lye brook feasibility analysis draft€¦ · lye brook originates in steep mountainous terrain east...

Lye Brook Berm Alternatives Analysis RFP Hartland, Vermont and Winthrop, Maine www.northstarhydro.com

Lye Brook Feasibility Analysis DRAFT

Hydrology and Hydraulics Report

Manchester, Vermont

Northstar Hydro, Inc. for: Bennington County Regional Commission

May 2019

Lye Brook Berm Alternatives Analysis RFP Hartland, Vermont and Winthrop, Maine www.northstarhydro.com

Lye Brook Feasibility Analysis DRAFT

Table of Contents

Lye Brook Feasibility Analysis ...................................................................................................... 1

Executive Summary .................................................................................................................... 1

Lye Brook Feasibility Analysis ...................................................................................................... 3

1.0 Introduction ...................................................................................................................... 3

2.0 Site Visits and Calibration Data ....................................................................................... 7

3.0 H&H Model Development and FEMA Comparison...................................................... 10

4.1 Hydrology ....................................................................................................................... 10

4.2 Hydraulics ....................................................................................................................... 14

4.3 Calibration....................................................................................................................... 18

4.4 FEMA FIS Data .............................................................................................................. 21

5.0 Alternatives Analysis .......................................................................................................... 23

5.1 Alternative 1: No Action ................................................................................................. 23

5.2 Alternative 2: 10-Foot Berm ........................................................................................... 27

5.3 Alternative 3: Replace 5-foot Driveway Culvert with Bridge ........................................ 28

5.4 Alternative 4: Resize Existing Richville Road Culverts ................................................. 30

5.5 Alternative Analysis Summary ....................................................................................... 32

6.0 Future Alternative Analysis and Design Proposal: ............................................................. 35

7.0 Conservation: ...................................................................................................................... 35

8.0 Public Participation: ............................................................................................................ 38

9.0 Conclusion: ......................................................................................................................... 39

References ................................................................................................................................. 40

APPENDICES .......................................................................................................................... 41

Lye Brook Feasibility Analysis Executive Summary Northstar Hydro, Inc. developed a 2-Dimensional flow (2-D) model of Lye Brook and the Batten Kill in the Town of Manchester, Vermont. The model aids in the evaluation the performance of an earthen berm on the right bank of Lye Brook from Route 7 to approximately Richville Road, and to assists in understanding the relationship between the berm and flooding on Richville Road. The scope of the project is related to the complexity of the ecosystem studied. Flow dynamics and potential engineering solutions could only be understood and identified as the study model was compiled and run.

The study area was an active alluvial fan until the late 1800’s when Lye Brook was channelized to its existing location. The model study area starts upstream of Route 7 on Lye Brook and continues to just downstream of the wastewater treatment plant on the Batten Kill. The downstream end of the model is below the Lye Brook/Batten Kill confluence and coincides with Batten Kill FEMA section AW. The model was calibrated for the 2-year and 100-year storm events based on field observations.

Field observations and available data show that the bed of Lye Brook is actively aggrading and has risen 3-4 ft in a period of less than 30 years, effectively lowering the berm by the same amount.

The analysis of the existing conditions (Alternative 1: No Action) shows that Lye Brook is perched above the adjacent wetland. The berm has deteriorated or been effectively lowered through aggradation in many locations such that non-storm level streamflows flow from Lye Brook to the adjacent wetland complex. The eastern driveway that crosses Lye Brook and Richville Road creates a bathtub effect ponding water upstream of that intersection such that Richville Road and the driveway overtop during the 2-year event.

Three alternatives were analyzed to address flooding impacts. Alternative 2 involves raising the berm to a height of 10-feet to match the existing VTrans berm just downstream of Route 7. Raising the berm limits flow to the adjacent wetland but does not prevent the overtopping of Richville Road or the driveway. Raising the berm would likely be a temporary solution due to constant streambed aggradation and creates environmental impacts by limiting flow to the adjacent wetlands.

Alternative 3 involves replacing the 5-foot driveway culvert with a 33-foot bridge designed to pass all of Lye Brook in the future. Alternative 3 assumes that at some point in the future, Lye Brook will avulse, leaving its existing channel for the lower wetland channel. The new bridge would be designed to carry that flow downstream of the driveway. With additional flow to the wetland south of the driveway and no modifications to the 16 ft Richville Road culvert, Richville Road would flood more significantly south of the new bridge. Alternative 3 at this point involves construction on private property and was therefore deemed unviable.

Lye Brook Feasibility Analysis 1

Alternative 4 involves replacing the existing culverts along Richville Road with a series of substantially larger box culverts. These culverts would minimize flooding on Richville Road, but would not prevent it. The driveway would remain a barrier to flow and under this scenario would effectively force a majority of Lye Brook’s flow through Richville Road to the western wetlands. A modified version of Alternative 4 was deemed the most economic and sustainable alternative for the immediate future.

The analysis highlights the challenges of containing flood flows on a relatively flat alluvial fan. Under ideal conditions without development or road crossings, the stream would regularly switch channels as it deposits materials eroded from the upstream mountains. The alternatives analysis suggests that the best long term course of action seems to be a combination of Alternatives 3 and 4. Additional capacity through the driveway is needed and based on current landscape evolution. The location of the existing 5-foot culvert appears to be a likely future location of Lye Brook. Under this scenario some additional capacity under Richville Road will still be required to prevent overtopping. Sketches and cost estimates of Alternatives 3 and 4 were developed through the 30% design level and are included in the appendices.

The Town should also look at property acquisition to work towards the long-term mitigation plan described above. In the immediate future, the existing culverts on Richville Road should be replaced with substantially larger culverts to prevent overtopping of Richville Road during smaller storm events. Additionally modeling of this alternative is recommended to ensure the effectiveness of the solution prior to incurring construction costs.

It is also recommended that the Town consider replacing the existing 16 ft Richville Road culvert with a bankfull width culvert. This may improve flooding on Richville Road south of the eastern driveway. Additional hydraulic modeling of this option is also recommended before going to the design phase.


Lye Brook Feasibility Analysis

1.0 Introduction This report was prepared by Northstar Hydro, Inc. (NHI) for the Bennington Country Regional Commission (BCRC) and the Town of Manchester, Vermont to assist the understanding of flow dynamics between Lye Brook, the Batten Kill, and interconnected wetland areas and structures. The study is funded by ANR Ecosystem Restoration Program (ERP) to conduct a feasibility analysis or preliminary engineering report and builds on previous work by UVM students for their capstone project. The scope of the project is related to the complexity of the ecosystem studied. Flow dynamics and potential engineering solutions could only be understood and identified as the study model was compiled and run. This report summarizes the collection of study information and the development and use of a hydrology and two-dimensional flow (2-D) hydraulic (H&H) model of the Lye Brook corridor from upstream of Route 7 to downstream of the confluence with the Batten Kill. The model was used to identify and eventually analyze four alternatives for mitigation of some flood hazards along the Lye Brook corridor.

Lye Brook crosses Route 7 south of the junction of Route 7 with Route 11/30 and southeast of the Manchester Center downtown area. Figure 1 shows the location of the project study area within Manchester. Figure 2 shows key infrastructure and hydrologic components within the project study area.

Figure 1: Location of Lye Brook project study area (yellow box) within the Town of Manchester. Outer red box shows the limits of the Town of Manchester.

Manchester Center

Rte 7

Rte 11/30

Study Area


Figure 2: Key features within the Lye Book study area. Lye Brook originates in steep mountainous terrain east of Route 7. Shaded elevation imagery from 2012 (Figure 3) shows Lye Brook is a steep erosive stream that is capable of significant sediment transport. Just east of Route 7, the Lye Brook channel slope becomes more gradual as sediment was historically deposited in an alluvial fan. Multiple former channel beds can be observed in the shaded elevation imagery.

Batten Kill

Route 7

Lye Brook Railroad

Berms and wetlands

Richville Rd.

