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TRANSCRIPT
Alternatives Analysis
Beaver Brook Flood Study Pelham, New Hampshire
PREPARED FOR
Town of Pelham
6 Village Green
Pelham, NH 03076
PREPARED BY
101 Walnut Street
PO Box 9151
Watertown, MA 02471
617.924.1770
January 14, 2015
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Contents
Introduction ................................................................................................................................................................................................. 4
Project Description ................................................................................................................................................................................. 4
Study Background .................................................................................................................................................................................. 5
Data Collection ............................................................................................................................................................................................ 6
Bridge and Stream Channel Survey ....................................................................................................................................................... 6
FEMA Studies ........................................................................................................................................................................................ 7
GIS Data ................................................................................................................................................................................................ 7
Previously Completed Studies ............................................................................................................................................................... 7
Other Data Sources ............................................................................................................................................................................... 7
Hydrologic Analysis ..................................................................................................................................................................................... 9
Methods ................................................................................................................................................................................................. 9
Flood Frequency Analysis ............................................................................................................................................................... 10
Historic Flood Flow Evaluation ........................................................................................................................................................ 10
USGS Regional Regression Analysis ............................................................................................................................................. 11
USGS Dimensionless Hydrograph .................................................................................................................................................. 13
Hydraulic Model Update ............................................................................................................................................................................ 14
Digital Terrain Model Creation ............................................................................................................................................................. 14
GeoRAS Model Framework ................................................................................................................................................................. 15
Bridge and Culvert Data ....................................................................................................................................................................... 16
Expansion and Contraction Coefficients ......................................................................................................................................... 16
Boundary Conditions ............................................................................................................................................................................ 17
Flood Flow Profiles and Hydrographs .................................................................................................................................................. 17
Existing Conditions Results ...................................................................................................................................................................... 18
Alternatives Analysis ................................................................................................................................................................................. 21
Willow Street ........................................................................................................................................................................................ 21
Old Bridge Street (Abbott Bridge) ........................................................................................................................................................ 22
Main Street ........................................................................................................................................................................................... 23
Downstream Impacts ........................................................................................................................................................................... 24
Recommendation ................................................................................................................................................................................. 24
Summary ................................................................................................................................................................................................... 26
Appendix A Figures ................................................................................................................................................................................... 27
Appendix B Survey ................................................................................................................................................................................... 54
Appendix C FEMA Documents ................................................................................................................................................................. 55
Appendix D Hydrology .............................................................................................................................................................................. 56
Appendix E HEC-RAS Results ................................................................................................................................................................. 57
Appendix E HEC-RAS Data Disc .............................................................................................................................................................. 58
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Tables
Table 1: Hydrologic Methods ..................................................................................................................................... 9
Table 2: Beaver Brook Hydrologic Analysis Results ................................................................................................... 10
Table 3: Beaver Brook FEMA FIS Hydrologic Summary ............................................................................................. 10
Table 4: Beaver Brook Historic Flood Flows .............................................................................................................. 11
Table 5: Watershed Characteristics for Tributaries to Beaver Brook Study Area ........................................................... 12
Table 6: Tributary Hydrologic Analysis Results .......................................................................................................... 12
Table 7: Tributary FEMA FIS Hydrologic Summary .................................................................................................... 12
Table 8: Bridge and Culvert Summary ...................................................................................................................... 16
Table 9: Downstream Boundary Condition - Rating Curve .......................................................................................... 17
Table 10: Existing Condition Comparison to FEMA Elevations .................................................................................... 18
Table 11: Existing Condition Comparison to FEMA Flow Profiles ................................................................................ 19
Table 12: Existing Condition Strucure Losses ........................................................................................................... 19
