions based on the November 5, 2002 ADCIRC
model runs. The M2D also included Goldsmith Inlet flow quantities based on the DYNLET
simulations for the same period. Typical flood tide conditions are shown in Figure 5.4. In the
figure, typical tidal velocities immediately offshore of the project area are approximately 0.30
m/s (1.0 fl/s). Just offthe Goldsmith Inlet Jetty, an area of increased velocity can be seen,
caused by flow concentration due to thejetty. This is the general ama of the scour hole seen in
the bathymetric measurements in the area.
Figure 5.5 shows a similar condition for the ebb tide. Again, typical tidal current velocities near
the project area am approximately 0.30 m/s, with increased velocities associated with the tip of
the Goldsmith Inlet jetty. This nearshore area of increased velocity extends to the northeast for
the ebb flow case, corresponding to the area of deep water just offshore of the project area, as
can be seen in the batbymetry in Figure 5.2.
These typical tidal current velocities over the offshore area are sufficient to transport the fine
sand lbund in the area. This supports the hypothesis that the deep trough observed of Yshore of
17
the beaches in the area are being actively scoured by tidal currents, and that beach material could
be lost offshore to tidal currents.
Figure 5.3 M2D Model Cells and Bathymetry Near Goldsmith Inlet
Figure 5.4 Typical M2D Results, Flood Tide Condition
18
Figure 5.5 Typical M2D Results~ Ebb Tide Condition
Figure 5.6 shows the same tidal condition in a close-up view of the Goldsmith Inlet area. In this
figure, a jet can be seen coming from the mouth of Goldsmith Inlet at the peak of the ebb current.
The model indicates that this ebb current is strong enough to transport sand seaward a maximum
of about 60 meters (200 feet). By the time thejet reaches the seaward end of the jetty, the
velocity has dropped to approximately 0.10 m/s (0.3 f/s), too weak to carry sand. This is
consistent with the observations in the field ora small ebb delta formed by the ebb currents;
however, inlet material would not be carried out to the bar east of the end of the jetty under
normal circumstances. An eddy to the east of the jetty can also be seen in the figure.
M2D runs were also made for the 33% (120 feet) and 50% (180 feet)jetty-shortened cases. It
was assumed that jetty fillet dredging took place down to an elevation of-1.5 meters MLLW,
but the majority of the fillet down to the deep offshore trough remains in assumed to be in place
after jetty shortening.
Figure 5.7 shows a case with the jetty shortened by 33%. Because the majority of the fillet is left
in place, the tidal and wave driven currents change very little when compared to the existing
condition case, except for a reduction in the currents near the tip of the existing jetty. The eddy
to the east of the jetty during the ebb tide has also moved landward due to the reduction in the
length of the jetty.
The 50% jetty shortening case is very similar to that seen in Figure 5.7, with the eddy moved
even closer to shore due to the shorter jetty.
19
Figure 5.6 Close-up of Goldsmith Inlet Area for Existing Jetty, Ebb Tide Condition
Figure 5.7 Close-up of Goldsmith Inlet Area, Jetty Shortened by 33% (120 ft), Ebb Tide Condition
20
The M2D model was also run with wave generated currents added to the tidal current condition
to examine the relative effects of wave driven currents on beach erosion processes in the project
area. An offshore wave condition with a zero-moment wave height of 0.67 meters (2 feet) from
the west was generated using the wave model STWAVE and propagated over the M2D
bathymetry. M2D then used the wave properties over the model grid to calculate additional
currents due to the wave, in addition to the tidal currents used in the previous case.
Figure 5.8 shows the currents during the flood tide period. Wave-generated cra'rents are only
significant in the nearshore area where waves are breaking. During the flood tide, the wave-
generated current opposes the tidal current. It can be seen that the nearshore current conditions
are much more complex than for the tidal current alone, with nearshore currents flowing to the
east while the tidal currents flow to the west.
Figure 5.8 M2D Results with Wave at Flood Tide Condition
For the ebb tide condition, shown in Figures 5.9 and 5.10, the wave driven currents are in the
same direction as the tidal currents. In this case, there are very strong nearshore currents,
exceeding 0.40 m/s (1.3 fps) in places. East of Goldsmith Inlet along the shoreline there are
irregular areas of increased current. These are due to the model cells causing steps in the
shoreline, and are an artifact of the model.
Figure 5.10 shows the Goldsmith Inlet ebb jet being displaced eastward by the wave-driven
currents, as compared to the jet going straight ()ut for the without wave case.
2l
Figure 5.9 M2D Results with Wave at Ebb Tide Condition
Figure 5.10 M2D Results with Wave (Close-up) at Ebb Tide Condition
6. Shoreline Modeling
The response of the shoreline to changes in shoreline structure geometry and beach nourishment
was modeled with the shoreline change model GENESES (The GENEralized model for
Simulating Shoreline change, Hanson and Kraus, 1989). This model uses local wave conditions
to calculate beach sediment transport and changes in shoreline position over time.
A wave time history at the shoreline was calculated from wind measurements at Avery Point,
CT, and depth information from NOS navigation charts of Long Island Sound. Four years of
data (1995 to 1999) at one-hour intervals were used to calculate a time history of offshore waves
using the shallow wave forecasting curves (Automated Coastal Engineering System, 2003). The
offshore waves were transformed to the shoreline using the wave transformation model
STWAVE. A wave spectrum was developed for each offshore wave case and then propagated
into the nearshore area where local bathymetric features strongly affect the wave characteristics.
The four years of wave data are assumed to be representative of the long-term wave conditions,
and were used for model calibration using historic shorelines, and future condition modeling.
Historic shorelines of the area were used for calibrating and verifying the GENESIS model.
Shorelines from 1955, 1964, 1969, 1976, 1980, 1993, 1998, and 2003 were available from
previous studies of the area.
The GENESIS model uses a grid that is set up so that the X-axis is a straight line along the
general alignment of the shoreline and the Y-axis is in the offshore direction. For the present
model the X-axis begins about 800 meters to the West of Goldsmith Inlet, and runs about 4000
meters (2.5 miles) east to the McCabe's Beach area. The shoreline was represented at an
increment (model cell size) of 25 meters (82 feet). All historic shorelines were interpolated so
that a shoreline x-y position was available at each 25-meter cell along the shoreline.
Representative samples of the shorelines are shown in Figure 6.1. The entire set of shorelines is
not shown on the plot for clarity. On the GENESIS grid Goldsmith Inlet is at location 800
meters, Bittner's Groin is at about 1900 meters, and Lockman's Groin is at about 3100 meters.
Based on the shoreline plots and other historical records, the jetty at Goldsmith Inlet was
constructed in 1963 and 1964, Bittner's Groin was built between 1964 and 1968, and Lockman's
Groin was built between 1980 and 1986.
23
Historic Shorelines, Goldsmith Inlet Area
5OO
~- 400 ,
o 300
o 200
._~
~ 100 '
0 500 1000 1500 2000 2500 3000 3500
Distance Alongshore, (m)
1998 1976 1969 1964 ...... 1955i
Figure 6.1 Representative Sample of Historic Shorelines in GENESIS Grid
4000
Model Calibration
The model was initiated with the 1955 shoreline, which is the available historical shoreline prior
to the construction of the Goldsmith Jetty. The model was then driven with the wave time
history described earlier and repeated until the year 1976. The jetty was added to the model at
the proper year (early 1964). Reasonable tuning of the model's input wave parameters produced
a good match to the historic shorelines of the western (updri~rt) fillet at the jetty, as shown in
Figure 6.2.
On the updrift side of the inlet, the model calculated an average net longshore transport rate of
19,500 cubic meters per year (25,500 cy/y). This is the average annual infilling rate required to
match the observed historical shorelines, based on the offshore bathymetry and berm height of
the shoreline. This rate of infilling is not consistent with all past estimates ofjetty fillet filling
made in previous studies; however, a rate close to the calculated value is required in order to
match the observed fillet growth. Previous studies may not have benefited from the detailed
bathymetric surveys of the vicinity used in the present study. The bathymetric surveys
accurately define the offshore depths, which govern the volume of fillet material required to
translate the shoreline a given amount seaward. Thus, the net transport rate determined here is
consistent with the input wave conditions, measured shoreline changes and bathymetric changes.
The lack of field wave measurements to verify the input wave conditions introduces a large
24
uncertainty as well. Personal communications with authors of prior sediment budgets fbr the
area (for example, those of the Aubrey Consulting, h~c. report) feel that an updated sediment
budget project that includes the more detailed bathymetric surveys could more accurately
validate the net annual longshore transport rate on the updrift side of the inlet.
Goldsmith Inlet- Genesis Simulation
300
280
260
240
220
200
180
160
140
120
100
200
300 400 500 600 700 800
'--~--Meas. 1976
...... M eas. 1969
'-'~Meas. 1964
--Meas. 1955
'~Mode11964
~'"""~Mode11969 i
~""~"~Mode11976
Distance Along Shoreline, meters
Figure 6.2 Goldsmith Inlet Jetty Fillet lnfilling Calibration
Following the calibration of the fillet area, the entire area to the east of Goldsmith Inlet was
validated using the historic shorelines. The wave and sediment transport parameters developed
in the jetty calibration were held constant, and other model parameters such as groin bypassing
and offshore losses required adjustment to provide a good correlation between the historic
shorelines and the model results. To optimize the model validation, some longshore smoothing
was necessary. It should be noted that the model smoothing affects the results especially in areas
near structures, which cause more model inaccuracy than in other areas. This smoothing can be
seen updrift of the inlet jetty and at the Lockman and Bittner groins, where the sand fillet
produced by the model is rounder and less peaked, making the beaches just to the west of the
structure wider than indicated in the measurements.
