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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

i Table of Contents \\vhb\proj\Wat-

LD\52327.00\reports\52327.00 Beaver Brook - Alternatives Analysis - Draft.docx

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

4 Introduction \\vhb\proj\Wat-


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:

5 Introduction \\vhb\proj\Wat-


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.

6 Data Collection \\vhb\proj\Wat-


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



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



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.

9 Hydrologic Analysis \\vhb\proj\Wat-


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.



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



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



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



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.

14 Hydraulic Model Updates \\vhb\proj\Wat-


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



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.



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.



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




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.

18 Existing Conditions Results \\vhb\proj\Wat-


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-



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


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


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



simulation are also presented as part of the alternative analysis in the following

chapter.

21 Alternatives Analysis \\vhb\proj\Wat-


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.



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



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.



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.



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.

26 Summary \\vhb\proj\Wat-


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.