Table 13: Willow Street - Bridge Alternatives ............................................................................................................ 22
Table 14: Willow Street – Alternative Model Results .................................................................................................. 22
Table 15: Abbott Bridge - Alternatives ...................................................................................................................... 23
Table 16: Abbott Bridge – Alternatives Model Results ................................................................................................ 23
Table 17: Main Street - Bridge Alternatives ............................................................................................................... 24
Table 18: Abbott Bridge – Alternatives Results .......................................................................................................... 24
Table 19: Preferred Bridge Alternatives .................................................................................................................... 25
Figures
Figure 1: Beaver Brook Watershed .......................................................................................................................... 28
Figure 2: Beaver Brook Study Reach ....................................................................................................................... 29
Figure 3: Historic Flood Flows – USGS Gage (010965852) ........................................................................................ 30
Figure 4: Template Hydrographs ............................................................................................................................. 31
Figure 5: Beaver Brook Flood Hydrographs .............................................................................................................. 32
Figure 6: 100-Year Tributary Flow Hydrographs ........................................................................................................ 33
Figure 7: 100-Year Study Hydrographs .................................................................................................................... 34
Figure 8: Model Framework and DTM ...................................................................................................................... 35
Figure 9: Alternative Analysis Model Framework and DTM ......................................................................................... 36
Figure 10: HEC-RAS Section – Mammoth Road ....................................................................................................... 37
Figure 11: HEC-RAS Section – Castle Hill Road ....................................................................................................... 38
Figure 12: HEC-RAS Section – Tallant Road ............................................................................................................ 39
Figure 13: HEC-RAS Section – Windham Road ........................................................................................................ 40
Figure 14: HEC-RAS Section – Main Street .............................................................................................................. 41
Figure 15: HEC-RAS Section – Old Bridge Street (Abbott Bridge) ............................................................................... 42
Figure 16: HEC-RAS Section – Willow Street ............................................................................................................ 43
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Figure 17: HEC-RAS Section – Unnamed Crossing ................................................................................................... 44
Figure 18: HEC-RAS Profile – Existing Condition ...................................................................................................... 45
Figure 19: Flood Map – Existing to FEMA ................................................................................................................. 46
Figure 20: HEC-RAS Profile – Existing and FEMA Study Flows .................................................................................. 47
Figure 21: HEC-RAS Profile – Existing and Natural Conditions ................................................................................... 48
Figure 22: HEC-RAS Section – Willow Street Alt1c ................................................................................................... 49
Figure 23: HEC-RAS Section – Abbott Bridge Alt2b .................................................................................................. 50
Figure 24: HEC-RAS Section – Main Street Alt1b ...................................................................................................... 51
Figure 25: HEC-RAS Profile – Preferred Alternative .................................................................................................. 52
Figure 26: Flood Map – Existing to Preferred Alternatives .......................................................................................... 53
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1 Introduction
The significant flooding that occurred during the Mother’s Day storm of 2006, followed in quick succession by flooding in 2007, 2008, and 2010, has heightened New
Hampshire citizens’ concerns about flooding. Pelham is no exception. Beaver Brook
has always been flood prone, as low gradient streams of this type are known to be.
But the frequency and depth of flooding has become a problem for residents along the
brook, with many convinced that the problem has grown significantly worse over the
years. Like so many watersheds in southern New Hampshire, the Beaver Brook area
has developed rapidly as Pelham and the surrounding communities have grown. This
development has had many benefits, but increased impervious cover within the
watershed, combined with the construction and modification of culverts and bridges
in the brook, along with a long history of modifications to the floodplain have had
unintended consequences for those living alongside the stream. Additionally,
hydrologists have documented a shift in rainfall intensity in the northeast over the
last several decades, which has led to more frequent and more intense flood events in
New Hampshire. These factors have created a problem that needs to be addressed to
respond to citizens’ concerns and to ensure appropriate protection of private property and public infrastructure while maintaining and enhancing the ecological integrity of
Beaver Brook.
Project Description
This report presents findings from the second phase of a flood study of Beaver Brook
in the Town of Pelham, New Hampshire. VHB conducted the preliminary study in
2013 to identify the causes and potential solutions to flooding associated with Beaver
Brook. This study uses newly collected data and includes a more detailed hydrologic
and hydraulic analysis to assess alternatives for flooding improvement. The following
report summarizes this study and includes an evaluation of existing flooding
conditions, an evaluation of potential mitigating measures, and recommendations for
future action. This updated study includes the following elements:
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Additional data collection and evaluation to refine previous hydrologic and
hydraulic analysis
Hydrologic analysis to develop flood hydrographs for evaluation of existing
and proposed conditions
Updates and refinements to the study area hydraulic model to include new
geometric data, boundary conditions, increased downstream extents and
unsteady flow capacity for a better reflection of existing conditions and to
provide a basis for alternatives analysis
Evaluation and recommendation of preferred alternatives for bridge
improvements at Willow Street, Main Street, and Old Bridge Street to reduce
future flood impacts
Study Background
The scope of this study includes the approximately 11 miles of Beaver Brook in
Pelham, New Hampshire plus an additional 1.5-mile downstream reach through
Dracut, Massachusetts to the Collinsville Dam. Beaver Brook flows approximately
35 miles from north to south with headwaters in Londonderry and ultimately
discharges to the Merrimack River in Lowell, Massachusetts. Pelham is located near
the downstream end of Beaver Brook. Beaver Brook drains a watershed of
approximately 50 square miles at its upstream (northern) boundary with Pelham
which increases to approximately 80 square miles at its downstream limit in Pelham.
Beaver Brook is characterized by a moderate gradient in its upper reach through
Pelham (above Golden Brook confluence) with a relatively narrow floodplain and a
clearly defined channel. Below the Golden Brook confluence, Beaver Brook is
characterized by a flat gradient with a wide floodplain and bordering wetlands.
Seven roads cross the Beaver Brook study reach via bridges, culverts, and an old
access road that now serves as a snowmobile bridge, located just downstream of
Pelham in Dracut, Massachusetts. Figure 1 and Figure 2 respectively show the Beaver
Brook watershed and the study area. Several of these structures are scheduled for
replacement in the next several years. This report provides recommendations for
revised openings for these structures to reduce flood impacts.