Trail


Figure 3: Shaded elevation map of Lye Brook showing the steep erosive upstream environment flowing into an alluvial fan and a shallower depositional environment. The approximate extents of the alluvial fan are outlined in orange. Several former channel beds can be observed in the alluvial fan region. For example one channel has been traced with a yellow line. A review of historic maps including USGS topography maps and a 1869 Manchester business listing shows that channelization of Lye Brook began prior to 1869 where Lye Brook was likely channelized to its current location east of Lye Brook Road. An 1869 map of Manchester shows Lye Brook dividing into three distinct channels downstream of the current location of Lye Brook Road. Figure 4 shows a trace of the Lye Brook channels from a georeferenced 1869 map. It appears that Lye Brook was further channelized into its existing location prior to 1900. With this channelization, Lye Brook no longer deposited sediment onto the alluvial fan, but instead deposits material in the streambed. The 1900 USGS map shows an elevation of 700 ft at the end of Lye Brook Road whereas the 2012 LiDAR shows a bed elevation of ~713 ft. Furthermore the 700 ft contour line is further downstream in the 2015 USGS topo maps than in the 1968 map (Figure 5). Some discrepancy is expected with change in technology, but this elevation change is likely evidence of streambed aggradation as well as berm reconstruction over the century.

Alluvial Fan

Steep Stream


Figure 4: Trace of 1869 stream channels (purple) compared to current National Hydrography Dataset (NHD - blue).

Figure 5: Migration of 700 ft contour line from 1900 to 1968 to 2015. 700-ft topographic lines were digitized from USGS maps. This migration of the 700 ft contour line is indicative of streambed aggradation.


NHI compiled and reviewed existing data related to study components for Lye Brook corridor including:

• FEMA Flood Insurance Studies • Bridge design plans for Route 7/Lye Brook Bridge and Railroad/Lye Brook Bridge. • UVM Lye Brook Flood Mitigation Design Project, May 1, 2016 detailing culvert sizes

and inverts for multiple culverts within the project area. • Precipitation Data from VT-BN-7 - MANCHESTER 2.8 ENE, VT for August 1-5, 2018 • USGS StreamStats, version 4.2.1, Basin Characteristics and Regression-based Peak Flow

Scenarios. • Historic USGS maps (1900, 1968, 1997, and 2015) and an 1869 Manchester Business

Listing Map. • GIS Database:

o Vermont Emergency Flood DFIRM o National Hydrography Dataset Flowlines and Water Bodies o Vermont ANR VSWI Wetlands Class Layer o Manchester Bridge and Culverts o VTrans VTR Railroad Valuation Sheets, ArcGIS Online, May 2018. o 2012 2-m Bennington County LiDAR and associated Hillshade Model o VCGI Tropical Storm Irene GIS Resources

2.0 Site Visits and Calibration Data On September 7, 2018 NHI, Ripple Natural Resources, LLC (Ripple) and members of the Lye Brook committee toured selected significant locations along Lye Brook. During this visit, flood markers from Hurricane Irene 2011 and an August 2018 storm were noted. On October 23, 2018 NHI revisited the corridor to review berm conditions and locate additional calibration markers from the August Storm. Figure 6 highlights critical infrastructure and/or flood locations along the Lye Brook corridor.


Figure 6: Significant notes from field visits. During the site visit, it was noted that Lye Brook appears to be actively aggrading and that the berm is no longer containing average flows much less flood flows. Figure 7 shows a location along the berm where the berm is no longer effective. The bank/berm is slightly above the channel bed. On a typical October day water was leaking into the adjacent wetland channel. The location of trees along the bank suggest that the stream is actively aggrading as trees do not typically grow that close to water. Figure 8 shows a panoramic of the 2015 berm repair located just upstream of the Figure 7 photo location. Of note is the elevation difference between the stream bed of Lye Brook and the adjacent wetland channel bed. Lye Brook is several feet above the wetland surface elevation on the west side of the berm.


Figure 7: Photo shows almost no berm at this location, channel aggradation and flow leaking into the adjacent wetland channels. Location is the right bank of Lye Brook south of the Cass Terrace development.

Figure 8: Panoramic of 2015 Repaired berm location showing elevation of Lye Brook channel several feet above the elevation of the adjacent wetland channel.

Wetland Channel

Repaired Berm


Figure 9 shows the condition of the berm on Lye Brook from Route 7 to the driveway that crosses Lye Brook. “Solid Berm” locations are where the berm is approximately 6 ft above the channel bed and is not expected to overtop during routine flood events. “Short Berm” locations are where the berm has a height above the channel bed of approximately 3 ft and show evidence of recent overtopping events. The VTrans berm is well above the channel bed (~10 ft), appropriately armored and is not expected to overtop for large storms including the 100-yr event.

Figure 9: Berm condition based on field visit and elevation data.

3.0 H&H Model Development and FEMA Comparison NHI compiled a 2-D hydraulic model in the software program SMS-SRH2D. The required model inputs are hydrology or flow into and out of the model domain, ground elevation, structures and surface materials. The following sections summarize the development of these inputs and the calibration of the model.

4.1 Hydrology

NHI used StreamStats to implement a regression-based statistical analysis of peak flows for Lye Brook and Batten Kill using equations for ungaged streams in Vermont. StreamStats is a web-based GIS application developed by the USGS that provides users access to key water resource planning data and implements state-based regression curves to estimate flow for ungaged, unregulated streams. USGS regression curves for Vermont are based on a statistical analysis of 145 regional stream gages. Drainage area, annual mean precipitation, and percent wetlands were


determined by USGS to be key parameters required to estimate flood flows in Vermont. For drainage areas greater than 0.3 square miles, VTrans’s Hydraulics Manual recommends the use of the USGS regression-based equations as the best available tool for estimating flood flows.

For Lye Brook, the watershed was delineated at the Lye Brook/Route 7 Junction and just upstream of the confluence of Lye Brook and the Batten Kill. The Batten Kill watershed was delineated just upstream of the confluence with Lye Brook. Table 1 shows the drainage areas for Lye Brook and the Batten Kill at the delineation points.

Figure 10: Drainage areas: Batten Kill (red watershed), Lye Brook (smaller purple watershed), dots show the mouths of the respective streams. FEMA’s 2015 Countywide Flood Insurance Study (FIS) provides additional estimates of streamflows, which were originally developed for the Town of Manchester’s 1985 FIS. The FEMA flood flows for both Lye Brook and Batten Kill were developed based on a drainage area-peak discharge relationship using the USGS gage described below on the Batten Kill. The Vermont regression equations as implemented in StreamStats are the recommended methodology for estimating flows on ungaged streams. Table 1 shows the estimated FEMA flows compared to the StreamStats estimates. Note that the drainage area for Lye Brook at the Batten Kill is different in FEMA vs. StreamStats.

57.7 sq. mi

8.13 sq. mi


Location Data Source

Drainage Area, sq.mi. Flow (cfs) at Recurrance Interval

Batten Kill @ Lye Brook 2-year 10-year 50-year 100-year 500-year FEMA – 2015 FIS 59.6 na 3,463 5,360 6,240 8,562 StreamStats 59.7 2,080 4,080 6,430 7,570 10,800

Lye Brook @ Batten Kill FEMA – 2015 FIS 9.5 na 735 1,475 1,825 3,000

StreamStats 8.9 444 931 1,530 1,830 2,670 Lye Brook @ Route 7

StreamStats 8.13 427 899 1,480 1,770 2,590 Table 1: Comparison of FEMA FIS flows vs. StreamStats regression estimates, FEMA as listed in 2015 FIS. USGS maintained a stream gage (USGS Station 1329000) on the Batten Kill approximately 8.1 miles downstream of the Lye Brook junction in the Town of Arlington, VT from 1928 until 1984 with peak flows recorded until 2011.

Based on current VTrans hydrologic investigation recommendations and the ratio of drainage areas, the gage is located too far downstream to be used in developing flows. However, it can serve as a check on regression-based flows particularly for the Batten Kill. At the gage, the two-year regression-based estimate is 48% higher and the 100-year flood is 80% higher than the gage data. The comparison suggests that actual peak flows on the Batten Kill may be less than those estimated by the regression equations. The regression equations were used as a conservative estimate of peak flows. Moreover, due to the elevation gradient along Lye Brook, the Batten Kill’s influence likely does not extend into the Richville Road area of the model. Thus using a conservative flow for the Batten Kill likely will not significantly impact modeling results along Lye Brook. Table 2 shows the comparison of gage peak flows to regression estimates at the Arlington gage. Note that the peak flow from Hurricane Irene at the Batten Kill gage is just below a 50-year event.

