Beaver Brook Flood Study

Transcription

1 Alternatives Analysis Beaver Brook Flood Study Pelham, New Hampshire PREPARED FOR Town of Pelham 6 Village Green Pelham, NH PREPARED BY 101 Walnut Street PO Box 9151 Watertown, MA January 14, 2015

2 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 Historic Flood Flow Evaluation USGS Regional Regression Analysis USGS Dimensionless Hydrograph Hydraulic Model Update Digital Terrain Model Creation GeoRAS Model Framework Bridge and Culvert Data Expansion and Contraction Coefficients Boundary Conditions Flood Flow Profiles and Hydrographs Existing Conditions Results Alternatives Analysis Willow Street Old Bridge Street (Abbott Bridge) Main Street Downstream Impacts Recommendation Summary Appendix A Figures Appendix B Survey Appendix C FEMA Documents Appendix D Hydrology Appendix E HEC-RAS Results Appendix E HEC-RAS Data Disc i Table of Contents

3 Tables Table 1: Hydrologic Methods... 9 Table 2: Beaver Brook Hydrologic Analysis Results Table 3: Beaver Brook FEMA FIS Hydrologic Summary Table 4: Beaver Brook Historic Flood Flows Table 5: Watershed Characteristics for Tributaries to Beaver Brook Study Area Table 6: Tributary Hydrologic Analysis Results Table 7: Tributary FEMA FIS Hydrologic Summary Table 8: Bridge and Culvert Summary Table 9: Downstream Boundary Condition - Rating Curve Table 10: Existing Condition Comparison to FEMA Elevations Table 11: Existing Condition Comparison to FEMA Flow Profiles Table 12: Existing Condition Strucure Losses Table 13: Willow Street - Bridge Alternatives Table 14: Willow Street Alternative Model Results Table 15: Abbott Bridge - Alternatives Table 16: Abbott Bridge Alternatives Model Results Table 17: Main Street - Bridge Alternatives Table 18: Abbott Bridge Alternatives Results Table 19: Preferred Bridge Alternatives Figures Figure 1: Beaver Brook Watershed Figure 2: Beaver Brook Study Reach Figure 3: Historic Flood Flows USGS Gage ( ) Figure 4: Template Hydrographs Figure 5: Beaver Brook Flood Hydrographs Figure 6: 100-Year Tributary Flow Hydrographs Figure 7: 100-Year Study Hydrographs Figure 8: Model Framework and DTM Figure 9: Alternative Analysis Model Framework and DTM Figure 10: HEC-RAS Section Mammoth Road Figure 11: HEC-RAS Section Castle Hill Road Figure 12: HEC-RAS Section Tallant Road Figure 13: HEC-RAS Section Windham Road Figure 14: HEC-RAS Section Main Street Figure 15: HEC-RAS Section Old Bridge Street (Abbott Bridge) Figure 16: HEC-RAS Section Willow Street ii Table of Contents

4 Figure 17: HEC-RAS Section Unnamed Crossing Figure 18: HEC-RAS Profile Existing Condition Figure 19: Flood Map Existing to FEMA Figure 20: HEC-RAS Profile Existing and FEMA Study Flows Figure 21: HEC-RAS Profile Existing and Natural Conditions Figure 22: HEC-RAS Section Willow Street Alt1c Figure 23: HEC-RAS Section Abbott Bridge Alt2b Figure 24: HEC-RAS Section Main Street Alt1b Figure 25: HEC-RAS Profile Preferred Alternative Figure 26: Flood Map Existing to Preferred Alternatives iii Table of Contents

5 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: 4 Introduction

6 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. 5 Introduction

7 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 6 Data Collection

8 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, Data Collection

9 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 Beaver Brook at North Pelham, NH for the water years 1987 to Data Collection

10 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 Tributarys 1 Log-Pearson Type III with area-weighted scaling USGS Regional Regression Analysis Historic Flood Flow Evaluation USGS Dimensionless Hydrograph Method 1 Golden Brook, Gumpas Road Brook, Tonys Brook, Gumpas Pond Brook, New Meadow Brook. 9 Hydrologic Analysis

11 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 ( ) 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 (mi 2 ) Q2 (cfs) Q10 (cfs) Q50 (cfs) Q100 (cfs) Source Upper Beaver Brook ,000 3,200 3,900 Scaled from USGS Gage Lower Beaver Brook ,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 (mi 2 ) Q10 (cfs) Q50 (cfs) Q100 (cfs) Q500 (cfs) Pelham-Windham Town Line ,501 2,560 3,185 4,925 Downstream from Golden Brook ,545 2,955 3,515 5,600 Historic Flood Flow Evaluation VHB reviewed the recorded flood flows at the USGS gage ( ) in North Pelham to develop flood flow hydrographs for Beaver Brook. VHB selected several 10 Hydrologic Analysis

12 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 Date 1 Peak Flow (cfs) 1996-Oct-22 1, Jun-15 1, Mar-23 1, Apr-02 1, May , Apr-17 1, Feb-26 1, 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 11 Hydrologic Analysis