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2 Data Collection
VHB collected and reviewed additional data to update and refine the existing
hydrologic and hydraulic models of the study area. This additional data included:
Bridge and stream channel survey
Federal Emergency Management Agency (FEMA) Flood Insurance
Studies (FISs)
Geospatial Information System (GIS) data
Previous studies
Bridge and Stream Channel Survey
To refine the hydraulic model, including channel and bridge geometry, VHB
completed a field survey for the following bridge/culvert crossings of Beaver Brook
within our study area in July of 2014:
Mammoth Road
Castle Hill Road
Tallant Road
Windham Road
Main Street
Old Bridge Street (Abbott Bridge)
Willow Street
Unnamed Crossing, Dracut, MA
As part of the survey, we collected the following information at each location:
Roadway profile
Structure sketches including dimensions and permanent elevations
2 upstream channel cross sections
2 downstream channel cross sections
2 internal channel cross section with the crossing
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Figures showing survey information for each structure are provided in Appendix B.
FEMA Studies
VHB obtained and reviewed the following FEMA FISs for Beaver Brook:
Hillsborough County FIS (33011C), New Hampshire; Effective Date
September 25, 2009
Middlesex County FIS (25017C), Massachusetts; Effective Date July 7, 2014
FEMA information used in VH”’s analysis is provided in Appendix C.
GIS Data
VHB collected hydrologic, topographic and GIS data from various online sources for
use in this study, including:
Light Detection and Ranging (LiDAR) topographic data for the North East
Project (2011/2012), with overall vertical accuracy listed at 0.3 feet
Topographic data from the Merrimack River Hydrologic Unit Code (HUC) 8
LiDAR Project (2012) with an overall vertical accuracy listed at 0.24 feet
Previously Completed Studies
VHB collected and reviewed the following studies and information concerning Beaver
Brook and related structures:
Beaver Brook Flood Study completed by VHB in October 2013
HEC-RAS model for Beaver Brook completed in support of proposed
development near Old Bridge Street and Route 31, provided by The H.L.
Turner Group Inc. (HL Turner)
Photos and Sketches from the Collinsville Dam Phase 1 Report provided by
HL Turner
Other Data Sources
VHB collected and reviewed the following surveys and information concerning
Beaver Brook and related structures
Survey for the Willow Street Bridge provided by Quantum Construction
Consultants, LLC (Quantum) in digital AutoCAD format on July 29, 2014
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Main Street Bridge Design plans dated March 1955 provided by New
Hampshire Department of Transportation (NHDOT)
Annual peak flows from United States Geologic Survey (USGS) flow gage,
titled 010965852 Beaver Brook at North Pelham, NH for the water years 1987
to 2012.
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3 Hydrologic Analysis
VHB updated the hydrologic analysis conducted as part of the previous study to
facilitate development of an unsteady flow hydraulic model. The previous study
evaluated peak flow conditions based on an analysis of river gage records. These
flows were suitable for use with the steady flow hydraulic model used with that
study. This study includes an unsteady flow hydraulic model which facilitates a
dynamic evaluation of the impact of conveyance and flood storage.
Methods
The unsteady flow model requires flow hydrographs (varied flow over time) as
opposed to a single peak flow as the primary hydraulic input. To update the
hydrologic analysis, VHB developed flood hydrographs for a range of return intervals
for the study area using a variety of techniques, including:
Flood frequency analysis of gage records
Evaluation and scaling of historic floods based on USGS gage records
Regional regression analysis of flood flows
Development of watershed hydrographs based on the USGS dimensionless
urban hydrograph
Table 1 summarizes the hydrologic methods used to estimate peak flood flows and
develop flood hydrographs for Beaver Brook and tributary streams in the study area:
Table 1: Hydrologic Methods
River Peak Flood Flow Flood Hydrograph
Beaver Brook Log-Pearson Type III with area-weighted scaling
Historic Flood Flow Evaluation
Tributarys1 USGS Regional Regression Analysis
USGS Dimensionless Hydrograph Method
1 – Golden Brook, Gumpas Road Brook, Tonys Brook, Gumpas Pond Brook, New Meadow Brook.
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Flood Frequency Analysis
VHB conducted a hydrologic analysis to estimate flood flows for multiple return
frequencies for Beaver Brook. This hydrologic analysis was conducted using the
following methods:
Log-Pearson Type III flood frequency analysis
Area weighting method for scaling flows
A USGS river gage is located on Beaver Brook, just upstream from Pelham, allowing
VHB to conduct a flood frequency analysis of gage records to evaluate flood flows in
Beaver Brook. Where gauge data is available for sufficient time periods (>25 years),
this is considered the most accurate way to estimate flood flows. The USGS gage at
North Pelham (010965852) includes a continuous record for the last 26 years. VHB
conducted a log-Pearson Type III flood frequency analysis of these records in
accordance with USGS Bulletin 17B methodology using the United States Army Corps
of Engineer (USACE) software HEC-SSP. VHB then used the watershed area
weighting method to adjust predicted flows to account for increased watershed areas
through Pelham. Table 2 shows the resulting predicted flood flows.
