HomeMy WebLinkAboutAtt 5C_Inundation_Mapping_June2025
MEMORANDUM
DATE June 30, 2025 JOB NO. 2022-0011-05
TO Eric Ohanian, PE
Project Manager
Tighe & Bond
One University Ave., Ste 100
Westwood, MA 02090
FROM Arden KT Herrin
Direct Phone: (508) 495-6271
aherrin@woodsholegroup.com
Upper Bass River: Inundation & Salinity Mapping
In support of the Massachusetts Department of Ecological Restoration (MA DER) ,project partners, and Tighe & Bond
(T&B), Woods Hole Group conducted a suite of model simulations to gain greater understanding of tidal dynamics, changes
in saltwater penetration in the system, and the effects of changes to two (2) road crossings and bog restoration on the
impacts of storm events following restoration. Restoration activities are expected to occur using a phased approach,
replacing the failing culvert at Weir Road (WR) with a bridge span and topological changes of the relic cranberry bogs and
Hamblin’s Brook section of the uppermost section of Bass River (Figure 1). Downstream of the relic cranberry bogs, the
river crossing at North Dennis Road (NDR) the existing 6-ft by 6- ft (W x H) box culvert is to be replaced by a single span
bridge and the existing roadway to be raised allow for increased tidal flows form Follins Pond into Mill Creek.
Figure 1. Upper Bass River Restoration area of study.
Miss Thatcher’s Pond
Weir Road
N. Dennis Road
Mill Pond
Follins Pond
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Model Domain and Boundary Conditions
Two (2) model grids were developed from the 2024 Environmental Fluid Dynamics Code Upper Bass River eXtended (EFDC-
UBRX) model using the existing bathymetry upstream of Mill Pond (Figure 2) and the Interfluve design elevations upstream
of Mill Pond (Figure 3). For the existing conditions (EC), the EFDC model used the existing hydraulic connections for all
simulations, while the restored elevations, the existing box culvert at NDR, and the clear span bridge for the Weir Road
(WR) only simulation and design elevations upstream of Mill Pond and a clear span bridge at both Weir Road and North
Dennis Road for design condition (DC) simulations. Table 1 lists the boundary condition and grid configurations modeled
for developing shapefiles provided to Tighe and Bond. The Critical Elevation simulation is the storm event that reaches
the low chord of the existing roadway (6 ft-NAVD88) to determine potential impacts due to the North Dennis Road bridge
replacement. A check mark (✓) in the right three (3) columns indicates a combination that was modeled. Time series for
boundary conditions for tidal forcing, Mean High Water Spring (MHWS), Mean High Water (MHW), and Highest Annual
Tide (HAT) are shown on Figure 4 and Figure 5. Time series for each of the storm events are shown on Figure 6.
Table 1. Model configurations and boundary conditions for each simulation.
Simulation AEP Recurrence Interval WSE (ft-NAVD88) Existing NDR + WR WR Only
MHWS N/A Monthly 2.77 ✓ ✓
Highest Annual N/A Annually 3.0 ✓ ✓
Critical Elevation 16.6% 6.0-year 6.0 ✓ ✓
Storm of Record (Carol) 10.5% 9.5-year 6.77 ✓ ✓
10-Year Event 10.0% 10-year 7.0 ✓ ✓ ✓
25- Year Event 4.0% 25-year 8.1 ✓ ✓
Figure 2. Existing elevations for Upper Bass River
N. Dennis Road
Miss Thatcher’s Pond
Mill Pond
Follins Pond
Weir Road
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Figure 3. Design elevations for Upper Bass River
N. Dennis Road
Weir Road
Miss Thatcher’s Pond
Mill Pond
Follins Pond
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Figure 4. Observed spring high tide in Follins Pond (blue).
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prin Tide
22.77 ft-NAVD88
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Figure 5. Observed tidal signal in Follins Pond (blue) and the highest annual tide from the New England Tidal Flood Profiles (orange).
23.0 ft-NAVD88
Salinity Output
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Figure 6. Time series of water levels for low frequency storm events.
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Time hours
ri cal leva on torm of Record P P P P
Inundation Extents, Duration and Salinity Penetration
Instantaneous salinity contours and water levels wereextracted from both the EC and DC tidal simulations at high tide on
the fifteenth day as shown Figure 5, while all other Water Surface Elevation (WSE) maxima for spring and annual tides
and storm events were extracted as the highest observed value at each computation cell over the entire simulation to
account for any potential time delay between the downstream boundary at Follins Pond and each computational cell. To
determine the time spent inundated during the critical elevation simulations for EC and DC at each computational cell
over the entire 3-day simulation, the time each cell had a depth greater than 0.015 m (0.083 ft) was summed such that
cells that were always wet (for example in the ponds) had a time inundated of 72 hours, and cells that were never
inundated (for example in the upland areas near the model domain boundary) had a time inundation of 0 hours. For
salinity mapping, four (4) ranges in Practical Salinity Units (PSU) were specified to illustrate freshwater (0.5 to 5 PSU) an d
sea water (greater than 25 PSU) and two (2) variations in brackish water; 5 to 15 PSU for predominantly freshwater, and
15 to 25 PSU for predominately seawater. Shapefiles for the simulations listed in Table 1 and the instantaneous salinity
contours, and Inundation time are shown in Appendix A. Figure 7 shows the inundation extents for for the monthly spring
tide. Figure 8 shows the inundation extents for both existing and design conditions during the highest annual tide. Figure
9 shows the inundation extents for Hurricane Carol, the Storm of Record (SOR). Figure 10shows the inundation extents
for all three (3) configurations during the 10% AEP event. Figure 11 and Figure 12 show instantaneous salinity contours
at high tide and duration of inundation during the 6 ft-NAVD88 critical storm elevation event respectively.
References
United States Geological Survey. (2021). 2021 USGS Lidar: Central Eastern Massachussetts. Washington, DC: USGS.
USACE. (1988). Tidal Flood Profiles New England Coastline. Concord, MA: New England Division of USACE.
Woods Hole Group. (2024). Upper Bass River Restoration Yarmouth, MA Hydraulic & Hydrologic Study. Bourne, MA:
Woods Hole Group.
Appendix A: Shapefiles provided to Tighe & Bond from Model Simulations
Figure 7. Modeled extents of inundation for existing (turquoise) and design (blue) conditions for a monthly spring tide.
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Figure 8. Modeled extents of inundation for existing (orange) and design (turquoise) conditions for the highest annual tide.
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Figure 9. Modeled extents of inundation for existing (green) and design (turquoise) condition for the critical elevation storm events.
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Figure 10. Modeled extents of inundation for existing (turquoise), Weir Road bridge only (green) and design (yellow) conditions for the
10% AEP storm event.
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Figure 11. Instantaneous salinity contours at high tide for existing (top) and design (bottom) conditions.
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Figure 12. Inuundation duration contours for the critical storm event (6 ft-NAVD88) tide for existing (top) and design (bottom)
conditions.
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