13 Table 5: Watershed Characteristics for Tributaries to Beaver Brook Study Area Tributary Drainage Area (mi 2 ) Mean April Precip (in) Wetlands (%) Stream Slope 1 (ft/mi) Golden Brook Gumpas Brook Gumpas Pond Brook Tonys Brook New Meadow Brook 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 Gumpas Road Brook Gumpas Pond Brook Tonys Brook New Meadow Brook 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 (mi 2 ) Q10 (cfs) Q50 (cfs) Q100 (cfs) Golden Brook ,025 Gumpas Road Brook Gumpas Pond Brook Tonys Brook No detailed study New Meadow Brook Hydrologic Analysis

14 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. 13 Hydrologic Analysis

15 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 14 Hydraulic Model Updates

16 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 begins at the downstream limits at the Collinsville Dam in Dracut, Massachusetts and continues upstream through Pelham to station 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. 15 Hydraulic Model Updates

17 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 VHB Survey Castle Hill Road Multiple Opening VHB Survey Tallant Road Bridge VHB Survey Windham Road Bridge VHB Survey Main Street Culvert NHDOT Plans & VHB Survey Abbott Bridge Culvert NHDOT Plans & VHB Survey Willow Street Multiple Opening Quantum & VHB Survey Unnamed Crossing Bridge 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. 16 Hydraulic Model Updates

18 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 Channel Invert Upstream from Spillway Spillway Invert Q10 FEMA Flow and WSE Q50 FEMA Flow and WSE Q100 FEMA Flow and WSE 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. 17 Hydraulic Model Updates

19 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 FEMA Elevation (feet) Study Elevation with Revised Flows (feet) Difference (feet) Structure Mammoth Road Castle Hill Road Tallant Road Windham Road Main Street Abbott Bridge Willow Street Unnamed Crossing 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 Existing Conditions Results

20 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 Study Elevation with FEMA Flows (feet) Study Elevation with Revised Flows (feet) Difference (feet) Bridge Mammoth Road Castle Hill Road Tallant Road Windham Road Main Street Abbott Bridge Willow Street Unnamed Crossing 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 Castle Hill Road Tallant Road Windham Road Main Street Abbott Bridge Willow Street Unnamed Crossing 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 19 Existing Conditions Results

21 simulation are also presented as part of the alternative analysis in the following chapter. 20 Existing Conditions Results

22 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 Alternatives Analysis

23 Table 13: Willow Street - Bridge Alternatives Bot. Width US Invert Low Open Alternative Abutment Type Span (ft) (ft) (ft)t Chord (ft) Area (sf) Existing Vertical Alt1a 2:1 Sloping Alt1b 2:1 Sloping Alt1c 2:1 Sloping Alt1d 2:1 Sloping Alt2a Combined Alt2b Combined Table 14: Willow Street Alternative Model Results Alternative Q50 WSE 1 Freeboard 2 (ft) Q100 WSE 1 Freeboard 2 (ft) Existing Alt1a Alt1b Alt1c Alt1d Alt2a Alt2b Natural na na 1 - Water surface elevation (WSE) taken 2 model cross sections upstream from Willow Street at RS 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 22 Alternatives Analysis

24 Willow Street Bridge. This analysis assumes the recommended Willow Street Alt1c has been constructed. Table 15: Abbott Bridge - Alternatives No. of US Invert Low Chord Open Alternative Span (ft) Arches (ft) (ft) Area (sf) Existing 2 na Alt1a 4 na Alt1b 8 na Alt2a Alt2b Alt2c Table 16: Abbott Bridge Alternatives Model Results Alternative Q50 WSE 1 Freeboard2 (ft) Q100 WSE 1 Freeboard2 (ft) Existing Alt1a Alt1b Alt2a Alt2b Alt2c Natural na na 1 - Water surface elevation (WSE) taken 2 model cross sections upstream from Main Street at RS 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. 23 Alternatives Analysis

25 Table 17: Main Street - Bridge Alternatives Abutment Bot. Width US Invert Low Chord Open Area Alternative Span (ft) Type (ft) (ft) (ft) (sf) Existing na na Alt1a 1.5:1 Sloping Alt1b 1.5:1 Sloping Table 18: Abbott Bridge Alternatives Results Alternative Q50 WSE 1 Freeboard2 (ft) Q100 WSE 1 Freeboard2 (ft) Existing Alt1a Alt1b Natural na Water surface elevation (WSE) taken 2 model cross sections upstream from Abbott Bridge at RS 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. 24 Alternatives Analysis

26 Table 19: Preferred Bridge Alternatives Preferred Bot. Open Area Bridge Abutment Type Span (ft) Alternative Width (ft) (sf) Willow Street Alt1c 2:1 Sloping Abbott Bridge Al2b 1 Vertical Main Street Alt1b 1.5:1 Sloping 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. 25 Alternatives Analysis

27 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. 26 Summary