Table 2: Beaver Brook Hydrologic Analysis Results
River Reach Drainage Area (mi2)
Q2 (cfs)
Q10 (cfs)
Q50 (cfs)
Q100 (cfs)
Source
Upper Beaver Brook 54.4 900 2,000 3,200 3,900 Scaled from USGS Gage
Lower Beaver Brook 73.1 1,100 2,300 3,800 4,500 Scaled from USGS Gage
VHB compared predicted flows to the current FEMA FIS. The predicted flows are
approximately 30 percent higher than the FEMA study flows for all recurrence
intervals. The FEMA FIS flood discharges for Beaver Brook were previously
computed in 1978 using regional discharge-frequency equations. Table 3 presents the
flood discharges from the FEMA FIS.
Table 3: Beaver Brook FEMA FIS Hydrologic Summary
Location Drainage Area (mi2)
Q10 (cfs)
Q50 (cfs)
Q100 (cfs)
Q500 (cfs)
Pelham-Windham Town Line 51.0 1,501 2,560 3,185 4,925
Downstream from Golden Brook 74.5 1,545 2,955 3,515 5,600
Historic Flood Flow Evaluation
VHB reviewed the recorded flood flows at the USGS gage (010965852) in North
Pelham to develop flood flow hydrographs for Beaver Brook. VHB selected several
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historic flood flow records to create representative flood hydrographs for Beaver
Brook. These historic flood flows are shown in Table 4.
Table 4: Beaver Brook Historic Flood Flows
Storm Date1 Peak Flow (cfs)
1996-Oct-22 1,580
1998-Jun-15 1,440
2001-Mar-23 1,170
2004-Apr-02 1,500
2006-May-142 2,940
2007-Apr-17 1,900
2010-Feb-26 1,510
2010-Mar-15 1,480
1 – Date of peak flow
2 – Storm of record
VHB extracted the gage record for each storm event. Figure 3 show the hydrograph
for each of these events. A review of the flood flow hydrographs with similar peak
magnitudes revealed similar hydrograph shapes including rising slope, peak
duration, and falling slope of the hydrograph. Exceptions were the May 14, 2006 and
April 17, 2007 storms,which were used as templates for the larger flood flow
hydrographs.
VHB developed these template hydrographs by averaging gage records for the storm
events as shown in Figure 4. VHB then extrapolated template hydrographs to match
the peak magnitude of the 2-, -10-, 50-, and 100-year storm events based on our
analysis described in the previous section. These hydrographs are shown in Figure 5.
USGS Regional Regression Analysis
VHB estimated flood flows from the tributaries to the Beaver Brook study area using
the USGS regional regression equations for New Hampshire. These equations were
developed by the USGS based on recorded flood discharges and drainage basin
characteristics. The drainage basin characteristics include drainage area, mean April
precipitation, percentage of wetland area, and main channel slope. VHB used the
USGS StreamStats web application to calculate watershed characteristics and used the
USGS National Stream Statistics (NSS) software to estimate peak flood flow based on
the USGS regression equations at desired recurrence intervals. The watershed
characteristics for the tributary streams are provided in Table 5
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Table 5: Watershed Characteristics for Tributaries to Beaver Brook Study Area
Tributary Drainage Area
(mi2) Mean April Precip (in)
Wetlands (%) Stream
Slope1(ft/mi)
Golden Brook 18.6 4.01 14.7 19.3
Gumpas Brook 1.53 3.97 8.39 60.0
Gumpas Pond Brook 3.79 3.96 9.36 78.4
Tonys Brook 0.96 3.98 7.28 92.9
New Meadow Brook 3.23 3.98 15.6 17.5 1 Stream slope calculated using the 10 and 85 method.
The estimated tributary flood flows obtained using the USGS regional regression
equations are shown below in Table 6.
Table 6: Tributary Hydrologic Analysis Results
Tributary Q2 (cfs) Q10 (cfs) Q50 (cfs) Q100 (cfs)
Golden Brook 300 600 900 1100
Gumpas Road Brook 45 105 180 220
Gumpas Pond Brook 110 240 395 475
Tonys Brook 35 85 140 170
New Meadow Brook 50 110 180 215
VHB compared these estimated tributary flood discharges to the current FEMA FIS.
Generally, the estimated flows are higher than the FEMA study with the exception of
New Meadow Brook. The FEMA FIS calculated flood discharges for Gumpas Road
Brook and New Meadow Brook by averaging regional regression equations with an
area-weighted log-Pearson Type III frequency analysis of three gages on similar
streams in Massachusetts. Golden Brook and Gumpas Pond Brook flood discharges
were estimated based on the Dimensionless Hydrograph Method. No detailed FEMA
study has been performed for Tony’s Brook. Table 7 presents the flood discharges
from the FEMA FIS.
Table 7: Tributary FEMA FIS Hydrologic Summary
Tributary Drainage Area (mi2)
Q10 (cfs) Q50 (cfs) Q100 (cfs)
Golden Brook 17.8 390 860 1,025
Gumpas Road Brook 1.3 54 78 88
Gumpas Pond Brook 3.7 200 345 425
Tonys Brook No detailed study
New Meadow Brook 3.7 184 275 345
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USGS Dimensionless Hydrograph
VHB computed hydrographs that represent average runoff of a specified peak
discharge based on the dimensionless-hydrograph method for the tributaries to the
Beaver Brook study reach. The dimensionless-hydrograph method is applicable for
most urban and rural streams throughout the United States. The noted exceptions are
drainage areas with flat topography, slow runoff areas, or streams with a more
complex (e.g. double-peak) hydrographs. None of the exception are applicable to the
study area.