Recurrence Interval Gage

Regression Est.

Percent Higher

Flow, cfs Flow, cfs 2_Year_Peak_Flood 3,280 4,870 48% 10_Year_Peak_Flood 5,600 9,350 67% 50_Year_Peak_Flood 8,210 14,600 78% 100_Year_Peak_Flood 9,510 17,100 80% 500_Year_Peak_Flood 13,000 24,100 85% Maximum_Peak_Flood (1936) 11,100 Recorded @ Gage

Irene Peak (8/28/2011) 7,640 Recorded @ Gage

Table 2: Batten Kill Stream Gage Arlington, VT, Drainage Area 152 sq. mi. One inflow boundary condition was selected for the hydraulic model for each stream. Flow into the model along the Lye Brook corridor is the average of flow computed at Route 7 and at the


Batten Kill Junction. Flow into the Batten Kill is the flow computed just upstream of the Lye Brook Junction.

Two conditions were modeled for the Lye Brook Corridor:

1. A moderate flood with a high probability of occurrence. A storm occurred on August 3-4, 2018 that generated 2.41 in. of rain in 24 hours as observed at the VT-BN-7 Manchester 2.8 ENE precipitation station located in Manchester Center. This corresponds to approximately a two-year rainfall event according to the Northeast Regional Climate Center (NRCC) Extreme Precipitation Table for the study area. Flood marks from this event were observed during the field visit: therefore this event was chosen as a calibration event and was used as the moderate flood with a high probability of occurrence.

2. A high magnitude flood with a low probability of occurrence. Hurricane Irene in August of 2011 dropped approximately 5.5 inches of water in the headwaters of Lye Brook. According to the NRCC this rainfall total represents a 50-year event. However, rainfall events and stream flow events do not correlate directly. In the Lye Brook watershed we only have rainfall data. Based on ground observations, super saturated soil conditions, and the flashiness of Lye Brook, stream flow for Irene was likely closer to a 100-yr event. Therefore the 100-year event was chosen as the high magnitude low probability of occurrence event for Lye Brook.

Due to the difference in drainage area and time of travel between the watersheds, Lye Brook and the Batten Kill are not likely to experience the same flood magnitude at the same time. Without gage data or a full hydrologic study on each stream it is difficult to determine flood flows on the Batten Kill when Lye Brook reaches its peak. The Batten Kill’s watershed is 6.7 times bigger than Lye Brook. If both rivers were modeled to peak at the same time, the backwater conditions of the Batten Kill would inundate and limit the flow dynamics of Lye Brook thus impacting understanding of flow on Lye Brook. Anecdotal comments from Town officials suggest that the Batten Kill crests several hours after Lye Brook. For these reasons, NHI chose to model the Batten Kill with a two-year event flood flow for all analyses. Lye Brook was modeled as a two-year flow event for the calibration/high probability event and a 100-year event for the high magnitude low probability event.

Hydrologic parameters were input into the hydraulic model as boundary conditions. Inflow boundary conditions were placed at the upstream ends of the model for both the Batten Kill and Lye Brook. The outflow boundary condition for the Batten Kill downstream of Lye Brook was set as an elevation. This elevation was determined based on normal depth, using topography at the model exit, outflow, and the slope of the FEMA model at FEMA cross section ‘AW’ which corresponds to the model downstream end.


Batten Kill Event/Lye Brook Event Frequency 2-yr/2-yr 2-yr/100-yr Lye Brook Inflow, cfs 436(1) 1800(1)

Batten Kill Inflow at upstream end of model, cfs 2080 2080 Batten Kill Flow at downstream end of model, cfs 2516(2) 3880(2)

10-yr FEMA Slope at Section AW ft/ft 0.083 0.083 Batten Kill Outlet Elev., ft 648.72 649.56

(1): Average of StreamStats estimate for Lye Brook at Route 7 and at the Batten Kill Junction (2): Sum of Lye Brook and Batten Kill inflows

Table 3: Hydrologic Boundary Conditions

4.2 Hydraulics

The Surface-water Modeling System Sedimentation and River Hydraulics – Two-Dimensional (SMS-SRH2D) model developed by the U.S. Bureau of Reclamation was selected to model the Lye Brook project area. SMS-SRH2D is a FEMA approved two-dimensional river hydraulics model. A 2-D modeling package was preferred for this project area because of significant lateral flow within the model area. One dimensional models are not designed to capture lateral flow dynamics.

The model has four primary inputs:

1. Topography: • 2012, 2-meter LiDAR for all areas above water surfaces • Field and aerial photography-based estimates of channel bathymetry for all areas

covered by water during the LiDAR acquisition 2. Surface Materials: Aerial photography and land-based estimation of surface materials for

the development of Manning’s “n” 3. Boundary Conditions: Inflows estimated by StreamStats and outflow elevations estimated

based on the normal depth and the slope of the FEMA profile as described in the Hydrology Section of this report.

4. Structures: a total of 17 structures are coded into the model. 4 were modeled as bridges with pressure flow and 13 were modeled as culverts in the Federal Highway Administration’s (FHWA) culvert analysis program HY-8. HY-8 is integrated with SMS-SRH2D.

Figure 11 through Figure 13 are graphical representations of the topography, boundary conditions, surface materials and structures that are integrated into the model. LiDAR-based topography was adjusted only in areas covered by water during acquisition. Adjustments were made by lowering the ground surface based on field visits and aerial photography-based estimates of channel bathymetry.


Data for culverts and bridges were collected through multiple sources. Figure 13 shows a map of the culverts and the gauge of the uncertainty of the data. Culverts and bridges circled green, with the exception of the Lye Brook/Richville Road culvert, were surveyed in the UVM study and subsequently adjusted to match the project datum based on the road centerline elevation. UVM elevations for the Lye Brook/Richville Road culvert did not agree with field observations, so Town Highway field reviewed the slope of the culvert and elevations were adjusted. Data for crossings circled purple were obtained through field measurements or in the case of the Railroad Bridge and Route 7 Bridge, old construction plans. Crossings circled red were estimated by GIS sources including aerial photography and a GIS database of Town maintained culverts. Crossings circled green represent the least uncertainty in data collection methods, whereas the red crossing represent the most uncertainty. Purple culverts were field verified but not surveyed and therefore have moderate uncertainty. Table 4 is a summary of culvert data used in the model.

Figure 11: The outer blue line represents the limits of the model. 2012 LiDAR topography is shaded from red (highest) to blue (lowest). Yellow dots and interconnected lines represent the locations of channels where estimated bathymetry was added to the model topography based on field and aerial photography observations. Blue arrows represent the inflow and outflow locations.


Figure 12: Materials map digitized from aerial photography. Each material is represented in the model by a different Manning’s “n”.


Figure 13: Map showing 17 modeled structures. Structures modeled as pressure flow are blue squares. Structures modeled as culverts are yellow dots. Outer circles represent relative data uncertainty. Green circles represent surveyed data, purple circles represent field measured data and red circles represent data estimated from available digital sources.


Culvert ID Culvert Name Culvert Size/Type

Diam., ft

Length, ft

Invert In, ft

Invert Out, ft Slope

Embed. (in)

1 Richville 16 ft 16' x8.25' Pipe Arch 5.5 78 647.8 646.2 0.020 6 2 Driveway 5 ft 5' CMP 5 40 652.1 651.8 0.009

3 Farm Bridge 26' x 6' Concrete Box 5.93 16 643.3 643.2 0.010 0.8

4 Culv. 0621 3' CMP 3 55 650.8 650.0 0.014 5 Culv. 0622 1.5' CMP 1.5 31 653.9 653.6 0.009 6 Culv. 0623 1.5' CMP 1.5 30 653.6 653.2 0.012 7 Culv. 0624 64"x43" Pipe Arch 5.5 38 655.4 655.3 0.001 6

8 Western Driveway 2' CMP 2 24 655.9 654.6 0.057

9 RR Crossing 2' HDPE 2 40 648.8 648.1 0.019

10 Farm Culvert 6'x3.2' Open Bottom Arch 3.17 13 646.0 645.9 0.010

11 Route 7 Culvert 3' CMP 3 160 711.0 704.0 0.044 12 Dry Culv. 0625 6' CMP 4 24 662.3 662.1 0.010 24 13 Upper RR 2.5' HDPE 2.5 24 658.0 657.0 0.042

Table 4: Culvert data as modeled.

