hydraulic assessment - gray brook - hillsborough, nb - 2014.04.14

CANADA

File No. 131-0M|141185

April 1 4,2014

Hillsborough Floodwaters Groupc/o Bruce Alcorn2887 Main StreetHillsborough NB E4H 2X8

Dear Mr. Alcorn:

As requested in your email of March 9,2014 please find enclosed a copy of the "GrayBrook Hydraulic Assessment" report prepared by R.V. Anderson Associates Limited.

Once reviewed please contact Mr. Robert Sharpe, Director of Design at 453-3939, toarrange a meeting with your group and representatives from my Department.

Sincerely,

C l^a*ssl-li -'--zClaude WilliamsMinister

Encl.

c.c. Hon. Michael Olscamp - Minister of Dept. of Agriculture, Aquaculture and FisheriesPatrick Armstrong, Mayor of HillsboroughWayne Steeves, MLA - AlbertCharles Boudreau, District Engineer - MonctonRobert Sharpe, Director of Design

Minister/MinistreTransportation and lnfrastructure/Transports et lnfrastructure

P.0. Box/(.P 6000 Fredericton New Brunswick/Nouveau-Brunswick E3B 5H1 (anada

&www.gnb.ca

New Brunswick Departmentof Transportation and lnfrastructureMarch 2014

GRAY BROOK HYDRAULIC ASSESSMENT

RVA No. 112319

Final Report


HILLSBOROUGH, NB

FINAL REPORT

Prepared for:

New Brunswick Department of Transportation and lnfrastructure

-This report is prctected by copyright and was prcpared by R.V. Anderson Associafes Linited for the accout t of the New BrunswickDepaftment of Tnnsportation and lnfnstrudurc. ft shail not be copied without permrsslrn The mateial in it /.eflecls our bestjudgment in light of the information available to R.V. Anderson Assocrales Limited at the time of prcpantion. Any use which a thitdpafty makes of this rcpoft, or any rcliance on or decisions to be made based on it, are the rcsponsibiry of such third parties. R.V.Anderson Associates Limitecl a@epfs no responsi'bility for damages, if any, suffered by any thid paty as a rcsuft of decisbns madeor actions based on this rcpoft."

R.V. AndersonAssociates Limitedangrneefing envrronmsnl rnfraslruclure

445 Urquha.t Crescent FredenclonNew Brunswrcl E38 SKit CanadaTel 506 455 2888 Fax 506 455 0193www rvanderson com

RVA 112319

March 10,2014

New Brunswick Department of Transportation and lnfrastructure TOC 1-1


TABLE OF CONTENTS

Paoe

!NTRODUCT|ON.......... ..............1-1

Background ................ ..............1-1

1.0

1.1

2.0

2.1

3.0

3.1

3.1.1

3.1.2

3.2

3.2.1

3.2.2

3.2.3

3.2.4

4.0

4.1

4.1.1

5.0

5.1

5.2

5.2.1

5.2.2

5.2.3

5.2.4

5.3

5.3.1

5.3.2

5.3.3

6.0

7.0

8.0

scoPE oF woRK..

Description........

GRAy BROOK WATERSHED.................. ......3-l

Description and Physiography..... ........3-1

Hydraulic Structures ................3-5

Gray Brook Aboiteau ..................3-5

Route 114 Culverts .....................3€

Floodplain and ln-line Storage Upstream of Route 114 ............ ...........3-7

Floodplain and ln-line Storage Downstream of Route 114............. ......3-9

GRAY BROOK HYDRAULIC MODEL ..........4.1

Model Validation.....

Gray Brook Hydrology .....5-1

Gray Brook Hydraulics. ............5-2

Petitcodiac River Tides. ..............5-3

Gray Brook Aboiteau ..................5-3

Route 114 Culverts .....................5-3

Ranking of Flooding Factors....... ...................5-4

Potential Flood Risk Reduction Options................ ......5-5

lncreasing Discharge Capacity of the Aboiteau ............ ....5-5

Upgrading Route l l4 Culverts .......... . . ..... . . . .. 5-6

Stormwater Retention in the Lower Gray Brook Watershed. ................5-7

coNcLUStoNs.....