This dimensionless-hydrograph method requires three components; peak discharge,
basin lag time, and dimensionless hydrograph ordinates. VHB calculated the peak
discharge as described in the previous section of this report. VHB calculated lag time
based on the watershed delineations and the LiDAR topographic datasets. VHB used
the NSS computer program to obtain the hydrograph ordinates and develop the
hydrograph for the desired recurrence intervals. The 100-year flood flow hydrograph
for the tributaries to Beaver Brook are shown in Figure 6 and Figure 7. The USGS
StreamStats exports and the NSS results are provided in Appendix D.
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4 Hydraulic Model Update
VHB updated the hydraulic model developed under the previous study to include
refined geometric data, extended downstream limits, and unsteady flow capability.
VHB used this model to refine evaluation of existing flood conditions and to evaluate
alternatives for flood improvement at the four bridge in downtown Pelham. VHB
updated the hydraulic model of the study area using the USACE’s Hydrologic Engineering Center – River Analysis System (HEC-RAS), version 4.1. This model is
widely used for this type of analysis and is the standard model used to develop FISs
for FEMA. Generally, The HEC-RAS model requires the following inputs:
Flood flow profiles (for steady flow analysis) and flood hydrographs (for
unsteady flow analysis) - see hydrologic analysis section
Representative channel/floodplain cross sections including geometry and
roughness information
Bridge and culvert data including geometry and other hydraulic parameters
Downstream boundary conditions
The following provides a brief description of how these parameters were
developed/updated.
Digital Terrain Model Creation
VHB developed a composite Digital Terrain Model (DTM) based on the best available
topography and survey information. VHB used the 3-Dimensional Analyst and
Spatial Analyst Extensions for ArcGIS 10.1 as well as AutoCAD Civil 3D 2012 to
combine the various topographic datasets and create the composite DTM. The
following lists the topographic dataset used in the DTM creations:
LiDAR data covering Pelham, New Hampshire and Dracut, Massachusetts
VHB field survey for the bridges within the survey area
VHB bathymetic survey for channel elevation surrounding the through each
structure
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VHB created channel breaklines
Willow Street Survey provided by Quantum
VHB digitized channel breaklines to burn the Beaver Brook channel into the terrain
model. Breaklines locations were chosen based on aerial photography and field
survey. Elevations along these breaklines were interpolated at a constant slope
between surveyed cross sections.
GeoRAS Model Framework
VHB used the HEC-GeoRAS extension for ArcGIS to process geospatial datalayers
and create a geometric data file for import into HEC-RAS. Figure 8 shows all GeoRAS
datalayers for the study reach with a locus through downtown Pelham shown in
Figure 9. VHB digitized the following datalayers within ArcGIS to generate the HEC-
RAS geometry file:
Channel centerline
Flow paths
Banks lines
Channel cross sections
Ineffective flow areas
VHB digitized the channel centerline for Beaver Brook based on the composite DTM,
field survey, and aerial photography. The channel centerline station is assigned
downstream to upstream. Station 00+000 begins at the downstream limits at the
Collinsville Dam in Dracut, Massachusetts and continues upstream through Pelham
to station 60+074.18 at the Pelham/Windham town line.
VHB developed left and right bank features to represent the approximate location of
the top of the channel banks. The left and right overbank features were developed to
represent the approximate center of mass of the overbank flow to calculate cross
section reach lengths.
VHB digitized cross section locations along the channel centerline, from left to right
facing downstream, to represent channel and floodplain geometry. Cross section
locations were chosen to represent changes in channel or floodplain geometry and to
capture conditions upstream and downstream of each crossing structure. Cross
sections were spaced periodically along the centerline and all cross sections were
drawn perpendicular to flow.
The roughness factors (Manning’s N values) for the existing stream and floodplain
conditions were estimated using various sources of data. The factors represent the
typical natural stream conditions, different vegetation along the river banks and
various types of overbank flow.
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VHB assigned permanent and non-permanent ineffective flow areas to portions of the
model cross sections. Permanent ineffective flow areas were assigned to areas that
would flood but provide no active conveyance. Non-permanent ineffective flow areas
were assigned to areas that are ineffective until a trigger water surface elevation is
reached and the flow area becomes effective.
Bridge and Culvert Data
VHB extracted internal bridge cross sections from the composite DTM and entered
roadway profile and culvert/bridge data based on field survey. VHB modeled arch
and elliptical openings as culverts while single span structures were coded in as
bridges. Structures with both culvert and bridge openings were coded as multiple
openings within HEC-RAS. VHB assigned the multiple opening analysis stagnation
points based on the cross section geometry and opening locations. The following
structures were included as part of this study.