4.3 Calibration

Calibration is an important step to ensure the model behaves in a similar fashion to observed data in the actual system. Model calibration provides further confidence that the results of alternatives analyzed in the model represent real-world scenarios. Initial model parameters are an estimate of field conditions based on engineering judgment. In calibration, those model parameters are adjusted within reason to match model results to field observations.

The calibration process also assists in identifying locations within the model where input data does not provide correct simulation of field conditions. Channels, materials and structures were adjusted within the model building and calibration process to match field conditions as closely as possible within the limits of available data.

As described above, the 2- and 100-year flood events also known as the 50% and 1% annual exceedance probability (AEP), respectively were selected as the calibration storms. Calibration markers at key locations were collected by surveying the Lye Brook Study Committee for recollection of Hurricane Irene and the summer storm, discussions with land owners adjacent to Lye Brook, and through observable field markers for the recent 2-yr storm event. The model was qualitatively calibrated by adjusting the Manning’s “n” for surface materials until the model generally matched observed flood conditions for both the 2- and 100-year events. Figure 14 shows calibration check locations. Table 5 shows the results of the calibration assessment.


Figure 14: Calibration check locations


Location Source Observations for 2-year storm

Ground Elev. Ft.

Water Surface Elev, ft.

Water Depth, ft

2 Vtrans Berm Field Observation Summer flood marker ~2 ft above Toe of Slope

712.6 714.3 1.7

3 Near Tire berm Field Observation ~3.5 ft flood marker 691.1 694.3 3.2 4 Water edge in yard Owner recollection location of water's edge 654.3 654.5 0.2 7 Richville Rd Between

culv. 0622 & 0623 (node 55701)

Town Highway recollections

Water overtopping between culvert 0622 & 0623 (node 55701)

656.6 657.2 0.2

Location Source Observations for 100-year storm

Elev. WSE Depth

1 Vtrans Berm Comment from Committee

Irene 1 ft below top of Old Vtrans Berm, berm raised 2 ft with reconstruction (per 10/17/2011 reconstruction sketch) Existing berm top 732.5 so prior top 730.5

730.5 728.9 -1.6

5 Richville Rd. flooding Town Highway recollections

Extent of water on road (N) Water covers NB lane 55 ft South

6 Richville Rd flooding Town Highway recollections

Extent of water on road (S) Water covers NB lane 156 ft South

7 Richville Rd Between culv. 0622 & 0623 (node 55701)

Town Highway recollections

Water overtopping between culvert 0622 & 0623 (node 55701)

656.6 658.1 1.46

Table 5: Calibration results at observed points.


4.4 FEMA FIS Data

NHI reviewed the current (2015) FEMA Flood Insurance Study (FIS) as well as prior studies for the Town and Village of Manchester. A summary of the review is presented below, highlighting several important differences between the FIS and the current NHI study. The original study for the Town of Manchester was prepared in 1976, but a copy was not located as part of this study and is not available at FEMA’s online site. That study was redone in 1985. Similarly, a study for the Village of Manchester was completed in 1975 and redone in 1986.

Flood boundaries for both the Town and Village were remapped in 2015 for the Countywide FIS for Bennington County.

For the Batten Kill, the following information was listed in the 2015 FIS. The hydraulic analysis was compiled for the 1976 and 1985 FIS studies using the HEC-2 hydraulic model. For the 2015 study, flows were recomputed using gage data and the hydraulics were rerun using the HEC-RAS model. The following table lists flow data for the Batten Kill as listed in the FIS reports. According to the FEMA FIS, flows are based on a drainage area-peak discharge relationship using the USGS gage described above on the Batten Kill.

Drainage Area, sq. mi. Q10, cfs

Q50, cfs

Q100, cfs

Q500, cfs

1985 FIS 75.1 3475 6950 8550 14150 57.6 3150 6275 7725 12775

2015 FIS Re-delineation 59.6 3463 5360 5240 8562

Table 6: FEMA FIS, Batten Kill flow history The Lye Brook hydrologic and hydraulic analyses are described in the 2015 FIS as re-delineation. This term specifically refers to remapping of prior FIS data onto new aerial photographs with no new analyses. According to the FEMA FIS, the hydraulic model was based on field survey of channel sections for the 1975 study with updated field survey for backwater analysis of structures as part of the 1985 study. The 1985 study does not mention which model was used for the hydraulic analysis, but HEC-2 would have been typical at the time. The 1985 study describes an August 1976 storm where the old Lye Brook Road Bridge was washed out and substantial damage occurred to the railroad tracks along the Batten Kill.

No new hydraulics were run for Lye Brook as part of the FEMA 2015 re-delineation. Flows are listed below.

Drainage Area, sq. mi. Q10, cfs

Q50, cfs

Q100, cfs

Q500, cfs

1985 & 2015 FIS 9.5 735 1475 1825 3000

Table 7: FEMA FIS, Lye Brook flow history


Notable in the FEMA analysis when comparing to this study: 1. Hydraulic models are based on surveys from 1976 and additional data from 1985. 2. Models were developed in either model HEC-2 or WSP2. 3. The 2015 study contains rerun of hydraulic model for Batten Kill using updated flows. 4. The 2015 study contains the 1985 model data for Lye Brook which does not include

Route 7 in its current location. 5. These conclusions are based on interpretation of limited information contained in the

2015 FIS for Bennington County. 6. Hydraulic models were run as 1-D flow which does not account for lateral flow

including overtopping or flow through Richville Rd or for flow through or over the railroad.

Flood profiles are shown in Figure 15 comparing FEMA FIS elevations to this study including 100-year flood level and stream bottom profile. Note erosion in the channel bed upstream of the Route 7 Bridge from 1985 to 2012 and deposition downstream of Route 7 by comparing the gray existing elevation line to the black FEMA elevation line. Table 8 highlights elevation changes at the FEMA cross sections, orange shading shows deposition and green shows erosion.

Figure 15: Comparison of FEMA model to Lye Brook 2D model, letters designate FEMA cross sections (Model Flow = 1800 cfs, FEMA Flow = 1825 cfs).

AB CDEFG

HI JK

L

MN

O

640

660

680

700

720

740

760

780

0 1000 2000 3000 4000 5000 6000 7000

Model GroundModel 100-yrFEMA GroundFEMA 100-yr

Rich

ville

Roa

d

Driv

eway

Farm

Tra

il

Railr

oad

Brid

ge

Old

Lye

Bro

ok R

oad

Rout

e 7

Brid

ge


Section Dist.

US (ft)

FEMA Ground (’76/’85 Survey

- ft)

NHI Model Ground (2012 LiDAR)

Ground Diff (ft)

FEMA 100-yr

(ft)

NHI Model 100-yr

(ft)

100-yr Head Diff.

FEMA-Model (ft)

A 252 642.5 643.0 -0.5 650.0 652.8 -2.3 B 295 642.6 643.7 -1.1 650.3 652.9 -1.5 C 565 643.2 644.6 -1.4 650.6 653.2 -1.3 D 600 643.5 644.6 -1.1 650.6 653.4 -1.7 E 615 643.5 644.7 -1.2 652.1 653.4 -0.2 F 1867 647.2 647.4 -0.2 653.7 654.1 -0.2 G 1929 647.4 647.4 0.0 661.2 654.1 7.2 H 2736 653.2 653.1 0.1 661.5 657.0 4.4 I 2776 653.3 653.5 -0.2 661.5 657.5 4.2 J 3376 657.4 658.9 -1.6 661.6 660.0 3.1 K 4001 662.3 665.2 -2.9 664.4 667.1 0.2 L 4761 672.9 676.3 -3.5 676.3 680.3 -0.5 M 6285 711.3 713.2 -1.9 716.1 718.4 -0.4 N 6319 711.4 714.5 -3.0 718.2 719.4 1.8 O 7394 767.9 752.6 15.3 772.8 757.1 0.4

Table 8: Comparison of FEMA model to LiDAR-based 2D model at Lye Brook FEMA Cross Sections (orange deposition from 1985 to 2012, green erosion).