The comparison of the measured shorelines and modeled shorelines for the model calibration is
shown in Figure 6.3.
25
Goldsmith Inlet- Genesis Simulation
300
25O
E~ 200
O
,J: 150
o¢ 100
800 1200 1600 2000 2400 2800 3200 3600 4000
Distance Along Shoreline, mete rs
Meas~ 1976 Moas. 1980 Moas, 1993 -- Moas. 2003 ,~-Mode11980 , rvbde11993 Model 2003
Figure 6.3 GENESIS Beach Calibration, Goldsmith Inlet to McOabe's Beach
The observed and modeled shoreline evolution can be compared more clearly by plotting the
distance from the model baseline to the shoreline position at discrete positions along the shore.
This has been done in Figures 6.4 through 6.6 for typical locations between the Goldsmith inlet
jetty and Bittner's Groin, between Bittner's Groin and Lockman's Groin, and east of Lockman's
Groin. For each figure, a typical position away from the immediate effect of groins orjetties was
chosen. It can be seen that although the measured shoreline positions vary over time more than
the modeled shorelines, the general trend over the model time period is reasonable compared to
the measured shorelines.
For each of the locations it can be seen that the most recent measured shorelines have reversed
the trend of erosion, and show accretion, or increased shoreline width. It is not known if this is a
temporary reversal of the type seen elsewhere in the measured shoreline history, or if something
fundamental has changed in the offshore region that has changed the erosion patterns. It is safer
to assume that the shoreline erosion will return to its long-term erosion rate at some point, until
future shoreline measurements show more definitively what the long-term trend will be. This is
the approach taken in the shoreline modeling, which assumes that the long term trends and
erosional processes will continue into the future.
26
Typical Shoreline Change
Goldsmith Inlet to Bittner Groin
125
100
75
25
0
1940 1960 1980 2000 2020 2040
Year
Measuredi
Modeled
Figure 6.4 Comparison of Measured to Modeled Shoreline Position (Typical Position between
Goldsmith Inlet and Bittner Groin).
Typical Shoreline Change
Bittner Groin to Lockman Groin
150
125
100
75
50
25
1940 1960 1980 2000 2020 2040
Year
Measured
Modeled
Figure 6.5 Comparison of Measured to Modeled Shoreline Position (Typical Position between
Bittner and Lochman Groins).
2?
Typical Shoreline Change
East of Lockman Groin
250 ,
225] -~.~
200
150
125
lO0
1940 1960 1980 2000 2020 2040
Year
~----~ Measuredi
Figure 6.6 Comparison of Measured to Modeled Shoreline Position (Typical Position East of
Lockman Groin).
The sediment transport rates and offshore beach losses calculated by the model are shown in
Table 6.1. The sediment budget results from the model calibration process have been compared
to the results of previous sediment budget calculations pertbrmed for the Town. In general, the
results have many similarities, especially in the losses to the offshore. The major difference is in
longshore transport rates in the eastern end of the area. The model calculations show that the
transport rates drop to the east due to the change in the shoreline orientation as the beach curves,
while the previous sediment budget assumes that they increase.
Another difference is the long-term accumulation of sediment in the Goldsmith Jetty fillet. The
calibration of the model adjusted the jetty bypassing and offshore losses so that beyond about
1976, the fillet volume remains approximately constant. This is consistent with the historical
shorelines.
28
Location
Table 6.1
Sediment Budget Based on Numerical Modeling
Source of Modeling
Determined from Quantities
Shoreline Modeling
(cy/yr)
Western Fillet at Goldsmith Inlet
Longshore Transport
Into Region
Accretion in Area
Loss to Offshore
-25000
6OO0
Based on calibration of
fillet filling rate
Fillet geometry has been
stable since 1970's
Calibration of GENESIS
Longshore Transport 19000 GENESIS model
Out
East of Goldsmith Inlet to Bittner Groin
- 19000 GENESIS model
Longshore Transport
Into Region
Accretion in Area
-4000
Loss to Offshore 4000
Calibration of GENESIS
Calibration of GENESIS
Longshore Transport 19000 GENESIS model
Out
Bittner Groin to Lockman Groin
Longshore Transport -19000 GENESIS model
Into Region
Accretion in Area -3000 Calibration of GENESIS
Loss to Offshore 9000 Calibration of GENESIS
Longshore Transport
Out
13000
GENESIS model
Lockman Groin to McCabe's Beach
Longshore Transport -13000 GENESIS model
Into Region
Accretion in Area
Loss to Offshore
Longshore Transport
Out
-5000
7000
11000
Calibration of GENESIS
Calibration of GENESIS
GENESIS model
29
Future Conditions, Existing Geometry
After completing the calibration of the GENESIS model for the area of interest, the model was
rm~ with existing shoreline structm'e geometry for 30 years to observe the expected shoreline
changes in the future. Based on observations, the majority of the wooden Bittner Groin was
destroyed in 2003. However, it is believed that the land on which the house is located is still
protected by bulkheads and revetments. Therefore, the Bittner Groin in the model was reduced
in length by t0 meters, to represent the loss of the wooden portion of the groin. The remainder
of the groin in the model was left to represent the protection to the shoreline provided by the
bulkheads and revetments around the house.
The results can be seen in Figure 6.7. The figure shows the model shoreline l¥om the Goldsmith
Inlet Jetty at station 800 to McCabe's Beach to the east at station 4000. Again, the model
smooths the accumulation of sediment to the west of the groins, especially Bittner's Groin,
spreading the material further to the west, but the model properly reproduces the average
shoreline change between the groins.
Goldsmith Inlet - Genesis Simulation
300
250
200
150
100
60
0
800 1200 1600 2000 2400 2800 3200 3600 4000
Distance Along Shoreline, meters
fVeas. 1993 Meas. 2003 ........ Model 2014 ~ Model 2024 Model 2034
Figure 6.7 Future Shoreline Conditions With Existing Geometry
It is difficult to estimate shoreline erosion rates from Figure 6.7. Therefore, the data from the
GENESIS model has been plotted in Figure 6.8 as the change in shoreline position, with the year
2004 as the beginning point, for ten years of data. This figure shows that over the 10-year period
typical shoreline erosion between the groins ranges between six and seven meters (20 to 23 t~eet,
or about two feet per year).
Goldsmith Inlet - Genesis Simulation
2
0
-6
-8
-10
800 1200 1600 2000 2400 2800 3200 3600 4000
Distance AJong Shoreline, meters
1 Year Change 3 Year ~ 6 Year ~- 10 Year
Figure 6.8 Difference in Shoreline Position, 2004 to 2014
Existing Condition with Beach Fill
The effect of applying a beach fill to the east of Goldsmith Inlet was examined with the
GENESIS model. A fill of 10,000 cubic meters (13,000 cy) was assumed to be applied over a
500 meter (1600 foot) stretch of beach, starting about 300 meters (1000 feet) to the east of the
inleL The fill was placed over a two-month period. This might simulate the placement of
dredged material from inlet dredging of from an inland source.
The results are shown in the shoreline change plot in Figure 6.9. Compared to the prior
alternative of an existing condition without the beach fill (Figure 6.7), it can be seen that the
beach fill has extended the beach seaward by over 2 meters after one year in the area of the
beach fill. After 10 years the beach in the vicinity of the fill is still more than 1.5 meters seaward
than the without beach fill case.
East of the Bittner Groin, the model predicts no significant impact on the beach width over the
10-year simulation period versus the no-fill case. This is likely due to the losses of sand to the
offshore along the area, the groins separating the beach into partially independent segments, and
the small quantity of material placed relative to the quantity of sand transport due to natural
processes.
31
Goldsmith Inlet - Genesis Simulation
8
6
4
2
0
-2
-6
-iB
-10
8O0
1200 1600 2000 2400 2800 3200 3600 4000
Distance Along Shoreline, meters
1 Year Change .... 3 Year ~ 6 Year - 10 Year
Figure 6.9 Difference in Shoreline Position With Beach Fill
33% Reduction in Goldsmith Inlet Jetty Length
The GENESIS model was used to examine the effect of shortening the Goldsmith Jetty by 33%
(37 meters or 120 feet) on the updrift and downdrift beaches. For the simulation of the updrifl
beach, no fillet dredging was assumed, in order to determine the shape and position of the
equilibrium beach with the shortened jetty.
The results are shown in Figure 6.10. It can be seen that the beach near the jetty erodes landward
approximately equal to the amount of the jetty shortening (37 meters). To the west the beach
adjustment becomes less and less, with very little beach width reduction 300 meters (1000 feet,
at station 500) west.