Table 8: Bridge and Culvert Summary
Structure Model Structure Type
Model River Station
Data Source
Mammoth Road Culvert 57350.88 VHB Survey
Castle Hill Road Multiple Opening 53118.73 VHB Survey
Tallant Road Bridge 51320.48 VHB Survey
Windham Road Bridge 32043.40 VHB Survey
Main Street Culvert 28402.75 NHDOT Plans & VHB Survey
Abbott Bridge Culvert 25544.26 NHDOT Plans & VHB Survey
Willow Street Multiple Opening 21883.28 Quantum & VHB Survey
Unnamed Crossing Bridge 5818.84 VHB Survey
Expansion and Contraction Coefficients
Model contraction and expansion coefficients were set to 0.1 and 0.3 respectively for
all cross sections, with the exception of the one cross section immediately upstream
and downstream of each bridge crossing For these cross sections, the contraction and
expansion coefficients were set to be 0.3 and 0.5 respectively.
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Boundary Conditions
VHB developed a downstream boundary rating curve for the downstream study
limits at the Collinsville Dam. The rating curve is based on survey data, FEMA FIS
flows, and FEMA profiles. The rating curve is shown below in Table 9.
Table 9: Downstream Boundary Condition - Rating Curve
Stage (feet) Flow (cfs) Description
97.64 0 Channel Invert Upstream from Spillway
112.7 1 Spillway Invert
116.3 1710 Q10 FEMA Flow and WSE
117.8 3223 Q50 FEMA Flow and WSE
118.5 3830 Q100 FEMA Flow and WSE
120.0 6080 Q500 FEMA Flow and WSE
1 – Water Surface Elevation (WSE)
Flood Flow Profiles and Hydrographs
VHB entered flow data from the hydrologic analysis presented in the previous
section. The steady flow simulations used flood profiles based on VH”’s updated
hydrologic analysis as well as the effective FEMA FIS flows for comparison. VHB
used the updated flood flow profiles for the existing condition and alternative
analysis presented in the following sections.
VHB entered 100-year flood hydrographs for the unsteady flow simulations. The
unsteady flow model evaluates dynamic changes in flood flows accounting for
floodplain storage and flood flow attenuation. This type of analysis evaluates the
potential impacts on downstream flood elevations based on the alternatives analysis
presented in Chapter 6.
Although steady and unsteady flow models rely on similar inputs, unsteady flow
models are significantly more complex set up and run and are very sensitive to
relatively minor input changes. VHB adjusted the hydraulic parameters for each
bridge/culvert crossing and model cross section to maintain model stability. As
needed, VHB modified the internal boundary curves for each crossing to create a
family of stage discharge rating curves for all tailwater conditions. VHB selected a 5
day unsteady flow simulation duration to fully capture the peaks flood elevations
from Beaver Brook and its tributaries.
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5 Existing Conditions Results
VHB used the updated HEC-RAS model to calculate flood profiles for the study area
under existing conditions for the 2-, 10-, 50-, and 100-year return period flows. Figure
10 through Figure 17 provide upstream and downstream model bridge/culvert
sections for the existing condition crossings in our study. Figure 18 presents a flood
profile of Beaver Brook for all storm events analyzed. Figure 19 presents the 100-year
recurrence interval (1% annual chance) floodplain boundary from this analysis as
compared to the FEMA floodplain boundary.
VHB compared the hydraulic model results to the effective FIS base flood elevations.
These results indicate that the predicted 100-year floodplain is, on average, 1.1 feet
lower than what is shown in the FIS; see Table 10. All elevations presented in this
report are in the NAVD 88 (feet) vertical datum. This result is attributable to the
greater detail topographic and crossing structure information used in this study as
compared to the FEMA FIS.
Table 10: Existing Condition Comparison to FEMA Elevations
Structure
FEMA Elevation
(feet)
Study Elevation with Revised Flows
(feet) Difference
(feet)
Mammoth Road 168.4 167.4 -1.0
Castle Hill Road 155.6 155.6 0.0
Tallant Road 152.5 151.5 -1.0
Windham Road 135.5 133.9 -1.6
Main Street 135.0 133.2 -1.8
Abbott Bridge 132.4 130.7 -1.7
Willow Street 129.9 128.0 -1.9
Unnamed Crossing 123.3 123.1 -0.2
1 - Elevations taken two model cross sections upstream from crossing.
VHB compared these predicted study elevations to the hydraulic model results from
the effective FIS flood profile simulation. These results indicate that the study 100-
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year floodplain is on average 1.0 feet higher than what is predicted using the FEMA
FIS flows in the updated model. Figure 20 presents a 100-year flood profile of Beaver
Brook comparing the study elevation with revised flows to the elevation with FEMA
flows. These results are compared in Table 11.
Table 11: Existing Condition Comparison to FEMA Flow Profiles
Bridge
Study Elevation with FEMA Flows
(feet)
Study Elevation with Revised Flows
(feet) Difference
(feet)
Mammoth Road 165.9 167.4 1.6
Castle Hill Road 154.1 155.6 1.5
Tallant Road 150.2 151.5 1.3
Windham Road 133.4 133.9 0.5
Main Street 132.4 133.2 0.8
Abbott Bridge 130.0 130.7 0.7
Willow Street 127.0 128.0 1.1
Unnamed Crossing 122.7 123.1 0.5
The hydraulic model results also indicate that many bridges in Pelham are undersized
and cannot effectively convey flood flows. These results show several feet of head loss
across each structure; see Table 12. Backwater from the downstream structures affect
the water surface elevations at Willow Street, Abbott Bridge, Main Street, and
Windham Road. Actual loss across these crossing are likely greater than shown
below.