5.0 Alternatives Analysis Several alternatives were evaluated to understand the existing hydraulic system and how modifications to structures may impact this system. Each is presented below. The accompanying figures show the extent of flooding for the 100-year event for each alternative. The extents of flooding for the two-year event are shown in the Appendix. Tables 10-12 show a comparison of the alternative results at key locations within the model.

5.1 Alternative 1: No Action

The No Action or existing conditions alternative model was built using the 2012 LiDAR base topography adjusted for areas covered by water during LiDAR collection. Available structure information was input as described above. The model was calibrated by adjusting materials parameters as described. The 2012 LiDAR data and the No Action Model do not include the small berm repair completed in 2015 by the Town Highway Dept.

The No Action alternative provides insight into the flooding problems observed by the Town of Manchester. Figure 17 is a map of the water depth with key findings highlighted. The primary observation is that the base of Lye Brook is perched 3-5 ft above the adjacent land to the west (Figure 18). During average to high flow events, Lye Brook loses water over the banks or low berms to the adjacent wetland. This water in the wetland either continues flowing south through a 5-ft culvert under the driveway or flows west under Richville Road via culverts or overtops Richville Road. The driveway also limits flow thereby creating a bathtub effect where water


surface elevations upstream of the driveway are up to two feet higher than those on the downstream side (Figure 19). Also noteworthy is that on the Batten Kill, the railroad limits flood flow from the Batten Kill to the wetland east of the railroad, much like Richville Road limits flow from Lye Brook into that same wetland complex.

Figure 16: 2-Year, No Action water depth color shading


Figure 17: 100-Year, No Action water depth color shading, black arrows show generalized flow vectors. Figure 18 is a cross section of the model upstream of the eastern driveway. The section shows the wetland channel west of Lye Brook is lower in elevation than Lye Brook. Figure 19 is a profile through the 5 ft driveway culvert. The cross section and profile show the water elevation (head) difference caused by Richville Road and the driveway. Table 10 shows key culverts in the study area and the model results. Yellow highlights show culverts that likely overtop and therefore likely do not meet minimum VTrans hydraulic standards.

Cross section (Figure 18)

Profile (Figure 19)

Berm Overtopping (Purple)

Richville Overtopping (Yellow)


Figure 18: Alt 1 No Action Cross section of model from west to east located 350 ft north of the eastern driveway.

Figure 19: Alt 1 No Action Profile of flow through 5 ft driveway culvert from upstream to downstream.

Lye Brook

Richville Rd.

Berm

Railroad

Batten Kill

Wetland Channel

Driveway

~4 ft head difference

~2 ft head difference


5.2 Alternative 2: 10-Foot Berm

Alternative 2 assumes a full reconstruction of the berm to mimic the VTrans section of the berm. The berm is assumed to be 10 feet in height from the base of the channel to the top of the berm. For modeling purposes and because the shape and construction of such a berm is extremely problematic in the field, this model assumes a near vertical berm. Figure 20 shows the proposed extent of the 10-ft berm.

Figure 20: Extent of 10-foot berm in Alternative 2 (yellow). The berm effectively eliminates most flow into the wetlands to the west. This may affect the viability of the adjacent Class 2 wetlands. At the driveway, all flow cannot successfully pass under the driveway bridge and therefore backs up into the wetland which is lower in elevation than Lye Brook. Richville Road overtopping north of the driveway is less than under existing condition, but overtopping at culvert 0621 (Richville Road downstream of the driveway) during the 100-year event is much more significant, 194 cfs verses 680 cfs with the berm. Flow velocities at the driveway bridge are erosive and would necessitate additional armoring at the existing driveway bridge.

Streambed aggradation would likely continue with the 10-foot berm. Continuing the aggradation rate of the last 30 years forward another 30 years, the 10-ft berm would effectively be reduced to 6-7 feet in height and the channel bed would be even further elevated above the adjacent land. In 50 years the Town would likely be facing a similar scenario to today where the effective height of the berm is no longer high enough to prevent flooding. The aggradation patterns may well be shifted downstream, and the channel below the driveway may well become the primary depositional area.


Figure 21: 100-year Flood Depth Alternative 2, 10-foot berm.

5.3 Alternative 3: Replace 5-foot Driveway Culvert with Bridge

Alternative 3 replaces the 5-foot driveway culvert with a 33-ft bridge. The bridge would be designed to carry a bank full width flood flow that would likely escape Lye Brook under a berm breach or channel avulsion scenario. This concept assumes Lye Brook will eventually abandon its existing channel that is higher in elevation than the adjacent lands and the current path of least resistance is through the existing wetland channel to the existing 5-foot culvert. Figure 22 shows the proposed alignment of the bridge and flood chute.

Richville Overtopping

Dry Wetlands


Figure 22: Alternative 3 Proposed Bridge Location (yellow line) and Flood Chute (arrow). Installing a 33-ft wide bridge at the existing 5 ft culvert location reduces but does not eliminate overtopping of Richville Road north of the driveway. During the 100-year event the new bridge conveys the majority of Lye Brook flow. South of the driveway, there is additional flooding and overtopping of Richville Road as more flows are channeled toward that crossing.

Adding the new driveway bridge eliminates the head drop across the driveway, but Richville Road still creates a significant barrier to flow. Richville Road overtops at culverts 0622 and 0623 (upstream of driveway) but only 212 cfs flows over the road verse 595 cfs in the existing condition. Since more of the flow is conveyed south of the driveway, there is additional overtopping on Richville Road at culvert 0621. Under existing conditions 192 cfs flowed over the road verses 562 cfs with the new bridge. Under this scenario, the existing 16 foot Richville Road culvert has a flow depth of 8.2 feet and a headwater to depth ratio of 1.0 indicating the culvert meets hydraulic design regulations even though it does not meet current environmental standards.


Figure 23: 100-year water depth Alternative 3, 33-foot Driveway Bridge

5.4 Alternative 4: Resize Existing and Add Richville Road Culverts

Alternative 4 looks to address Richville Road overtopping by increasing the size of the existing Richville Road culverts (Figure 24). The goal of the increased size is not only to increase the amount of flow that can pass under Richville Road, but to prevent culvert clogging and road overtopping during flood events. Figure 24 shows the potential locations for replacement culverts.

Increased Flow


Figure 24: Alternative 4 proposed upgraded culvert locations (green). Each culvert replacement is assumed to be a 12 ft span by 4 ft rise concrete box culvert embedded 12 inches. It is estimated that each of these culverts under ideal conditions can convey approximately 260 cfs. The inverts of each culvert remain the same as existing conditions. Multiple model run iterations were completed to minimize Richville Road overtopping. Ultimately, the model indicated a need for five culverts north of the driveway to mostly eliminate overtopping and placed as follows: two at 0622, two at 0622.5 and one at 0623. Only one culvert was required south of the driveway, at 0621.

Under existing 100-year event conditions, 655 cfs flows across Richville Road (overtopping plus flow through culverts 0622 & 0623). With the five new culverts, 1138 cfs passes through Richville Road. Most of this flow is through the culverts, but there is still some overtopping. This is because the existing driveway so restricts downstream flow that flow to the west, under and over Richville Road, is the path of least resistance. The wetland on the west side of Richville Road becomes the limiting condition and increases approximately 1-foot with the

0624

0623

0622

0621

0622.5


added flow. Moreover, the modeling effort showed that as the number of culverts increased, the effectiveness of each culvert diminished due to high water surface elevations at the wetland outlet.

Figure 25: 100-year water depth, Alternative 4, Resized Richville Culverts

5.5 Alternative Analysis Summary

The goal of this project was to assess alternatives that would be the most effective and sustainable solution to minimize flooding on Richville Road while impacting the fewest properties. Four alternatives were analyzed through this project: 1) No Action, 2) 10-Foot Berm, 3) Replace 5-foot Driveway Culvert with Bridge and Create Flood Chute, and 4) Resize Existing Richville Road Culverts.

Based on this goal, Alternative 2: 10-Foot Berm is ruled out as it does not decrease the total flow across Richville Road and is not environmentally or economically sustainable in the long term.