Figure 6.11 shows the change in the shoreline position with time. The large adjustment near the
jetty location (station 800) is due to the artificial rounding off of the fillet in the model, which
will not occur in nature. The maximum reduction near the jetty will be equal to the jetty
shortening length, 37 meters. This figure more accurately shows the reduction of beach width
west of the jetty. At 100 meters (330 feet) west the modeled beach width reduction is
approximately 25 meters (80 feet), while at 200 meters (660 feet) the reduction is about 12
meters (40 feet).
To the east of the jetty, between stations 800 and 900, the model indicates an increase in beach
width due to the jetty shortening. The increase in beach in this area can be seen to peak at about
20 meters (65 feet) in year 3, and then decrease as the excess material from the fillet is
transported out of the area.
32
Goldsmith Inlet
- Genesis Simulation
3OO
250
200
150
100
50
0
0
100 200 300 400 500 600 700 800
Distance Along Shoreline, meters
Figure 6.10 Fillet Shoreline Positions After Shortening Jetty by 33%.
Goldsmith inlet - Genesis Simulation
900
-20 -
-4-0
-60
-80
0 100 200 300 400 500 600 700 800 900
~stance Along Shoreline, meters
........ 1 Year Change ~ 3 Year ..... 6 Year 10 Year
Figure 6.11 Change in Fillet Shoreline Positions After Shortening Goldsmith Jetty by 33%
(Note: The large adjustment near the jetty location (station 800) is due to the artificial rounding off of the
fillet in the model, which will not occur in nature. The maximum reduction near the jetty will be equal to the
jetty shortening length, 37 meters.)
The shoreline movement distances correspond to the landward displacement of the shoreline
(approximateLy the seaward edge of the berm or the landwardmost contour associated with
hydraulically-driven sand transport). The same year-to-year variation in shoreline location and
33
shape will exist after adjustment to the shortened jetty as exists under the present conditions.
Extreme storm conditions may create beach recession in the area in excess of the long-tem~
average shoreline positions predicted here.
After determining the expected shoreline equilibrium shape of the jetty fillet due to jetty
shortening, the shoreline to the east of Goldsmith Inlet was modeled assuming that fillet
dredging will remove most of the excess fillet material before the jetty is shortened. In the 33%
jetty shortening case, approximately 17,000 cubic meters (22,000 cy) of material is assumed to
be removed from the fillet area. This volume corresponds to the shoreline moving landward 37
meters (120 feet) at the jetty, decreasing linearly to no removal 300 meters west of the jetty.
Dredging from the top of the berm down to -1.5 meters (-5 feet) NAVD is assumed. For the
analysis it was assumed that the material dredged from the inlet fillet was not spread on the
beach to the east, but instead disposed of elsewhere off the project area.
It was also assumed in the shortened jetty case that the offshore losses in the fillet area are
reduced to 3500 cubic meters per year from the 5000 cubic meters per year for the existing case.
This reduction is due to the bench at -1.5 meters NAVD assumed in the fillet dredging. This
bench will allow longshore transport to travel along the fillet and around the jetty with fewer
losses down the steep slopes to the tidal current channel.
The results are shown in Figure 6.12. In this case, the beach immediately to the east of
Goldsmith lnlet has slightly slower erosion rates as compared to the existing case, due to the
increased jetty bypassing (reduced offshore losses). This reduced erosion results in 6 meters (20
feet) of erosion after l 0 years near station 1200 fbr the shortened jetty case, as compared to about
8 meters (25 feet) for the existing case. East of Bittner's Groin the erosion rates stay
approximately the same. This case could be rerun using present-day offshore loss rates, which
would probably restore the downdfift erosion to the rates indicated i-hr the no-shortening case.
Goldsmith Inlet- Genesis Simulation
~ 8
E
_o 2
~ 0
O
~ -2
~ -8
o -10
800
~'~ /
t200 1600 2000 2400 2800 3200 3600 4000
Distance Along Shoreline, meters
1 Year Change ~3 Year 6 Year 10 Year
Figure 6.12 Change in Shoreline Position due to Shortening Goldsmith Jetty by 33% (120 ft).
34
50% Reduction in Goldsmith Inlet Jetty Length
An analysis of shortening the Goldsmith Jetty by 50% (55 meters or 180 feet) was carried out in
the same manner as for the 33% reduction case. For the initial condition, no adjustment of the
fillet area shoreline is made, in order to determine the long-term equilibrium shoreline position.
The fillet erodes so that after 10 years the beach width near the jetty has decreased by 55 meters,
with the reduction decreasing to zero at station 450, which is 350 meters (1150 feet) to the west
of the jetty (Figure 6.13). At 100 meters (330 feet) the beach width reduction is approximately
36 meters (120 feet), at 200 meters (660 feet) the reduction is 20 meters (66 feet), and at 300
meters (1000 feet) the reduction is about 5 meters (16 feet).
As for the 33% reduction case, the shoreline movement distances correspond to the landward
displacement of all beach elevation contours, such as Mean Lower Low Water, Mean High
Water, ben'n edge, etc (Figure 6.14). The same year-to-year variation in shoreline location and
shape will exist after adjustment to the shortened jetty as exists under the present conditions.
Extreme storm conditions may create beach recession in the area in excess of the long-term
average shoreline positions predicted here.
Goldsmith Inlet - Genesis Simulation
3OO
25o
200
0
0 1 O0 200 300 400 500 600 700 800 900
Distance Along Shoreline, meters
Existing Fillet -~ lyr ...... 3yr ~6yr lOyr
Figure 6.13 Fillet Shoreline Positions After Shortening Jetty by 50%.
35
Goldsmith Inlet
- Genesis Simulation
40
2O
0
-20
-60
-80
0 1 O0 200 300 400 500 600 700 800 900
Distance Along Shoreline, meters
Figure 6.14 Change in Fillet Shoreline Positions After Shorteuing Goldsmith Jetty by 50%.
After determining the expected fillet shoreline equilibrium shape due to jetty shortening, the
shoreline to the east of Goldsmith Inlet was modeled assuming that fillet dredging will remove
most of the excess fillet material before the jetty is shortened. In the 50% jetty shortening case,
approximately 25,000 cubic meters (32,000 cy) of material is assumed to be removed fi'om the
fillet area. This volume corresponds to the shoreline moving landward 55 meters (180 feet) at
thejetty, decreasing linearly to no removal 350 meters west of the jetty. Dredging from the top
of the berm down to 1.5 meters (-5 feet) NAVD is assumed. For the analysis it was assumed
that the material dredged from the inlet fillet was not spread on the beach to the east, but instead
disposed of elsewhere off the project area.
It was also assumed in the shortened jetty case that the offshore losses in the fillet area are
reduced to 1500 cubic meters per year from the 5000 cubic meters per year for the existing case.
This assumed reduction is due to the bench at 1.5 meters NAVD assumed in the fillet dredging.
This bench will allow longshore transport to travel along the fillet and around the jetty with
fewer losses down the steep slopes to the tidal current channel.
The results are shown in Figure 6.15. In this case, the beach immediately to the east of
Goldsmith Inlet has accreted up to 12 meters after 10 years as compared to very little change in
this area for the existing case, due to the increased jetty bypassing (reduced offshore losses). The
erosion rate between Goldsmith Jetty and Bittner's Groin is approximately 6 meters after 10
years, as compared to about 8 meters (25 feet) fbr the existing case. As compared to the 33%
jetty reduction case, for the 50% jetty reduction case area of maximum erosion is further to the
east and is shorter~ East of Bittner's Groin the erosion rates stay approximately the same.
36
Goldsmith Inlet - Genesis Simulation
16
14
12
10
8
6
4
2
0
-2
-4
~6
-8
-10
800 1200 1600 2000 2400 2800 3200 3600 4000
Distance Along Shoreline, meters
1 Year Change = 3 Year 6 Year 10 Year
Figure 6.15 Change in Shoreline Position Due to Shortening Goldsmith Jetty by 50% (180 ft).
37
7. Summary of Findings
Existing Shoreline Erosion
Hydrodynamic modeling indicates that tidal currents contribute significantly to the formation
and maintenance of a deep channel in front of the project area beaches, and play an important
role in beach erosion. The modeling is supported by measurements of steep beach slopes and the
deep channel just offshore the beaches. The numerical hydrodynamic modeling demonstrates
that tidal and wave-driven currents are sufficiently strong in this area to transport sand, and that
the currents are asymmetrical, with more transport to the east. However, the formation of eddies
under certain wave and tidal conditions (only a representative subset of these have be modeled
here to demonstrate the processes) can cause a significant variability in transport and subsequent
shoreline erosion. Shoreline change modeling shows that wave-driven beach transport is
reduced from west to east due to shoreline angles, requiring offshore losses to explain the
historical beach erosion.
Hydrodynamic modeling of shortened jetty alternatives indicate that eddies which form along
beaches east of the inlet during ebb tide conditions, move closer to the downdrift shorelines for
shortened jetties, potentially increasing the rate of sediment transport and possible shoreline
erosion. The effect, not possible to model in concert with the shoreline change modeling, could
offset the reduced erosion rates due to jetty shortening shown in the shoreline change modeling
west of Bittner's groin.