Table 12: Existing Condition Strucure Losses
Bridge Upstream Downstream Difference
Mammoth Road 167.4 164.34 3.1
Castle Hill Road 155.6 153.17 2.4
Tallant Road 151.5 149.28 2.2
Windham Road 133.9 133.34 0.6
Main Street 133.2 131.01 2.2
Abbott Bridge 130.7 128.38 2.3
Willow Street 128.0 125.69 2.3
Unnamed Crossing 123.1 120.98 2.1
VHB evaluated the influence of the four crossings: Willow Street, Abbott Bridge, Main
Street, and Windham Road on Beaver Brook flood stages through Pelham. VHB
created a natural condition hydraulic model which removed these four crossings
and their bounding cross sections. All other crossings were modeled in their existing
condition. This hydraulic analysis predicted a flood profile representing the most
achievable flood stage reduction by replacing these four crossings. This natural
condition flood profile is shown in Figure 21. Results from this natural condition
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simulation are also presented as part of the alternative analysis in the following
chapter.
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6 Alternatives Analysis
VHB evaluated alternative bridge openings to improve flood flow conveyance
through the Main Street Bridge, the Abbott Bridge, and the Willow Street Bridge. VHB
developed the bridge opening geometries for Willow Street and Main Street in
coordination with Quantum and NHDOT. VH”’s alternatives analysis evaluated
results from steady state simulations for the 50-year and 100-year flood profiles.
Unsteady state simulations were used to evaluate the potential impacts on
downstream flood elevations.
The existing condition hydraulic model demonstrates that water surface elevations at
each bridge location are affected by backwater from the undersized downstream
crossings. To evaluate the performance of each alternative, VHB assumed that
construction of the recommended downstream crossing bridge improvements had
been completed. These alternatives and the model results are presented below.
Willow Street
VHB evaluated several alternative openings for the Willow Street Bridge in
coordination with Quantum. Willow Street Alt1a thru Alt1d assume a single (varying)
span structure with sloping (2 horizontal : 1 vertical) bridge abutments. Alternative 1
would maintain the existing channel invert and low chord bridge elevations. Willow
Street Alt2a and Alt2b assume a combination sloping (2h:1v) and vertical abutment at
varying spans and elevations. The alternatives and the model results are represented
in Table 13 and Table 14.
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Table 13: Willow Street - Bridge Alternatives
Alternative Abutment Type Span (ft) Bot. Width
(ft) US Invert
(ft)t Low
Chord (ft) Open
Area (sf)
Existing Vertical 40 35 116.2 126.4 390
Alt1a 2:1 Sloping 75 35 116.2 126.4 560
Alt1b 2:1 Sloping 80 40 116.2 126.4 620
Alt1c 2:1 Sloping 100 60 116.2 126.4 820
Alt1d 2:1 Sloping 120 80 116.2 126.4 1030
Alt2a Combined 80 40 116.0 126.7 660
Alt2b Combined 90 45 116.0 127.8 810
Table 14: Willow Street – Alternative Model Results
Alternative Q50 WSE1 Freeboard2
(ft) Q100 WSE1
Freeboard2 (ft)
Existing 127.6 -1.1 128.0 -1.6
Alt1a 126.4 0.0 127.2 -0.8
Alt1b 126.2 0.2 127.0 -0.6
Alt1c 125.9 0.5 126.5 -0.1
Alt1d 125.7 0.7 126.3 0.1
Alt2a 126.2 0.5 126.9 -0.2
Alt2b 126.1 1.7 126.8 1.0
Natural 125.5 na 126.0 na 1 - Water surface elevation (WSE) taken 2 model cross sections upstream from Willow Street at RS 22141.28 2 – Freeboard measured from low chord elevation
Hydraulic analysis results indicate that Willow Street Alt1c (100-foot wide/sloping
abutment span) and Alt1d (120-foot wide/sloping abutment span) would provide
about 0.5-feet of freeboard during the 50-year flood flow. The predicted flood stages
for these alternatives would also be within 0.5 feet of the natural condition water
surface elevation. VHB recommends Willow Street Alt1c (100-foot wide/sloping
abutment span) or a larger span structure. Figure 22 provides the upstream and
downstream bridge/culvert model sections for this alternative.
Old Bridge Street (Abbott Bridge)
VHB evaluated alternatives for the Abbott Bridge that included adding stone arches
(Alt 1) as well as supplementing the existing arches with a single span opening (Alt 2).
The existing channel invert and bridge low chord elevations are maintained for all
Abbott Bridge Alternatives. The existing condition model results indicate that water
surface elevations at Abbott Bridge are affected by backwater from the existing
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Willow Street Bridge. This analysis assumes the recommended Willow Street Alt1c
has been constructed.