Increased Flow & Water surface elev.


Alternative 3: New Driveway Bridge is the most sustainable alternative considering the natural evolution of Lye Brook, though it has the effect of shifting the location of Richville Road flooding downstream without reducing the total flow across the road. Alternative 3 combined with a replacement of the 16 ft Richville Road culvert may be the most environmentally sustainable solution. However Alternative 3 involves Town construction on personal property and shifts flooding to developed downstream properties, making Alternative 3 logistically and economically unviable at this time.

Alternative 4: Resized and Add Richville Culverts significantly reduces Richville Road flooding but would change flow patterns from the existing condition and effectively reroute most of the Lye Brook flows to the west of Richville Road. In this analysis Alternative 4 represents the maximum number of culverts require to mitigate flooding. A smaller number of culverts would mitigate some flooding but not reroute flows as significantly. Given the options, a modified Alternative 4 seems to be the most economical and sustainable solution immediate solution.

100-Year Road Overtopping

Location

Alt 1 Alt 2 Alt 3 Alt 4 Flow (cfs)

Flow (cfs)

Flow (cfs)

Flow (cfs)

Richville Road - North of Driveway 595.1 162.1 211.7 6.9 Richville Road - South of Driveway 193.6 680.2 562.0 0.0

Richville Total Overtopping Flow 788.7 842.3 773.7 6.9 Flow Overtopping Driveway 183.8 155.1 60.6 5.8

Table 9: 100-year Richville Road Area Roadway Overtopping


100-Year 100-Year 100-Year 100-Year Alt 1 - Existing Alt 2 - Berm Alt 3 - New Driveway Bridge Alt 4 - New Culverts

Culvert Name Flow (cfs)

US Elev (ft)

DS Elev (ft)

Diff. (ft)

Flow (cfs)

US Elev (ft)

DS Elev (ft)

Diff. (ft)

Flow (cfs)

US Elev (ft)

DS Elev (ft)

Diff. (ft)

Flow (cfs)

US Elev (ft)

DS Elev (ft)

Diff. (ft)

Richville 16ft 532.0 655.4 654.1 1.3 658.4 656.1 654.2 1.9 634.9 656.0 654.2 1.8 294.7 654.5 654.1 0.4 Driveway Bridge 447.9 1031.6 287.1 313.2 Driveway 5 ft (Bridge Alt 3) 139.6 658.3 656.2 2.1 73.4 657.6 657.1 0.5 976.5 657.6 657.2 0.3 118.8 657.6 655.5 2.0 Culv 0621 37.7 655.6 654.2 1.4 65.4 656.3 654.2 2.1 58.6 656.1 654.2 1.9 182.6 654.4 654.3 0.2 Culv 0622 39.0 658.2 655.7 2.5 18.8 657.6 655.1 2.5 22.1 657.7 655.3 2.5 352.4 656.9 656.6 0.3 New Culv 622.5 - - - - - - - - - - - - 550.4 657.6 656.7 0.8 Culv 0623 21.3 658.2 655.9 2.3 12.4 657.6 655.1 2.5 12.5 657.7 655.2 2.6 228.5 657.5 656.9 0.6 Culv 0624 74.2 660.4 659.0 1.4 0.0 74.2 660.4 659.0 1.4 201.6 660.1 659.4 0.7 2-Year 2-Year 2-Year 2-Year Alt 1 - Existing Alt 2 - Berm Alt 3 - New Driveway Bridge Alt 4 - New Culverts

Culvert Name Flow (cfs)

US Elev (ft)

DS Elev (ft)

Diff. (ft)

Flow (cfs)

US Elev (ft)

DS Elev (ft)

Diff. (ft)

Flow (cfs)

US Elev (ft)

DS Elev (ft)

Diff. (ft)

Flow (cfs)

US Elev (ft)

DS Elev (ft)

Diff. (ft)

Richville 16ft 313.3 653.4 652.8 0.6 397.9 653.9 652.9 1.0 345.7 653.6 652.9 0.8 148.5 652.8 652.6 0.2 Driveway Bridge 107.0 386.8 107.0 117.1 Driveway 5 ft 104.4 657.2 655.3 1.9 -5.7 655.1 655.1 0.0 280.0 655.8 655.8 0.0 49.8 655.5 654.9 0.6 Culv 0621 24.4 653.6 652.0 1.6 32.2 654.1 652.7 1.4 27.1 653.8 652.7 1.1 4.2 652.9 652.9 0.0 Culv 0622 11.4 657.1 654.6 2.6 0.0 656.3 653.9 2.5 0.0 656.3 653.9 2.4 5.4 656.2 654.9 1.3 New Culv 622.5 - - - - - - - - - - - - 109.7 655.7 655.6 0.1 Culv 0623 12.1 657.2 654.7 2.5 5.6 655.1 654.5 0.6 8.6 655.9 654.6 1.3 49.3 655.4 655.3 0.1 Culv 0624 54.1 659.6 658.9 0.8 0.0 657.9 658.5 -0.6 54.1 659.6 658.9 0.8 121.6 659.4 659.2 0.2

Table 10: Summary table of flows and elevations at key study culverts, shaded cells represent locations of roadway overtopping.


6.0 Conservation: Town should consider property buyouts as a means of preserving land in preparation for the natural evolution of Lye Brook migrating to a lower elevation. Figure 28 shows properties subject to 100-yr flooding mapped with Class II wetlands and potential flow paths of Lye Brook.

Given historical evolution patterns of Lye Brook, its current elevation above its adjacent wetland, and the condition of the old berm, Lye Brook may during some future storm avulse and change course. If an avulsion were to happen, Lye Brook could reroute into the wetland behind Cass Terrace and flow parallel to Richville Road in the existing mapped Class II wetland area. Lye Brook may rejoin the existing Lye Brook at some point upstream of the 16 ft Richville Road culvert, but this is difficult to predict. Predicting the future path of a river following an avulsion is a gross estimation and would depend significantly on flow conditions at the time.

Figure 26: Properties subject to 100-yr Lye Brook Flooding with property lines (Purple) and Class II Wetlands (blue hatch). Potential Lye Brook future flow routes (blue) if Lye Brook changes course from its current location.


Many, but not all of the properties subject to Lye Brook flooding have development restrictions as Class II wetlands. These parcels mapped as Class II wetland have a 50 ft buffer and are regulated under the State of Vermont Wetlands Rule. Most development activities which would be proposed in this general area would require a wetland permit from the State, and depending on the area of impact, a permit from the US Army Corps of Engineers. Development potential is limited on these parcels with some potential uses, such as a driveway to an upland area for a building envelope are suitable, depending on associated impacts within the wetland boundary and its 50 foot buffer.

The natural vegetation of the parcels is well established and will help slow down flood waters and reduce erosion throughout this overall floodplain/wetland system. The area is primarily mapped as containing hydric soils (poorly drained) limiting the installation of residential wastewater systems. Additionally, a portion of the wetland complex is associated with the floodplain of Lye Brook and a mapped Special Flood Hazard Area (SFHA) subject to local and federal regulations. Very little restoration, if any, would be needed in the wetland other than reconnecting Lye Brook to the floodplain/wetland system by eliminating the berm or allowing it to naturally degrade.

While future development potential in the Lye Brook flood zone is limited by existing regulations, the Town may want to consider purchasing existing properties to help mitigate future conditions as described above.

7.0 Future Alternative Analysis and Design Proposal: Based on the above analysis, Alternative 4 Resized Richville Culverts appears to be the most economical immediate solution to address Lye Brook’s evolution and Richville Road flooding. Alternative 4 should be modified by reducing additional culverts and only replacing the existing culverts with larger culverts. Additional hydraulic analysis of Alternative 4 should be completed prior to final design or construction to determine the effectiveness of these modifications prior to construction expenditures.

The Town should investigate property buyouts using FEMA Flood Hazard Mitigation funding as part of a future analysis in recognition that continued aggradation of the channel and the wetland over time may increase flood risks to some properties even with the above proposed infrastructure improvements. Properties most impacted by the flood events are highlighted in Figure 27.