RECOMMENDATIONS ........7-1

........8-1

Gray Brook Hydraulic AssessmentRVA 112319

FINALMarch 10,2014


1.0 INTRODUCTION

The New Brunswick Department of Transportation and lnfrastructure has requested that

R.V. Anderson Associates Limited perform an hydraulic assessment of the Gray Brook

watershed in Hillsborough, New Brunswick. The objectives of this assessment were to model

the flood dynamics of Gray Brook, to investigate the possible causes of flooding of the low

lying areas and to assist the Department in developing a strategy to maintain service at the

Route 114 watercourse crossing during high flow conditions.

1.1 Background

Gray Brook crosses a low-lying section of Route 114 (Main Street) in Hillsborough and has

experienced flooding a number of times during the past few years. These events saw flood

waters overtop Route 114, impeding traffic flow and impacting business located within the

floodplain. Due to the complex interaction of natural storage and hydraulic structures along

Gray Brook, the Department requested that an hydraulic model be developed to assess the

watershed hydrology and Gray Brook flood dynamics.

The following sections describe the Gray Brook watershed, the hydraulics of the Route 114

culverts, the discharge gates at the aboiteau to the Petitcodiac River, the river tidal influence

and the storage in the marshes and Ducks Unlimited impoundment. This information was

used to evaluate the most likely causes of flooding and to recommend an option for the

Department to maintain service and traffic flow along Route 114 during storm events.

't-1

Gray Brook Hydraulic AssessmentRVA 1123'19

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2.0 SCOPE OF WORK

2.1 Description

The scope of work for the hydraulic analysis of Gray Brook included the following:

r a review of available mapping to assess the general site drainage patterns and to

identify significant tributaries to Gray Brook,

o a L|DAR survey of the lower sections of the Gray Brook watershed to define critical

elevations and to calculate the marsh storage volumes,

. an elevation survey to measure the diameters and inverts of the main culverts within

the watershed and at the aboiteau,

. the use of water level loggers to collect hydraulic grade line data for model

calibration,

r the development of a PCSWMM model of the Gray Brook watershed including the

culverts, storage and control structures,

. a site reconnaissance of the Gray Brook watershed during high flow conditions to

inspect the culverts, aboiteau and floodplains and drainage patterns to ensure that

the hydraulic modelaccurately reflects field conditions,

. a review of photographs and videos taken by local residents during past flooding

events,

r calibration of the hydraulic model using Environment Canada rainfall data and water

level readings collected by NBDOT,

o the use the model to identify potential hydraulic bottlenecks and other factors related

to the flooding of Gray Brook,

. an assessment of the effects of the tidal water level fluctuations of the Petitcodiac

River on flood levels and stormwater attenuation along Gray Brook, and

o hydraulic modeling of options that could be implemented by the Department to

reduce the risk of flooding and maintain traffic flow on Route 1 14.

The Department coordinated all survey and L|DAR data collection, installed and maintained

the water level loggers and provided the resulting data for use in these analyses.

2-1


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New Brunswick Department of Transportation and lnfrastructure 3-1

3.0 GRAY BROOK WATERSHED

3.1 Description and Physiography

3.1.1 Watershed and Subcatchments

The Gray Brook watershed covers approximately 16 km2 and is the largest within the Village

of Hillsborough. The main branch of the watercourse flows through the center of the Village

prior discharging to the Petitcodiac River. The watershed is characterized by primarily

wooded area and a golf course in the upper basin, low-density commercial and residential

development in the lower basin and wetlands and marshes in the floodplain sections of Gray

Brook. Figure 1 presents the extents of the Gray Brook watershed and the modeled

subcatchment boundaries, while Figure 2 presents additional detail in the lower watershed.

Figure 1. Approximate Gray Brook watershed and subcatchment

The Gray Brook watershed was divided into four (4) subcatchments for this study. This was

done to route the runoff from the different areas of the watershed through the major drainage

infrastructure. The four subcatchments were as follow:

-/boundaries.