Goldsmith Inlet Shoaling
The influence of shortening the Goldsmith Inlet jetty on shoaling of Goldsmith Inlet and pond
will depend on the details of dredging the jetty fillet, and future storms in the area in the years
following the shortening. If no dredging of the fillet is undertaken before the jetty is shortened, a
large quantity of material will move eastward over the first one or two years after shortening,
leading to frequent inlet blockages. The amounts and duration of the blockages will depend on
the severity and timing of storms from the west, but blockages will be more frequent and severe
than for the present case. If complete dredging of the jetty fillet removes all fillet material down
to the tidal current channel bottom and back behind the head of the shortened jetty, then the
amount of material bypassing the jetty will remain similar to the present condition. This will
result in inlet shoaling very similar to the present case where the inlet shoals quickly under even
small storm conditions such as those described earlier on Page 3. If the jetty shoal is dredged
down to a reasonably accessible depth of-1.5 meters NAVD (-2 feet MLLW), jetty bypassing
will increase by approximately about fifteen percent (3000 cubic yards per year) for the 33%
shortening case, due to less material being lost offshore. For the 50% shortening case, jetty
bypassing will increase by about twenty percent on average. These increases in bypassing will
most likely result in a minor increase in inlet shoaling compared to the present case. It should be
noted that the Goldsmith Inlet modeling shows that maintaining proper pond water levels will
depend highly on the degree to which the inlet channel and flood shoal are dredged.
38
Shoreline Response to Jetty Shortening:
Four cases were examined using the shoreline change model in this report: no shortening (no
action), no shortening with beach fill, 33% jetty shortening, and 50%jetty shortening. Pertinent
results can be summarized in the following table.
Beach width changes are given Existing plus Beach 33% Jetty 50% Jetty
relative to the no-shortening/nofill Fill East of Inlet Shortening ( 120') Shortening (180')
case 10 years into the future (no beach (no beach
placement) placement)
Accretion given as +
Erosion given as -
Key Assumptions · 13,000 cy placed · No beach fill · No beach fill
over 500m of · Offshore losses · Offshore losses
beach east of inlet, reduced by 1500 reduced by 3500
cy/yr cy/yr
Significantly Affected Longshore 0 fl Approx 1000 I2 Approx 1150 il
Updript Distance
Beach Width just West of Jetty 0 Pt -120 fi -180 ft
Beach Width 330 fl West of Jetty 0 Pt -80 ft -120 ft
Beach Width 660 fi West of Jetty 0 fl -40 Pt -66 il
Beach Width 1000 ft West of Jetty 0 fi 0 Et -16 ft
Beach Width 330 ft East of Jetty I 0 ft + 3 il +39 ft
Beach Width 1300 il East of Jetty +3 il + 3 tt +16 ft
Beach Width 2600 il East of Jetty +7 ft +3 Pt 0 ft
Beach Width 3900 fl East of Jetty 0 ft 0 ft 0 ft
(East of Birtner)
Based on the cases simulated, it is recommended that possible additional alternatives for
simulation might include the placement of sand removed from the updrift fillet onto the beach
east of the inlet for the jetty-shortening cases. This action could provide sand supply to
downdrift beaches to the east. Additional alternatives might also include simulations without a
reduction in offshore losses for the jetty-shortening cases to examine a more conservative
assumption, in case jetty bypassing is not increased due to disruption of sediment bypassing
pathways due to jetty shortening. Finally, lesser shortening of the jetty might be considered
given the extent of the updrift effects associated with the 33% reduction case if the beach width
reductions appear excessive.
As mentioned earlier, model input wave data are calculated using simplified techniques rather
than wave hindcasting that is validated using long term wave measurements. Sediment transport
rates and shoreline changes of the magnitudes calculated in this study could significantly change
if wave climate is varied. The modeling does, however, provide a basis for comparing
approaches, it is recommended that post-construction monitoring be performed of all
potentially-impacted areas to allow the Town to react and adjust expectations as long term
shoreline evolution and storm impacts occur.
39
8. Potential Shoreline Flooding and Wave Impacts
Previous sections of this report examine the consequences of shortening the jetty at Goldsmith
Inlet by as much as 50%. The evaluation has provided estimates of modified shoreline locations
in the fillet area to the west of the jetty due to jetty shortening, as well as other findings.
In lieu of evaluating additional jetty-shortening alternatives, this section presents an evaluation
of potential increases in flooding and wave action in the fillet area west of the inlet associated
with estimated changes in shoreline position if the jetty were to be shortened. This evaluation
uses readily available storm water level and wave information from the Federal Emergency
Management Agency (1998). These storm conditions were developed over the last two decades
by FEMA and its contractors and may not be consistent with estimates that are more
contemporary. FEMA also reports their criteria at widely spaced intervals along the coast of
Long Island Sound, requiring that conditions at Goldsmith Inlet to be estimated from adjacent
locations included in the report. Despite these shortcomings, the FEMA criteria do provide a
means by which the relative effects of varying extents of jetty shortening can be estimated in an
approximate sense.
The shoreline change modeling study examined historical shoreline evolution in the Goldsmith
Inlet area and, using local wave, tide and other physical conditions, estimated long-term changes
to the local shorelines due to shortening the Goldsmith inlet jetty 33% (120') and 50% (180').
Figure 8.1 shows the position of the shoreline in 1955 and 2003 overlain on an aerial photograph
of the Goldsmith Inlet area. The shoreline shown in this and subsequent figures is the mean high
water shoreline interpreted from aerial photography. The 2003 shoreline was developed from
aerial photography for this study and the 1955 shoreline was developed by Fields, Bosma and
Byrnes (1999). Beach profile data indicate that this shoreline corresponds to an elevation of
approximately +5' NAVD '88 (North American Vertical Datum of 1988). The photograph is
shown for illustration only and has been geographically rectified to an accuracy of about 20 feet.
4O
Figure 8.1 Historic ShoreLines (photograph rectification and shoreline position estimates +/- 50 ft).
The previous section indicated that the mean high water shoreline (about +5 feet NAVD) would
recede approximately to the locations shown in Figure 8.2 for the 33% and 50% jetty shortening
cases. Also shown in the figure are the approximate locations of the current jetty head (seaward
end) and base (landward end), along with the location of the head for the two shortened jetty
cases. The receded shoreline positions are those that are expected to occur after the shorelines
have equilibrated after the jetty is shortened. The time required to roach equilibrium will depend
on whether or not dredging of the fillet is carried out at the time o£jetty shortening, the extent of
the dredging, and the details of the wave climate tbllowing the shortening. It could take up to
several years to reach the equilibrium shape.
Due to the various assumptions made in the shoreline change modeling, it is estimated that the
horizontal accuracy of the shoreline positions is approximately 50 feet. It can be seen in Figure
8.2 that the most probable position of the receded shorelines cuts through the area currently
covered by dunes and grasses. This removal of the dune, along with the reduction of beach
width, potentially would increase the likelihood of flooding and wave action landward of the
beach. This potential increase in flooding and wave height is estimated in this study.
41
Figure 8.2 Approximate Shoreline Positions Due to Jetty Shortening.
Note that the 2003, 1955, 33% and 50% shorelines represent the position of the +5 ft NAVD
contotrr)
Wave and Water Level Conditions
In order to estimate the potential increase in water levels or wave heights landward of the shore,
extreme water levels and offshore wave heights are required. Typically, site specific, detailed
water level and wave data are developed for this type of analysis using hindcasting techniques
and verified using field measurements. Because the level of effort required to perform such a
study is beyond the budget and scope of the present study, existing data sources of ston-n wave
and water level conditions were acquired. This approach provides an estimate of the magnitude
of nearshore conditions and the variation of those conditions with sto~xn severity. Therefbre, the
results here are presented as increases in nearshore storm water levels and wave conditions due
to jetty shortening relative to those that coincide with the most recent surveyed shoreline (2003)
condition.
The Federal Emergency Management Agency (FEMA) has conducted Flood Insurance Studies
of the Long Island Sound area, which contain estimates of extreme wave and water levels. The
most recent available study for Suffolk County, New York, including the Town of Southold, is
dated May 4, 1998. Since this source contains the most data available, it will is used as input to
the present analysis.
42
The FEMA data are presented at discrete transects along the Long Island Sound coastline.
FEMA's Transect 46, representing the shoreline from the Town of Riverhead/Town of Southold
corporate limits to Mill road extended, is closest to Goldsmith Inlet. The water level data for
Transect 46 are shown in Table 8.1. The elevations used in the FEMA study are given in
National Geodetic Vertical Datum of 1929 (NGVD29). NGVD has been superseded as a vertical
datum by NAVD, which is used for the present study. The NAVD datum is approximately 1.0
foot above the NGVD datum. Therefore, the elevations in Table 8.1 must be lowered 1.0 foot to
be consistent with the NAVD datum.
Ii Table 8.1FEMA Water Level Data
etum Period, Years 10 50
tillwater Elevation, Ft. NGVD29 8.7 9.78.7
tillwater Elevation, Ft. NAVD88 7.7
100 500
10.6 12.4
9.6 11.4
The FEMA study lists 100-year return period maximum wave crest as +16 feet NGVD (+15 feet
NAVD), but no wave height information for other recurrence levels is provided. This is the
elevation of the wave crest as it begins to break in shallow water offshore of the beach.