A combination of Alternatives 3 and 4, as shown in Figure 28, four new 12’ x 4’ box culverts and the new driveway bridge may provide the greatest long range benefit, but has significant limitations including Town development on private property and increased downstream flooding of existing development. Long term this design concept would plan for the evolution of Lye Brook and minimize future modifications to Lye Brook, but would require acquisition of properties by the Town before being seriously considered.


Any future analysis should also include replacing the existing 16-ft Lye Brook culvert under Richville Road with one that fully spans the channel.

Figure 27: Properties most impacted by Lye Brook flooding (shaded green).


Design Plans and Cost Estimates

Preliminary, 30% design plans and cost estimates were developed for the proposed 12’ x 4’ culverts under Richville Road and a 33-foot driveway bridge. Preliminary design plans and full cost estimates are included in the appendix. Each 12’ x 4’ culvert is expected to cost $126,000 installed. Replacing them at the same time and ordering several of the same culvert may result in cost savings. The estimated cost of adding a 33-foot bridge at the driveway is $111,000.

Figure 28: Long Term Design Alternative

8.0 Public Participation: This feasibility study was completed in consultation with the Lye Brook Study committee with representatives from the following positions:

• Town of Manchester, Town Manager • Town of Manchester, Public Works Director • DEC River Management Engineer, Southwestern Region • Bennington County Conservation District, Regional Manager • Bennington County Regional Commission, Emergency Management Program • Bennington County Regional Commission, Environmental Program Manager • Northstar Hydro, Inc., Principal and Senior Engineers


• Ripples Natural Resources, Principal Engineer

The feasibility study was presented to the Town of Manchester for community input on May 2, 2019.

9.0 Conclusion: Northstar Hydro, Inc. prepared a hydrologic and hydraulic analysis including a 2-D flow model of Lye Brook and the Batten Kill in the Town of Manchester, Vermont in an effort to understand the relationship between the Lye Brook berm and flooding on Richville Road. The study area was an active alluvial fan until the late 1800’s when Lye Brook was channelized to its existing location. The upstream portions of the model start upstream of the Route 7/Lye Brook Bridge and just downstream of the wastewater treatment plant on the Batten Kill. The downstream end of the model coincides with Batten Kill FEMA section AW downstream of the Lye Brook/Batten Kill junction. The model was calibrated for the 2-year and 100-year storm events based on field observations. Field observations and available data show that the base of Lye Brook is actively aggrading and has risen 3-4 ft in a period less than 30 years effectively lowering the berm by the same amount.

The analysis of the existing conditions (Alternative 1: No Action) shows that Lye Brook is perched above the adjacent wetland. The berm has deteriorated or been effectively lowered through aggradation in many locations such that non-storm level streamflows flow from Lye Brook to the adjacent wetland complex. The eastern driveway that crosses Lye Brook and Richville Road creates a bathtub effect ponding water upstream of that intersection such that Richville Road and the driveway overtop during the 2-year event.

Three alternatives were analyzed to address flooding impacts. Alternative 2 involves raising the berm to a height of 10-feet to match the existing VTrans berm just downstream of Route 7. Raising the berm limits flow to the adjacent wetland but does not prevent the overtopping of Richville Road or the driveway. Raising the berm would likely be a temporary solution due to constant streambed aggradation and creates environmental impacts by limiting flow to the adjacent wetlands.

Alternative 3 involves replacing the 5-foot driveway culvert with a 33-foot bridge designed to pass all of Lye Brook in the future. Alternative 3 assumes that at some point in the future, Lye Brook will avulse, leaving its existing channel for the lower wetland channel. The new bridge would be designed to carry that flow downstream of the driveway. With additional flow to the wetland south of the driveway, Richville Road would flood more significantly south of the driveway. Alternative 3 at this point involves construction on private property and was therefore deemed unviable.

Alternative 4 involves replacing the existing culverts along Richville Road with a series of substantially larger box culverts. These culverts would minimize flooding on Richville Road, but would not prevent it. The driveway would remain a barrier to flow and under this scenario would effectively force a majority of Lye Brook’s flow through Richville Road to the western wetlands. A modified version of Alternative 4 was deemed the most economic and sustainable alternative for the immediate future.


The analysis highlights the challenges of containing flood flows on a relatively flat alluvial fan. Under ideal conditions without development or road crossings, the stream would regularly switch channels as it deposits materials eroded from the upstream mountains. The alternatives analysis suggests that the best course of action seems to be a combination of Alternatives 3 and 4 in the long term. Additional capacity through the driveway is needed and based on current landscape evolution, the location of the existing 5-foot culvert appears to be the future location of Lye Brook. Under this scenario some additional capacity under Richville Road will still be required to prevent overtopping. Sketches and cost estimates of Alternatives 3 and 4 were developed through the 30% design level and are included in the appendices.

The Town should also look at property acquisition to work towards the long-term mitigation plan described above. In the immediate future, the existing culverts on Richville Road should be replaced with sustainably larger culverts to prevent overtopping of Richville Road during smaller storm events.

References Andrew Carter, Yifan Li, Xuguang Shen, & Carli Shroyer, University of Vermont CE 185/186 Capstone Design Project, Lye Brook Flood Mitigation Design Project, May 1, 2016.

Atlas of Bennington County Vermont, Manchester Business Listing, 1869.

ESRI ArcMap, ArcGIS Desktop, Version 10.5 for Desktop

Federal Emergency Management Agency. Flood Insurance Rate Map Panel 50003C0191D and 50003C0193D. Bennington County, December 2, 2015.

Federal Emergency Management Agency. Flood Insurance Study. Bennington County, Vermont. 50003CV000A, December 2, 2015.

Historical air photographs from Google Earth on-line library.

National Operational Hydrologic Remote Sensing Center (NOHRSC), Precipitation Data from VT-BN-7 - MANCHESTER 2.8 ENE, VT for August 1-5, 2018.

Stormwater Modeling System (SMS), SRH-2D, v12.3.2, August, 2018.

U. S. Dept. of the Interior, U. S. Geological Survey. Estimated Discharges at Selected Annual Exceedance Probabilities for Unregulated Rural Streams in Vermont. SIR 2014 5078.

USGS Gage record, Gage Number 1329000, Batten Kill at Arlington, Vermont. https://waterdata.usgs.gov/nwis/dv/?referred_module=sw&site_no=01329000

USGS StreamStats v4.2.1, https://streamstats.usgs.gov/ss/

USGS Topography Maps, 1900, 1968, 1997 & 2015.

USGS. High Water Marks from Flooding in Lake Champlain from April through June 2011 and Tropical Storm Irene in August 2011 in Vermont. Data Series 763. Prepared in cooperation with FEMA.


Vermont Agency of Transportation, Hydraulics Manual, May 28, 2015

Vermont Agency of Transportation, VTR Valuation Sheets, Bennington to Burlington, from ArcGIS Online, May 31, 2018.

Vermont Center for Geographic Information, Bare Earth Lidar Tiles: 18TXN570780, 18TXN570795, 18TXN585780, 18TXN585795. 2012.

Vermont Center for Geographic Information, Hurricane Irene Data, https://vcgi.vermont.gov/data/irene

APPENDICES Figures 28-30: 2-year Water Depths for Alternatives 2-4

30% Design Alternatives and Cost Estimates

UVM CAD Drawings


https://vcgi.vermont.gov/data/irene

Figure 29: 2-year water depth Alternative 2, 10-foot berm


Figure 30: 2-year water depth Alternative 3, New 33-foot Driveway Bridge


Figure 31: 2-year water depth Alternative 4, Resized Richville Road Culverts


LYE BROOK

MANCHESTER, VERMONT

SH

EE

T ID

__

__

RICHVILLE ROAD TYPICAL

CULVERT SECTION

CLIENT:

PROJECT & LOCATION::

SHEET TITLE:


AutoCAD SHX Text

RICHVILLE ROAD ELEVATION TO BE MAINTAINED

AutoCAD SHX Text

6" BITUMINOUS PAVEMENT

AutoCAD SHX Text

12'X4' PRECAST CONCRETE BOX CULVERT (TYP)

AutoCAD SHX Text

BEDDING

AutoCAD SHX Text

EXISTING 18" HDPE CULVERT (TYP)