FINALMarch 10, 2014

Edgelts


the western portion of the watershed includes the Hillsborough Golf Club and is

routed through the 1.8 m diameter culvert under Golf Club Road near Taylor Lane,

the northwest portion of the watershed includes Fairview Avenue, Pleasant Street

and Academy Street and is routed into the upper marsh through culverts below Golf

Club Road,

the southwest portion of the watershed includes the Gray Brook Marsh Wildlife

Habitat and extends from Route 114 (in the lower subcatchment) past Taylor Lane (in

the upper subcatchment), and

the portion of the watershed east of Route 114 and upstream of the Petitcodiac River

dyke.

The physiography of the Gray Brook subcatchments is summarized in Table 1.

3-2

Table 1. Gray Brook Subcatchment Physiography

Subcatchment

Drainage

Area

(rm')

Drainage

Length

(m)

Slope

("/r)Description

A. Upstream (west) of Golf

Club Rd near Taylor Ln6.76 3300 3.7

Primarily wooded hilly terrain

with golf course

B. Northwest of Golf Club

Road to Route 114, incl.

Fairview Avenue

2.75 2860 3.2

Partially wooded, hilly terrain,

residential and commercial

development

C. Southwest of Golf Club

Road to Route 114, incl.

section of Taylor Ln

3.84 4700 3.5

Partially wooded, hilly tenain,

sparse residential

development, Gray Brook

Marsh Wildlife Habitat,

marshland

D. Downstream (east) of

Route 1142.60 3000 0.5

Primarily marshland,

commercial and residential

development along Route 114

and Steeves St

Overall Watershed 16.0 7000 1.7

Hilly tenain in upper basin,

marshland in lower basin, low

density commercial and

residential development


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Figure 2. Aerial Photo of Lower Gray Brook watershed'

Gray Brook HYdraulic AssessmentRVA 112319

FINALMarch 10, 2014


3.1.2 Wetland Storaoe

Wetland and marsh storage volumes play a vital role in attenuating flow while the Petitcodiac

River tides are elevated and when the discharge capacity of the culverts or aboiteau pipes

may be exceeded. lf sufficient storage volumes exist, the likelihood of flooding during these

peak conditions is greatly reduced. Surface areas and storage volumes for the low-lying

areas and wetlands in the lower Gray Brook watershed were calculated using L|DAR survey

data. This high resolution data uses millions of points to define the surface contours and

was critical to obtaining accurate estimates of potential storage volumes.

Table 2 presents a summary of the Gray Brook channel and floodplain storage volumes

upstream of Route 114 (floodplain and Ducks Unlimited impoundment) and downstream of

Route 114 (to the dyke and aboiteau).

Table 2. Stag+Storage Summary

Elevation(m, NAD83 geod. datum)

Storage Volume (m3)

Upstream of Route 114 Downstream of Route 114

3.5 0 3,700

4.0 0 12,700

4.5 150 30,900

5.0 14,400 65,200

5.5 72,600 152,400

6.0 149,600 627,000

The data presented in Table 2 indicates that the runoff storage volume increases

significantly between elevations of 5.5 m and 6.0 m (all elevations are referenced to the

NAD83 geodetic datum). ln comparison, Route 114 is overtopped when water reaches an

elevation of approximately 5.9 m. This elevation and stage-storage relationship are

important to consider when evaluating the capaci$ to store runoff during high tides.

34


FINALMarch 10, 2014


3.2 HydraulicStructures

There are a number of hydraulic structures within the Gray Brook watershed, with the main

structures that influence the water levels in the lower reaches of Gray Brook being the

discharge gates at the aboiteau and the culverts at Route 114. Table 3 presents general

information for these structures, while the following subsections provide additional details.

3.2.1 Grav Brook Aboiteau

Aboiteaux are typically constructed sluices or culverts through dykes with one-way gates to

allow the drainage of water from upstream but to prevent salt water from flowing inland and

entering marshland. Within a few years of construction, runoff would leach the salt from the

soils upstream of the aboiteau, leaving fertile field and marshlands for farmers. The current

function of the Gray Brook aboiteau is to provide protection from tidal flooding of the

Petitcodiac River.

Discussions with the New Brunswick Department of Agriculture, Aquaculture and Fisheries

indicate that the aboiteau was not likely designed or intended to discharge the peak runotf

from the Gray Brook watershed.