In order to estimate offshore wave heights for a range of recurrence periods, wind speeds for
storm events in Long lsland Sound are taken from a recent hindcast study performed for
Asharoken, New York, which is farther to the west on Long Island Sound (U.S. Army Corps of
Engineers, 2004). The wind characteristics are representative of over-water conditions and are
accurate to the degree possible based on validation using marine and coastal wind observations,
but lack continuous high-quality offshore measurements. Offshore wave heights were then
calculated from each of the wind speeds and maximum fetches at the offshore end of the beach
profile using shallow water forecasting curves (U.S. Army Corps of Engineers, 2003). The wind
speeds and resulting wave heights (not crest elevation) are shown in Table 8.2. These wave
values are used as being representative of conditions at the seaward end of the beach profiles.
Wind Speed, mph
Wave Height, ft.
Wave Period, sec.
Table 8.2 - Extreme Wind and Wave Data
Return Period, Years 2 25 50 100
88 98 108
10.7 13.9 17.6
4.0*
7.0*
10
74
7.0
7.9
* h~terpolated from other return period values
8.5
8.9
9.2
Storm hydrographs were created from the estimates of wave height by noting that storms of this
magnitude last about two days and temporally can be approximated by a sin-squared function, as
shown in Figure 8.3. This figure is for the 100-year wave height and water level hydrographs,
and is typical of the hydrographs for other levels of storm recurrence except for the value of the
peak wave and water level.
43
100 Year Hydrographs
20,0
18.0
16.0
14,0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
12 24 36 48
Time, Hours
~Wave Height
Water Level
Figure 8.3 Typical Storm Hydrograph
Local knowledge of flooding of the Goldsmith Inlet area was solicited from the Town of
Southold. This was important because the majority of the physical data collected was not site
specific, and was not considered reliable enough to provide highly accurate estimates of
shoreline flooding and wave action. Local knowledge allows adjustments to be made to the
analysis so it is consistent with observations during storm events. The fbllowing notes were
taken from conversations with the Town of Southold's Engineer:
The Town of Southold Highway Department reported that the Goldsmith Parking Lot itself does not
really flood. Wave action has come over the top of the parking area but the flooding occurs a few
hundred yards south at a low point in the road. This low point is approximately adjacent to the shoal that
has been established at the southerly end of the existing channel. Highway personnel stated that since the
mid seventies (1975/1976), this flooding has occun'ed 12 to 15 times during storm events such as
Northeasters and/or hurricanes. In +/- 50 % of these storm events, the road has been made impassible for
small vehicles and cars. Tracks with a high wheelbase had been more successfi, d navigating the flooded
road.
Based on the local information it appears that flooding of the Goldsmith Parking Lot area is rare.
Beach Profiile Data
Beach profile data in the jetty fillet area are available 1¥om field surveys that were pertbrmed by
OCTI in 1998 and 1999. The relevant profiles include E15,just to the west of the jetty in the
parking lot ama, and El4, approximately 700 feet to the west of the jetty in the dune area. Figure
8.4 shows a comparison of beach profile measurements near Goldsmith Inlet.
Profile data extending further offshore and to greater depths than available in Figure 8.4 are
desirable for the present analysis. Additional data across the shoal seaward of Goldsmith Inlet
44
obtained fi'om offshore surveys conducted by OCTI in 1999 are appended to the nearshore
survey profile data, as shown in Figure 8.5 for profile El4. The offshore portion of profile El5
is similar.
Beach Profiles, Goldsmith Inlet
10
O
,~ -20
-3O
-200 0 200
400 600 800 1000
Distance Offshore, ff
1200
1400
Figure 8.4 Nearshore Profile Data at Goldsmith Inlet, 1998-1999.
45
Beach Profile - Goldsmith Inlet
2o
10
0
> -10
Z
E -20
0
-~ -30
-4O
~5O
-6O
-2000 0 2000 4000 6000 8000 10000 12000 14000
Distance Offshore, ft
'-~-- E14 - Fall 1999 -- Et4 -Spring '98
-- Offshor- e Extension
Figure 8.5 Beach Profile with Offshore Extension
Modeling of Beach Erosion and Inland Hydraulic Conditions
The analysis of wave action and flooding in the fillet area of the Goldsmith Jetty was carried out
using the co~nputer model SBEACll (Storm Induced Beach Change Model, Version 2.0, Coastal
and Hydraulics Laboratory, USACE, Vicksburg, MS). This model uses values of beach profile,
wave and water hydrographs, and other wave information such as period and direction, and
calculates the change in the beach profile and maximum water levels at the shoreline due the
storm waves.
The model was initially run with the hydrographs and the beach profiles presented previously in
this report. For the initial runs, it was assumed that the waves approached directly towards the
beach from Long Island Sound. This is the most conservative assumption, leading to the greatest
nearshore water levels and erosion of the beach.
When the model was executed with the two- and ten-year hydrographs, extensive flooding was
predicted in the parking lot area. There are several possibilities regarding the difference in these
initial model runs and local observations of flooding of the parking lot being rare. The first is
that the wave and water hydrographs obtained from FEMA reports are overly conservative for
the Goldsmith lnlet area. This is entirely possible given that FEMA would like to optimize
public safety by erring on the side of conservatism when estimating the severity of coastal
flooding. The second is that, rather than the waves approaching the beach directly from offshore,
they actually travel obliquely to the beach in thc olfshore region, and then refract as they
approach the beach. This is a reasonable assumption because the largest storm waves are
generated by winds from the northeast, or by waves out of the west where the fetch of Long
Island Sound is longest.
Adopting the second assumption, the model was run with an offshore wave angle of 80 degrees
from shore-normal, almost parallel with the shoreline. This resulted in insignificant flooding in
the two-year event and minor flooding in the ten-year event in the parking lot area, which are
consistent with the local observations.
The existing profiles, shown in Figures 8.4 and 8.5, were then adjusted to represent a beach
profile that would exist after being modified by the removal of the seaward end of the Goldsmith
Inlet Jetty. Because the width of the beach profile in the fillet decreases with distance along the
beach to the west of the jetty, but the shape of the profile is similar along the fillet, modeling the
effect of the 33% (120 feet) and 50% (180 feet) jetty removal requires simulating a range of
modified beach profiles. The profiles El4 and El5 were translated landward by a series of
intervals at intervals from 20 feet to 180 feet. Typical modified profiles are shown in Figure 8.6.
To perform this translation, the profiles were shifted above an elevation of 8 feet NAVD, while
keeping them constant below that elevation. This elevation is assumed to be at the lower end of
possible fillet dredge elevation options, and is below storm induced profile changes and is
probably about the depth of closure in the fillet given that the wave heights are limited by the
offshore shoal. The deep trough in the profile is assumed attributable to tidal currents.
47
Modified Goldsmith Inlet Fillet Profiles
-20
-200 0 200 400 600 800
Distance Offshore, ft
IOO0
--Existing 80' Dredge 120' Dredge - 180' Dredge
Figure 8.6 Modified Inlet Fillet Profiles, Dune Area
The modified profiles were then modeled for each recurrence interval of storm water level and
wave height listed in Table 2, and the results tabulated. As noted previously, the lack of site-
specific criteria and field measurements for validation causes the results to be less reliable than
this type of analysis more typically performed to provide predictions of wave and high water
damage to coastal structures. Therefore, the results of the analysis are presented as relative
changes' in wave heights landward of the existing shoreline, rather than the absolute wave
heights themselves.
The results are presented in Figures 8.7 and 8.8. These plots show the increase in local wave
heights at a reference line 200 feet landward of the existing shoreline, relative to the existing
case. In these figures the modified case results have been divided by the existing case results, so
that a value of 1.0 represents no change from the existing case, and a value of 2.0 represents a
doubling in wave height as compared to the existing case. The values for the 10-year and 2-year
recurrence events have not been plotted because they cause negligible flooding for all of the
profile geometries modeled.
For the parking lot area, the 33% and 50% reductions in jetty length correspond to beach width
reductions of about 120 feet and 180 feet respectively. Farther to the west, in the dune area, the
beach width reduction is less than this because of the taper in the shoreline change, as can be
seen in Figure 8.2. At the eastern end of the dune area, the 33% and 50% jetty length reductions
correspond to beach width reductions of about 100 feet and 160 feet. At the western end of the
dune area, the 33% and 50% jetty length reductions correspond to beach width reductions of
about 40 feet and 60 feet respectively.
In the parking lot area, it can be seen that tbr a decrease in the shoreline widths of 40 and 80 feet,
the increase in wave heights is insignificant as compared to the existing condition. This is
because the geometry of the profile doesn't change significantly in the parking lot area tbr these
48
reductions in beach width, and the wave height is governed by the local water depths. In the
grassy and dune area, the increase in wave heights is more significant for the 40 foot and 80 foot
beach width reductions because the existing beach profile is protected by a small dune, which is
eliminated by the reduction in beach width.
As the reduction in shoreline width reaches 120 and 180 f~et, the increase in wave heights
becomes more significant, with an increase of up to twice the existing condition in the parking
lot area, and up to three times the existing condition in the grassy and dune area.