AutoCAD SHX Text

BOX CULVERT TYPICAL SECTION NOT TO SCALE

AutoCAD SHX Text

LIMIT OF PAVEMENT REPLACEMENT (TYP)

AutoCAD SHX Text

GRANULAR BACKFILL OF STRUCTURES

AutoCAD SHX Text

C-1

AutoCAD SHX Text

1 of 2

** OPINION OF PROBABLE COST **Lye Brook, Manchester Center VT

BCRC

RICHVILLE ROAD BOX CULVERTBased on January 2019 Concept Plans

DESCRIPTION OF THE ITEM UNIT OF ESTIMATED ESTIMATEDMEASURE QUANTITY COST

GENERAL1 Mobilization ( 8% ) of remaining construction costs LS 1 6,457$2 Survey Layout LS 1 1,000$3 Clearing and Grubbing LS 1 775$4 Erosion Prevention and Sediment Control LS 1 500$5 Site Restoration LS 1 500$6 Traffic Control (assume signs, flaggers, 3 days) LS 1 2,400$7 GENERAL SUBTOTAL 11,632$89 CROSSING10 Precast Concrete Box Culvert 12x4x48' LS 1 63,500$11 Common Excavation (203.15) CY 453 4,467$12 Crushed Stone for bedding (301.35) CY 35 1,123$13 Stone Fill Type III (613.12) CY 2 91$14 Granular Backfill for Structures (204.30) CY 80 3,224$15 -$16 -$1718 CROSSING SUBTOTAL 72,404$1920 ROADWAY21 Bituminuous Concrete Pavement (406.25) TONS 25 3,133$22 -$23 -$24 -$25 -$26 -$27 -$28 ROADWAY SUBTOTAL 3,133$2930 CONSTRUCTION SUBTOTAL 87,169$31 ADD 20% CONTINGENCY 17,434$32 SUB-TOTAL 104,603$3334 USE 105,000$3536 TESTING AND ENGINEERING37 Material Testing LS 1 2,000$38 Survey and Final Engineering LS 1 10,000$39 LS 1 2,000$40 LS 1 3,000$41 TESTING AND ENGINEERING SUBTOTAL 17,000$42 ADD 20% CONTINGENCY 3,400$43 SUB-TOTAL 20,400$4445 USE 21,000$464748 Prepared: 2/14/2019 CONSTRUCTION SUBTOTAL 105,000$49 Printed: 2/14/2019 TESTING AND ENGINEERING SUBTOTAL 21,000$50 Prepared by: MTM51 TOTAL 126,000$

10,000$

1,000.00$

UNITPRICE

6,456.95$

45.26$

775.00$500.00$500.00$

2,400.00$

63,500$9.86$

32.09$

40.30$

2,000$

127.86$

Bid - Phase Engineering 2,000$Construction - Phase Engineering 3,000$

This estimate is our professional opinion of probable construction cost. Actual costs may differ due to factors over which Ripple has no control, including the costand availability of labor, equipment, and materials, market conditions, or the Contractor's method of pricing. Ripple makes no warranty, express or implied, withrespect to the accuracy of this opinion of probable cost relative to actual costs.


4' SCOUR PROTECTION

33' SPAN

5' CLEAR

OPENING

LY

E B

RO

OK

MA

NC

HE

ST

ER

, V

ER

MO

NT

SHEET ID

____

DR

IV

EW

AY

B

RID

GE

TY

PIC

AL S

EC

TIO

N

CL

IE

NT

:

PR

OJE

CT

&

L

OC

AT

IO

N::

SH

EE

T T

IT

LE

:


AutoCAD SHX Text

DRIVEWAY BRIDGE TYPICAL SECTION NOT TO SCALE

AutoCAD SHX Text

EXISTING GRAVEL DRIVE

AutoCAD SHX Text

CAST IN PLACE REINFORCED CONCRETE ABUTMENT (TYP)

AutoCAD SHX Text

NAIL-LAMINATED WOOD DECK WITH WEARING TREADS

AutoCAD SHX Text

STEEL STRINGERS

AutoCAD SHX Text

GRANULAR BACKFILL FOR STRUCTURES

AutoCAD SHX Text

RESTORED STREAMBED MATERIAL

AutoCAD SHX Text

LIMITS OF EXCAVATION

AutoCAD SHX Text

BEDDING 18" THICKNESS

AutoCAD SHX Text

C-2

AutoCAD SHX Text

2 of 2

** OPINION OF PROBABLE COST **Lye Brook, Manchester Center VT

BCRC

PRIVATE DRIVE NEW BRIDGEBased on January 2019 Concept Plans

DESCRIPTION OF THE ITEM UNIT OF ESTIMATED ESTIMATEDMEASURE QUANTITY COST

GENERAL1 Mobilization ( 8% ) of remaining construction costs LS 1 5,505$2 Survey Layout LS 1 1,000$3 Clearing and Grubbing LS 1 750$4 Erosion Prevention and Sediment Control LS 1 500$5 Control of Water LS 1 1,000$6 Site Restoration LS 1 500$7 Traffic Control (foot traffic for homeowner) LS 1 500$89 GENERAL SUBTOTAL 9,755$

1011 CROSSING12 Cast In Place Reinforced Concrete Abutments CY 25 15,000$13 Prefab Steel Stringers, Rail, and Wood Decking (delivered) LS 1 30,000$14 Common Excavation (203.15) CY 415 4,092$15 Crushed Stone for bedding (301.35) CY 22 706$16 Granular Backfill for Structures (204.30) CY 85 3,426$17 Assembly and Installation of Structure DAY 2 6,000$18 Stream Channel Construction/Restoration DAY 1 3,000$19 Disposal of Existing Structure LS 1 1,000$2021 CROSSING SUBTOTAL 63,223$2223 ROADWAY24 Crushed Gravel for Roadway CY 42 1,344$25 -$26 -$27 -$28 -$29 ROADWAY SUBTOTAL 1,344$3031 CONSTRUCTION SUBTOTAL 74,323$32 ADD 20% CONTINGENCY 14,865$33 SUB-TOTAL 89,187$3435 USE 90,000$3637 TESTING AND ENGINEERING38 Material Testing LS 1 2,000$39 Survey and Final Engineering LS 1 10,000$40 LS 1 2,000$41 LS 1 3,000$42 TESTING AND ENGINEERING SUBTOTAL 17,000$43 ADD 20% CONTINGENCY 3,400$44 SUB-TOTAL 20,400$4546 USE 21,000$474849 Prepared: 2/14/2019 CONSTRUCTION SUBTOTAL 90,000$50 Printed: 2/14/2019 TESTING AND ENGINEERING SUBTOTAL 21,000$51 Prepared by: MTM52 TOTAL 111,000$

1,000.00$

UNITPRICE

5,505.39$

600$

9.86$32.09$

750.00$500.00$

1,000.00$500.00$500.00$

30,000.00$

This estimate is our professional opinion of probable construction cost. Actual costs may differ due to factors over which Ripple has no control, including the costand availability of labor, equipment, and materials, market conditions, or the Contractor's method of pricing. Ripple makes no warranty, express or implied, withrespect to the accuracy of this opinion of probable cost relative to actual costs.

10,000$

40.30$

2,000$

Bid - Phase Engineering 2,000$Construction - Phase Engineering 3,000$

3,000.00$3,000.00$1,000.00$

32.00$


Northbound Grass Edge

Northbound Road Edge

Road Centerline

Southbound Grass Edge

Culvert InletElev: 994.07 ft

Culvert OutletElev: 993.70 ft

Northbound Grass Edge Northbound Road Edge

CenterlineSouthbound Road Edge


Culvert OutletElev: 989.495ftCulvert Inlet

Elev: 997.12 ft

Southbound Road Edge

Culvert 0624

Culvert 0623

Telephone PoleElev: 999.69 ft








Bridge InletElev: 992.775 ft

Bridge OutletElev: 992.73 ft

Center of Driveway 2


Culvert InletElev: 992.34 ftNorthbound Road Edge


Centerline

Southbound Grass EdgeSouthbound Road Edge

Northbound Road Edge


Centerline


Southbound Road Edge

Culvert 3 Culvert 4

Culvert 5


lye brook feasibility analysis draft€¦ · lye brook originates in steep mountainous terrain east...

Documents