All four pipes at the Gray Brook aboiteau are fitted with flap gates at the discharge ends to

allow runoff to drain during low tide and prevent the Petitcodiac River from flowing into the

marsh during high tides. Two additional aboiteaux are located on the Hillsborough marsh;

one to the north and one to the south of Gray Brook. These adjacent aboiteaux were

observed during high flow conditions and were not noted to discharge runoff from the Gray

Brook watershed. lt is possible, however, that there could be a mixing of runoff between the

watersheds when the marsh is inundated during flood conditions. This could not be


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Table 3. Aboiteau and Route 114 Culverts

Location Number ofPipes

Diameter(m)

Upstreamlnvert

(m)

Top of Road /Dyke(m)

Aboiteau Culverts2lower 1.1 2.5

8.32 upper 1.1 4.1

Route 114 Culverts 3 1.7 3.1 5.9

sizes and inverts we rre suryeyed by NBDOT crews.

New Brunswick Department of Transportation and lnfrastructure J-b

confirmed, so for the purposes of this study, Gray Brook was considered to function

independent of the neighboring watersheds.


' a:r

3.2.2 Route 114 Culverts

Route 114 is a provincially designated road that is maintained by the Department of

Transportation and seryes as Main Street for the Village of Hillsborough. This road is the

main traffic artery through the Village and provides access to Hopewell and Riverside-Albert.

Gray Brook has overtopped Route 114 a number of times during severe wet weather events,

so the capacity of the culverts was of particular importance in the Gray Brook hydraulic

analysis.

The Gray Brook crossing of Route 114 is comprised of three (3) 1.7 m diameter culverts.

The culverts are located at the low point along the road, in close proximity to Broadview

Power Sports and PRL Shine Power Wash, with Patty's Place Restaurant and a few homes

also located nearby. Gray Brook flood waters have encroached on these properties in the

past.

FINALMarch 10,2014


3.2.3 Floodolain and ln-line Storaqe Uostream of Route 1'14

The terrain upstream of Route 114 is a wide floodplain along Golf Club Road, divided by the

Ducks Unlimited berm and outlet structure. Gray Brook is a narrow and well defined channel

during low flow conditions (Photo 2). This floodplain, however, becomes inundated by runoff

storage during storm conditions (Photo 3).

iv'ff.t',f/

3-7

Photo 2. Gray Brook upstream of Route 114 during low flow conditions.

u F}.T rllr

r{lrl

f:'

Photo 3. Flood waters upstream of Route 114 (Route 114 and Broadview Power Sports

Ltd. in background).


FINALMarch 10, 2014


The average elevation of the floodplain between Route 114 and the Ducks Unlimited

impoundment is approximately 4.5 m. This is only 0.3 m lower than the crown of the Route

114 culverts (a.8 m) indicating that the majority of the floodplain storage occurs after the

culverts flow full.

Photos 4 and 5 below show the Ducks Unlimited impoundment (also known as the Gray

Brook Marsh Wildlife Habitat) and outlet structure during high flow conditions.

3-8

Photo 4. Ducks Unlimited impoundment, looking toward outlet structure.

!

Photo 5. Ducks Unlimited impoundment outlet structure, looking upstream.


FINALMarch 10, 2014


3.2.4 Floodplain and ln-line Storaqe Downstream of Route 114

Gray Brook has a well defined channel through the marshlands downstream of Route 114 to

the aboiteau (as shown in Photo 6). There are two (2) adjacent watercourses and aboiteaux

located to the north and south of Gray Brook. lt is likely that the floodplain may be shared by

all three (3) watercourses during flooding conditions, however during average flow conditions

they were observed to function independently.

s+/

$;"

3-9

'r$i/_1

,_) v

Photo 6. Arial photo of Gray Brook downstream of Route 114.


FINALMarch 10, 2014


On average, the Gray Brook channel is approximately 2 m deep; therefore overflow into the

floodplain does not occur until high flow conditions. A review of the LiDAR topographic data

indicates that flooding of the marsh initiates between elevations 5.0 and 5.5 m. This is more

than 0.5 m higher than the floodplain elevation upstream of Route 114. The implication is

that flood conditions may first become apparent upstream of Route 114, then in the lower

marsh.