Increase in Wave Height Due to Dredging,
(Modified Divided by Existing Case, Parking Lot)
2.50
2.00
1,50
0,50
0.00
0 40 80 120 180
Decrease In Shoreline Width, ft
[] 100 Year
B50 Year
B25 Year
Figure 8.7 Increase in Wave Height in Parking Lot Area Due to Jetty Shortening
Increase in Wave Height Due to Dredging,
(Modified Divided by Existing Case, Dune Area)
3.50
3.00
2,50
2,00
1,5o
1.00
0.50
0.00
0 40 80 120 180
Decrease in Shoreline Width, ft
[] 100 Year
B 50 Year
B 25 Year
Figure 8.8 Increase in Wave Height in Dune Area Due to Jetty Shortening.
49
The reduction in beach width due to jetty shortening and fillet dredging will also impact the
distance a major storm will erode the beach and dune areas. For the reduced beach width
scenarios, the beach will start back closer the parking lot and houses along the shoreline, and
may also have greater storm erosion because the profile drops down to a lower elevation
landward of the dune. For the parking lot area, the shoreline erosion behind a given shoreline is
fairly constant because the elevation of the profile remains constant landward of the shoreline.
Figure 8.9 shows the shoreline erosion in the dune areas fbr various storm events and decreased
shoreline widths, in general, the erosion in the two-year event is approximately 20 feet; in
the 10-year events, it is roughly 40 to 60 feet; and in the 50-year storms the erosion is 60 to
100 feet. This erosion distance is added to the assumed decrease in shoreline width to give the
total distance the +5 foot contour will move landward during a given stom~ intensity for an
assumed reduction in shoreline width.
· 4- '
Storm Erosion of 5 Contour
"~ :
._.q ,2,0 25O
200 Fl50 Year
O61
[] 0¥ear
'; [ 400 []2 Year
~ 5o [~
0 40 80 120 180
Width of Shoreline Dredging, ff
Figure 8.9 Increase in Storm-Induced Shoreline Erosion, Dune Areas.
It can be seen in Figure 8.4 that the elevation at the landward end of transect El4 is slightly
above +10 feet NAVD. Based on the distance t¥om the shoreline, this is near the seaward side of
the house closest to the transect line. This is slightly greater than the FEMA 100-year still water
elevation. The SBEACH analysis indicated that wave set-up would increase the still water
elevation by about one foot during the 100-year storm. Therefore, if the elevations at the
landward end of transect El4 is representative of the ground elevations near the homes along the
shoreline, the water depth during the 100-year storm at the homes will be from one to two feet.
According to FEMA guidelines, still water depths of three feet or greater are required to support
waves large enough to damage structures. Therefore, it appears that structural damage will not
occur in the lO0-year ston'n for the existing condition or jetty shortening conditions.
Other types of damage to the homes that might occur due to major storms may include water
damage due to wave crests exceeding the elevation of the building ground floors, and wind-
50
borne spray entering the structure around door and window openings. This type of damage
would likely increase for the jetty shortening cases because the shoreline would be closer to the
structures, with less distance for wave runup to dissipate and less distance for the air-borne water
to travel before reaching the houses. For example, for the 100-year storm case, the distance from
the storm recession shoreline to the homes would decrease from approximately 400 feet for the
existing jetty case to about 200 feet for the 50% jetty removal case. The exact consequences of
this decreased distance to the shoreline on wave crest flooding are not possible to calculate given
the uncertainties in the ground elevations in the vicinity of the structures and the unknown first
floor elevations of the structures. Wind-borne spray effects are even mom difficult to quantify
due to a lack of established procedures for calculating this type of damage.
A qualitative estimate of the increase in damages due to wave crests exceeding the first floor
elevations of the buildings and wind-borne spray are given in Table 8.3. This table is based on
engineering judgment of the potential for damage, not precise predictions based on the results of
modeling or established coastal engineering procedures. In the table, minor damage represents
minor amounts of water on the ground floor flooring, water stains around doors, windows or
other openings, and possible warping or buckling of the ground floor covering. No structural
damage would be expected. Modest damage would consist of more significant water damage,
with standing water on the ground floor and staining and water damage to the lower part of the
ground floor walls. The actual amount of damage for both cases is sensitive to the elevations of
the ground floors, which is not known.
Table 8.3 - Estimate of Damage on West Side of Goldsmith Jetty Due to Jetty Shortening
Storm Return Period Existing Condition 33% Jetty 50% Jetty
Shortening Shortening
10 None None None
25 Unlikely Unlikely Minor
50 Unlikely Minor Modest
100 Minor Modest Modest
In order to help visualize the changes in the shoreline positions due to the reduced jetty lengths,
the approximate locations of the shorelines are overlain on the aerial photograph of the
Goldsmith Jetty area. Figure 8.10 shows the historic and receded shorelines (due to jetty-
shortening) that were presented earlier in Figure 8.2. Also shown on Figure 8.10 is the
approximate location of the +5 foot contour alter the shoreline have adjusted to a jetty length
reduction of 50% (180 feet) and a 50-year storm event.
51
Figure 8.10 Assumed Reduced Beach Width Shoreline Locations.
The storm simulations presented above assume that the large waves are generated by westerly
winds along the greatest fetch of Long Island Soun& Although this modeling approach cannot
simulate the effects of waves from the north and northeast (another storm direction), these waves
could also impact the shoreline after the removal of the seaward end of the jetty because the jetty
would not be blocking as much of the incoming waves. This impact is expected to be relatively
small in major storm events because 1) the jetty elevation is low (approximately +5 feet NAVD)
compared to water elevations during major events, and 2) refraction bends the waves around so
that by the time they reach the end of the jetty they are traveling more directly onshore. Waves
coming directly over the jetty during major storms from the northeast will probably be
significant in the pm-king lot area, but the increase in wave action in the fillet area due to jetty
shortening is expected to be relatively minor.
As mentioned earlier, the inputs for this analysis were taken from publicly-available sources.
Input water levels were taken from FEMA reports where underlying data were developed several
years ago and likely don't include the impacts of more recent (for example, 1990's) storm
periods. Input wave data are calculated using simplified nomograph techniques rather than wave
hindcasting that is validated using long tem~ wave measurements. Finally, the assessment of
flooding potential is calibrated using anecdotal evidence in the parking lot area, rather than
explicit measurements at inland locations there and farther to the west. It is recommended that
post-construction monitoring be performed of all potentially-impacted areas to allow the Town to
react and adjust expectations as long term shoreline evolution and storm impacts occur.
52
9. Decision Matrix
Based on the discussions presented above, the following table summarizes the potential impacts
on the Town of Southold and parts of the study area. The table is meant to aid the Town in
assessing the advantages and disadvantages of each approach to jetty-shortening and associated
dredging. The potential impacts are accompanied by arrows that indicate a positive impact (up
arrow), a negative impact (down arrow), or neutral impact (sideways arrow) on that particular
area or group. There may be additional impacts and other areas affected, and the perception of
positive, negative or neutral impact may be different for other interest groups or areas.
Area or No Action 33% Shorter 50% Shorter Dredging
Interest Jetty Jetty
Group
Pond *-, Continued ,[ May cause ,[ May cause { May erode flood
growth of flood increased growth increased growth shoal due to
shoal of flood shoal of flood shoal increased tidal flow
(reducing easterly (reducing easterly (erode marshes)
transport) transport)
? Maintains
flushing of pond
Inlet $ Dredging ,l, Possible ~L Possible $ Possibly
required to increased dredging increased dredging increases (jets)
maintain sediment loss to
flushing the offshore
Updrift $ Minor inland ,[ Minor inland
Area West flooding and flooding and
of Inlet possible increased possible increased
wave attack in wave attack in
approx. 25-yr approx. 10-yr
event event
Downdrift *-, Continue ~' Approx. 25% T Approx. 25% { Possibly increase
Area East of present transport increase in increase in losses to offshore;
Inlet trends along transport to east transport to east may reduce
beaches to the side side transport to east
east
I Less sand losses "Less sand losses "Lessens
to offshore to offshore likelihood of an
inlet breach
~-* No effect is *-, No effect is elsewhere
calculated east of calculated east of
Bittner Bittner
Town ,, Liability of ? Lessened I Lessened ,[ Occasional
degraded jetty liability with liability with expenditures
repaired jetty repaired jetty
53
10. References
Federal Emergency Management Agency (FEMA), May 1998. Flood Insurance Study, Suf~tblk
County, New York (All Jurisdictions).
Fields, L., Bosma, K.F., and Byrnes, M.R., 1999. Historical Shoreline Change Analysis:
Western Town Line to Horton Point, Southold, New York. Final Report for the Town of
Southold, Aubrey Consulting, Inc., East Falmouth, MA, 92 p. + appendices.
U.S. Army Corps of Engineers, Engineering Research and Development Center, 2003. Coastal
Engineering Manual.
U.S. Army Corps of Engineers, New York District, 2004. Sediment Transport Analysis, Village
of Asharoken, New York (draft under review).