Table 15: Abbott Bridge - Alternatives
Alternative No. of
Arches Span (ft)
US Invert (ft)
Low Chord (ft)
Open Area (sf)
Existing 2 na 117.8 127.2 310
Alt1a 4 na 117.8 127.2 610
Alt1b 8 na 117.8 127.2 1210
Alt2a 2 40 117.8 127.2 690
Alt2b 2 60 117.8 127.2 880
Alt2c 2 80 117.8 127.2 1070
Table 16: Abbott Bridge – Alternatives Model Results
Alternative Q50 WSE1
Freeboard2 (ft)
Q100 WSE1 Freeboard2
(ft)
Existing 130.4 -3.2 130.7 -3.5
Alt1a 127.5 -0.3 128.4 -1.2
Alt1b 126.9 0.3 127.6 -0.4
Alt2a 127.3 0.0 128.1 -0.9
Alt2b 126.9 0.3 127.7 -0.5
Alt2c 126.7 0.5 127.5 -0.3
Natural 126.6 na 127.1 na 1 - Water surface elevation (WSE) taken 2 model cross sections upstream from Main Street at RS 25868.69 2 – Freeboard measured from low chord elevation
Hydraulic analysis results indicate Abbott Bridge Alt2b (supplemental 50-foot span)
could provide at least 0.5 feet of freeboard during the 50-year flood flow. The
modeling predicts there is no practicable way to achieve sufficient freeboard by
replicating the existing stone arches. However, additional arch opens could provide
up to 3.5 feet of flood stage reductions. VHB recommends Abbott Bridge Alt2b
(supplemental 50-foot span) or a larger span structure. Figure 23 provides the
upstream and downstream bridge/culvert model sections for this alternative.
Main Street
VHB evaluated several alternatives for the Willow Street Bridge in coordination with
NHDOT. Main Street Alt1a and Alt1b assume a single span structure with sloping
(1.5h:1v) bridge abutments This analysis assumes the recommended Willow Street
Alt1c and Abbott Bridge Alt2b have been constructed.
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Table 17: Main Street - Bridge Alternatives
Alternative Abutment
Type Span (ft)
Bot. Width (ft)
US Invert (ft)
Low Chord (ft)
Open Area (sf)
Existing na na 35 120 128.0 360
Alt1a 1.5:1 Sloping 100 70 120 128.0 710
Alt1b 1.5:1 Sloping 100 70 120 130.3 870
Table 18: Abbott Bridge – Alternatives Results
Alternative Q50 WSE1
Freeboard2 (ft)
Q100 WSE1 Freeboard2
(ft)
Existing 132.8 -4.8 133.2 -5.2
Alt1a 129.7 -1.7 130.5 -2.5
Alt1b 129.4 0.8 130.1 0.2
Natural 129.0 na 129.5 1 - Water surface elevation (WSE) taken 2 model cross sections upstream from Abbott Bridge at RS 28737.79 2 – Freeboard measured from low chord elevation
Hydraulic analysis results indicate Main Street Alt1b (100-foot wide/sloping abutment
span) could provide at least 0.5 feet of freeboard during the 50-year flood flow and
pass the 100-year flood. VHB recommends Main Street Alt1b (100-foot wide/sloping
abutment span) or a larger span structure. Figure 24 provides the upstream and
downstream model bridge/culvert sections for this alternative.
Downstream Impacts
VHB ran unsteady state simulations, using the previously presented 100-year flood
discharge hydrographs, to estimate potential impacts on downstream flood elevations
Results from this hydraulic analysis indicate that the alternatives could increase flood
elevations by as much as 0.04 feet immediately downstream from Willow Street with
an average increase of 0.02 feet from Willow Street to the downstream limits of the
hydraulic model.
Recommendation
VHB evaluated several bridge opening geometries to improve flood flow conveyance
through the Main Street Bridge, the Abbott Bridge, and the Willow Street Bridge. VHB
evaluated the performance of each alternative against the model results from the 50-
year and 100-year steady state flood profiles. Based on this analysis, VHB
recommends the bridge alternatives shown in Table 19 to more effectively convey
flood flows and reduce predicted peak flood elevations.
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Table 19: Preferred Bridge Alternatives
Bridge Preferred
Alternative Abutment Type Span (ft)
Bot. Width (ft)
Open Area (sf)
Willow Street Alt1c 2:1 Sloping 100 60 820
Abbott Bridge Al2b1 Vertical 60 60 880
Main Street Alt1b 1.5:1 Sloping 100 70 870
1 –Alternative in addition to the existing twin stone arches.
Figure 25 provides the predicted 100-year flood profile for the preferred alternative
compared to the existing conditions. Figure 26 shows the predicted 100-year flood
boundary in the vicinity of these bridge location.
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7 Summary
This report presented the findings from the second phase of a flood study of Beaver
Brook in the Town of Pelham, New Hampshire. The study is based on more detailed
analysis and newly collected data to better evaluate the causes and potential solutions
to flooding associated with Beaver Brook. VHB performed a more detailed hydrologic
and hydraulic analysis as an evaluation of alternatives for flooding improvement.
This report assessed the bridge openings under existing conditions and determined
many existing bridges cannot handle estimated flood flows, and these bridges
influence flood elevations at upstream structures.