54
Appendix: Responses to Review Comments
Appendix A. Responses to Comments from
New York State Department of State, Coastal Resources Division
(July 30, 2004)
Review Comnlents on the
Interim Report concerning Jetty Shortening Alternatives, Goldsmith Inlet, Bay and Adjacent Shorelines
by Offshore & Coastal Technologies, Inc.
Dated 20 J~une 2004
1. Pg. 2: "...the jetty at Goldsmith Inlet was constructed in 1962 and 1963...". To om' knowledge,
construction on the Goldsmith h~let jetty began in November 1963 and was complete by February 1964.
The 1996 workshop reports provide documentation of this timing, changed
2. Pg. 2: Under model calibration, "The jetty was added at the proper year (1962)." should be revised per
the information above, changed
3. Pg. 2, Model Calibration: The model calculated m~ average net longshore transport rate near the jetty
of 25,500 cy/y. Leatherman et al (1997) estimated this volume at 25,000 cy/y based on casual
observation from a limited set of air photos. The estimate by Aubmy Consulting, lnc., (February 1999)
was based on digital analysis of fill accumulation at the jetty in the air photo records. Aubrey estimated
the net longshore transport rate at 8,000 cy/y. Aubrey also estimated potential error in their methods at
+/- 35%, for a maximum potential net transport rate of 10,800 cy/y. Aubrey states this estimate "...likely
represents a minimum longshore transport rate for the study area since some sediment can be expected to
be transported through and arolmd the end of the jetty." Nonetheless, the difference between 8,000 and
25,500 cy/y is large. Can you describe a possible range of accuracy for tile model? Do the wave climate,
sediment size and dredging history at the inlet correlate well with the OCTI transport estimate? Does the
model allow for variation in transport direction'? This estimate is important fbr decisions regarding inlet
management and investigators must be careful in reporting volume estimates and estimate confidence.
A rate similar to the value calculated by the model is required in order to match the observed
fillet growth. Previons studies may not have benefited from the detailed bathymetric surveys of
the vicinity used in the present study. The bathymetric surveys accurately define the ofl~hore
depths, which govern the volume of fillet material required to translate the shoreline a given
amount seaward. Thus, the net transport rate determined here is consistent with the input wave
conditions, measured shoreline clnanges and balhymetric changes. The lack of field wave
measurements to verify the input wave conditions introduces a large uncertainty as well.
Personal communications with authors of prior sediment budgets R~r the area (for example, those
of the Aubrey Consulting, inc. report) indicate that an updated sediment budget that includes the
more detailed bathymetric surveys could more accurately validate tile net annual longshore
transport rate on the updrifl side of the inlet.
4. Pg. 3: "To optimize the model validation, some longshore smoothing was necessary." This suggests
the model could not accurately characterize some shoreline conditions without adjustment. If so, what
was the deviation(s), where did it occur and what kind of smoothing was necessary? It should be noted
that the model smoothing alTects the results especially in areas near structures, which cause fillet
55
geometries that are often different from model-produced shapes. This smoothing can be seen
updrifl of the inlet jetty and at the Lockman and Bittner groins, where the sand fillet produced by
the model is rounder and less peaked, making the beaches.iust to the west of the structure wider
than indicated in the measurements. The efl~ct of smoothing is very localized and does not
significantly affect trm~sport rates.
Page 3, last sentence: looks like the words "sand the" should be reversed (typo?). changed
5. Pg. 4, fig. 2.3: The scale of the graphic in the report is too small to compare the shorelines or visualize
model accuracy. An additional figure has been added to illustrate the shoreline changes.
6. Pg. 4, second paragraph. "The model is adjusted so that beyond about 1976, the fillet volume remains
approxittmtely constant." Is this to say the operator specifies a model condition that accurately reflects
physical reality, or that the model will not automatically begin bypassing sediment once the jetty becomes
filled? If the model will not mimic hydrologically driven sediment transport it calls into question the
ability of the model to predict outcomes with any confidence. A variety of.jetty bypassing and jetty
permeability factors are available within GENESIS to obtain realistic model calibrations. The amount of
material transported through and over the jetty, as well as otPshore losses, must be adjusted so that the
model matches the observed shoreline shape and rate of fillet growth.
According to Kraus and Morgan (draft) contimling downdrift erosion after the jetty filled was
associated with flood shoal accumulation, including formation of the spit that can be observed at the
present time along the east side of the jetty. The model adjustment does not seem to account ibr this
flood shoal accumulation, nor fk)r periodic dredging. The optimization ol'the model perfom~ance to
measured shorelines implicitly inctndes fuatures and phenomena that can't be explicitly represented in the
model.
7. Pg. 5, Table 2. I. For the column heading "Past Shoreline Change Analysis", a citation should be
provided to the source of this estimate.
In the column headed "Source of Modeling Quantities", the word "calibration" is used several
times. [s the amount of calibration necessary to get the model to reflect historic records
"reasonable"? ls the calibration necessary related to natural geologic variation, weather or
possibly error(s) in initial assumptions (longshore transport rate?)? Yes, the calibration of
modeling parameters is meant to get the model to reproduce historic records to the best degree
possible. The calibration is needed because Genesis does not inclade many shoreline processes
(it is a one-line model) and it is driven by numerically-derived waves that are coupled to shoreline
changes osing calibration parameters.
Do the sediment budget quantities in Table 2.1 reasonably reflect wave conditions, sediment size,
bottom slope, etc.? The sediment budget quantities are determined by the model once its calibration
parameters were ac[justed to best fit the measured shorelines. This implies a reasonable reflection of wave
conditions, sediment size, elc.
Is the 6000 cy/y "Loss to Ofl~hore" at Goldsmith Inlet an accurate estimate? Is this amount likely
to remain constant regardless of inlet management alternatives? i'4o, the loss to the of~khore
would change in response to management ahematives. For example, if the jetly were shortened,
oftkhore transport may change and longshore transport may also change.
56
At Goldsmith Inlet, the "Soume of Modeling Quantities" indicates "Fillet geometry has been
stable since 1970's." While it is true the fillet has been filled since that time, as mentioned above (p. 4
cmm'nent), development of flood shoals including a bypassing spit on the east side of the seaward end of
the jetty continued to form after this data and accumulate sediment. This is an important sediment
transport pathway to understand because dredging management of the inlet anticipates leaving the spit to
encourage bypassing. Agreed.
Is them a way to estimate sediment volumes for cells in Table 2.1 with historic shoreline
recession rotes to see if they correlate well? Aubrey (1999) documents recession rates. Would a
validation like this be useful? The anthors of that report will readily a£,n'ee that their results would likely
change if they had an oppor:unity to update their sediment budget with mom detailed bathymetric data
and otber types of data sources. That type of work should be done in conjunction or in addition to this
work.
P. 7, Existing Condition with Beach Fill. The last paragraph states "East of Bittner Groin, the model
predicts no significant impact on tbe beach width over the 10-year simulation period versus the no-fill
case. Where does the fill go? Is it lost to offshore transport as it migrates east, or is it distributed so
thinly over a large area that the model cannot track it? Offshore losses increase east of Bittner Groin.
P. 9, 33% Reduction in Goldsnfith Inlet Jetty Length. The modeling work was postponed until a dredging
plan from Suffolk County was completed. I understood from the Town and County that dredging was
modified so that the inlet channel would be oriented in a northeasterly direction mitrdcking the natural
conditiou, not parallel to thejetty. Is this condition meaningful in the model? It does not seem to be
represented in Figure 2.T The shomline change modeling is a t-tine model and doesn't represent the 2-D
or 3-D configoration of the channel.
P. 10, second paragraph. The modeled jetty reduction condition included removing excess fillet material
from the system. Ideally, the Department of State would prefer this material to be bypassed to the
downdrift beach to reduce erosion impacts. If additional model rm~s are performed in the future, as the
project is more well defined, the Department recommends modeling placement of dredged material on the
downdrift beach. This is consistent with one of the recormnendations at the end of the report, noted
P. 10, third paragraph. OffShore losses in the fillet ama are reduced from 5000 cu meters/year to 3500 cu
meters/year in the model. What is the basis fbr the 5000 cu meters/year estimate? Is the reduction due to
the assumption some material will be accumulating in the post project jetty fillet? Is die assumption
reasonable? This reduction is due to the bench at 1.5 meters NAVD assumed in the fillet
dredging. This bench will allow longshore transport to travel along the fillet and aronnd the jetty
with fkwer losses down the steep slopes to the tidal current channel. Additional tans can be made
at the option of the town to examine the sensitivity ol'~msnlts to this assmnption, which is reasonable.
P. 10, third paragraph. A "bench" at -1.5 meters mllw is assumed in the model as a post project condition.
Does OCTI have experience with such a forn~ation? Can OCT1 provide any recommendations on the
size/shape of such a feature? According to Kraus and Morgan (draft) a spit formation on the east side of
the jetty serves as a sediment bypassing pathway to the downdrift beach. How would the proposed
"bench" function in combination with this spit? The bench would be due to a limitation on the depth to
which the removal of the fillet can be accomplished. Yes, its presence would actually help to allow
transport to travel alongshore rather than being lost to the of{~shore. This would feed the bypassing spit.
P. I0, third paragraph. Regarding dredging pimps fbr the project, NYS-DEC has previously objected to
dredging below the water line, although they are reviewing this position. The Town and consultant
57
should be aware of this situation mid consider the optimal test condition if any additional model scenarios
are performed, noted
P. 10, third paragraph. A tidal cun'ent charnel is mentioned. Does this reference the inlet channel itself
or an offshore feature extending seaward from the inlet? The "tidal current channel" is a deep channel
running parallel to shore just offshore of the area. Does smwey data support the presence of such a
channel? See figure 3.4 of the revised report. According to Kraus and Morgan (draft) "The area offshore
on both sides of Goldsmith Inlet has a steep gradient, with a slope of approximately 1:10 from the beach
to a depth of approximately 18 ft NAVD88 (to approximately 700 ft offkhore)." Actually the shore-
parallel channel is deeper than -18 fl. No inlet channel is mentioned, but a lobe of sediment extending
northeast from the end of the jetty at a depth of 6 to 10 ft (NAVD88) is apparent in figures.
P. 11, first paragraph. The report suggests the jetty shortened alternative could be rerun with erosion rates
east of the inlet restored to existing rates. For the Town's purposes, this would demonstrate conditions if
extra sediment currently trapped behind the jetty, but exposed to transport if the jetty were shortened,
were removed by dredging. Noted.
P. 14, Summary of Findings. 330 feet west of the inlet is the location of maximum recession under the
jetty shortening scenarios modeled in the results table. Are these estimates the maximmn recession from
model results, or is there location to the east or west with slightly greater recession? From model results.
P. 14, Summary of Findings. Can the shorelines of recession under the various scenarios be provided to
the Town as a GIS product that could be imported into a map of the area to determine the relationship
between model results, property lines and structures? Yes.
C:lDocuments and Setti~gSlbl~enderg!My Documet~ts~Wpdocs~N~ShorelSouthoM~rojc, ctl2OO41Cmts_Octi, wpd
58
New York State Department of State
Coastal Resoumes Division
September 19, 2005
Comments on the
Interim Report Concerning Jetty Shortening Alternatives
Goldsmith Inlet, Bay and Ad,jacent Shorelines
dated 12 May 2005
by Offshore and Coastal Technologies, Inc.
1. pg. 3, third para.: "The inlet channel centerline survey data... It was reported by local officials that a
small storm f?om the east had greatly filled in the dredged channel between the dredging and the
survey..." Since the Town is concerned about long term maintenance of the inlet, and given observations
such as these, is it possible for the report conclusions to say something about how the channel will tend to
behave with the proposed project? Sentence added in the conclusions paragraph of Section 7: ['bis will
result in inlet shoaling very similar to iine present case where the inlet shoals quickly trader even
small storm conditions such a.~ thos;e described earlier on Page 3.
2. pg. 9, second para, on the effect of tidal currents on nearshore sediment transport, ls this effect
reflected in the sediment budget, Table 6.1 ? The sediment budget reflects changes to the shoreline
based on modeling thai is refilled as closely as possible to measured shorelines. The measured
shorelines include ~be effect of' ti&~l curreuts: however, the sedimen~ transport model was nol
capable o£ irtcluding tidal currem-driven transport. 'Ihns. non 4ypical tidal currents, like those
ge~erated by storms, could cause short term and anmml trat/sport rates that are much dill'erent
than those in Table 6.i, which are considered long term averages and reflective ot'the measured
shorelines used in calibrating the model,
3. pg. 15, second para. "This is the general area of the scour hole seen in the bathymetric measurements
in the area." If the jetty is shortened, will this scouring tend to persist? If so, is it Pavomble or det~Smental
fbr sediment bypassing'? The feature may move shoreward when ~he.jetty is shortened if, as
theorized, it is related to the presence of the.jetty. /t is nol appareut whether or how it would
impact sediment bypassiug.
4. pg. 22, second para. "This is the average annual infilling rate required to match the observed historical
shorelines, based on the offshore bathymetry and berm height of the shoreline." The current berm height
is probably substantially higher than in 1976 when the jetty fillet appeared filled. Subtraction of the
additional volume of sand that accumulated in the berm after 1976 could substantially reduce the report
estimated annual average longshore transport. Agreed.
5. pg. 31, second para. "To the west the beach adjustment becomes less and less, witb very little beach
width reduction 300 meters (1000 leer, at station 500) west.*' Where is this location relative to the
boun 'daries of adjacent private properties? See t~igure 8.2.
6. pg~ 32, Genesis Simulation. On page 31, third para., the report notes Genesis artificially rounds offthe
jetty fillet, showing a reduction that could not occur in reality. Is it possible to insert some line or other
indication into the graph on page 32 indicating the limit of actual reduction? Otherwise, despite the
59
disclaimer on page 31, some readers are likely to be misled by the graph. A note has been added to the
caption so readers are aware of tile issue.
7. pg~ 33, first para. "For the analysis it was assumed that the material dredged from the inlet fillet was
not spread on the beach to the east, but instead disposed of elsewhere offthe project area." Could the
report make any generalized statement about whether it would be beneficial (in temps of erosion and
sediment management) to place the sediment on the downdrift beach? Knowledge that this sediment
could help offset futura erosion could help decision makers at a later date with project formulation. A
uote has been added to Section 7 (last paragraph) as 2>flows: Based ou the cases simulated~ it is
recommended that possible additiooal alternatives for simt~lation might ioclude the placement of
sand removed fi'om tile t*pdriti fillet onto the beach east of the inlet fbi' the jetty-shortening cases.
This action could provide sand supply to dowr~dril~ beaches lo the east.
8. pg. 34, second para. Similar to comment #5, above, where do tile adjacent private property lines fall
relative to the shortened beach widths mentioned (at 100, 200 and 300 meters west ofthejetty)? See
Figure
9. pg. 38, second para. "...simalations without a reduction (which we think is reasonable) iu offshore
losses for the jetty shortening cases." What does the phrase "which we think is reasonable" mean? Do
you expect offshore losses to remain the same if the jetty is shortened, or does it mean you think it is a
reasonable option to model, or something else? The simulations with jetty shortening reduced the rate of
o~t~hore losses slightly. As stated in Section 7, "Addition d alternatives might also ioclude
simulations withm~t a reduction irt ofl~horc losses flu' the.jetty-shortening cases to examine a
more conservative assumption." This means it is a ~easonably more conserw~tive option to
model. On page 37 the repo~ states jetty b~assing will increase by about 15% if the je~y is reduced by
I/3, and bypassing would increase by 20% if the jetty is reduced by Va. How does ~is agree with the
paragraph on pg. 38 stating you think it is reasonable no reduction will occur? The esthnates uf 15% and
20% increased bypassing tbr 1/3 and ~5 shortening of the jetty, respectively, are based on a reasonable
estimate ora reduction in of[~hore losses fbr those cases. It is also reasonable to examine the more
conservative assumplio~* in another simulatioa that oiTshore losses may not decrease, thereby
increasingjetty bypassing, which is wh it is mentioned tha~ an additional (more conservative} simulation
might be prudenl.
10. pg. 40, second para. "It can be seen in Figure 8.2 that the most probable position of the receded
shorelines cuts through the area currently covered by dunes and grasses. This removal of the dune..."
There is no basis to assume the dune will be removed, since it fom~ed naturally. It is more likely to
reform in a more landward location. The report should not assume no dune ~vill exist, contrary to the
evidence. If necessary, a replacement dune could be constructed with sand removed from the jetty fillet
to supplement the natural process. Dune I~elbrmatior~ ;md/or replacement dunes are methods that cutdd
help in {briber prolecting inland localions. Man-made dunes may or may not provide protection anti}
fi~lly vegetated. Natural dime relbrmafion will lake a significant time during which the area's exposure
will be more similar to that simulated.
1 l. pg. 41, second para. "...(FEMA) has conducted Flood Insurance Studies...". "The most recent
available study for Suffblk County, New York, including the Town of Southold, is dated May 4, 19987'
Is this an old stody that has been amended, in which ease the original hydrologic information would date
back quite a bit, or is it a new hydrologic study'? This could make a significant difference in confidence
because these FEMA studies are re-dated every time an amendment occurs but the underlying data could
be decades old. Correct. The underlying hydra,die inlbrmation is relatively old. The scope was w'ritten
60
such that publicly-available data are to be ~lsed Ibr this application rather titan generale new hindcasl-
based h/i~irlilatioll tha~ is validated nshig contemporary measurelne[lls.
12, pg. 42. FEMA data am extracted from a hydrologic study transect at the Town border with
Riverhead. I'm fairly certain a FEMA lransect exists near Mattituck also. Depending on the date of the
studies, one or the other might be better as a data source. Do you know whether flood levels at the FEMA
transects lend to increase or decrease to the west? If so, it could have some bearing on how the study
water levels are interpreted. Tile choice was based on tile FEMA study description ol lbe reach that is
represented by/be selected transect. Fbe selection ora second transect would require sirmdatiol~s to be
reran ill o~der to determine how tile changes in conditions would ef'f~.~ct the results of lhe cross-shore
modeling. In general, extreme water levels tend to increase with distance into t.ong Island somtd which
would make erosion and immdation higher than in the present simtdalions.
61