Photo 7 (below) was taken during a site inspection on May 17,2011 and shows the water

levels upstream of the aboiteau to be above the crown of the upper discharge pipes

(elevation 5.2 m). Flood waters had not yet overtopped the channel banks into the marsh

floodplain. This reinforces the observation that much of the marsh floodplain storage is not

effective until elevation 5.5 m. This is important to consider when analyzing the overtopping

of Route 114 al elevation 5.9 m.

3-10

Photo 7. Water level upstream of aboiteau is above the discharge pipes.


FINALMarch 10, 2014


4.0 GRAY BROOK HYDRAULIC MODEL

Detailed hydrologic modeling and hydraulic analysis of the Gray Brook watershed was

performed using the PCSWMM software package. This software is based on the US EPA

Storm Water Management Model (SWMM), which is widely used for planning, analysis and

design related to stormwater runoff, channel hydraulics, and sewer systems. PCSWMM was

used to produce a dynamic representation of the hydraulic behavior of Gray Brook to

illustrate:

. the flows and water fluctuations during a range of storm events,

o the performance of culverts and control structures during normal and peak flow

conditions,

o the filling and draining of in-line and floodplain storage along Gray Brook, and

o the effects of the Petitcodiac River tides on the Gray Brook water levels.

Although static models could be used to model Gray Brook and identify hydraulic

bottlenecks, the dynamic representation offered by PCSWMM best represents the flooding

process (including the dynamic storage and draining of runoff in the flood plains) to show

potential causes and locations of flooding.

4.1 ModelValidation

The PCSWMM model needed to be validated to ensure it properly represented the hydraulic

behavior of the Gray Brook during average and peak flow conditions. Water level loggers

were installed along Gray Brook in the following locations:

. downstream of the aboiteau to record the tides and tailwater elevations,

o upstream of the aboiteau to record water levels in Gray Brook and observe the cycles

of filling and emptying of the marsh storage, and

. upstream of Route 114 to record water levels in Gray Brook and to observe the

potential effects of the culverts on restricting flow capacity along Gray Brook.

The level loggers were installed by Department personnel and remained in place between

September 26 and November 18,2011.

The PCSWMM model was calibrated to mirror the measured water level increases in

response to rainfall (measured at Nappan), the rising and falling of water levels in response

to the Petitcodiac River tides and the hydraulic grade line between Route 114 and the


FINALMarch 10, 2014

4-1


aboiteau. This validated that the modelwas acceptable to perform the hydrologic and

hydraulic analyses required for Gray Brook.

4.1.1 Water Level Loooer Measurements

Water level readings recorded between September 26 and November 18,2012 effectively

illustrate the storage effects and water surface profile along the lower section of Gray Brook.

Graphs on the following page present water level readings for two (2) heavy rainfall events

that occurred while the loggers were in place. The level readings were converted to

geodetic water surface elevations to compare the results at each locationland to determine

the water surface profile between the aboiteau and the upstream side of Route 114.

The following interpretations are made following review of the water level data:

water levels in the marsh and at Route 114 increase slowly folloriving rainfall due to

the lag time of the watershed, attenuation through the Ducks Unlimited

impoundment and storage in the lower marshes,

water levels and flows remain elevated for 2 to 3 days following rainfall,a

a runoff is unable to discharge through the aboiteau during high ti{e, resulting in

increased water levels as runoff is stored,

runoff stored within the channel and marsh discharges to the Peiitcodiac River

between tide cycles, however it takes a number of cycles to completely drain the

stored runoff, and

. the surcharged water levels are not sensitive to the absolute magnitude of the high

tide (i.e. little difference in upstream water levels for a 6 m or 7 m tide),

The water level loggers show that the hydraulic grade line is relatively flat between the outfall

and Route 114 during high flow conditions, particularly during high tide. The implication is

that flood levels at Route 114 are mainly influenced by the water levels in the lower marsh

and at the aboiteau as well as the volume of runoff stored between tide cycles.

4-2


FINALMarch 10,2014

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Figure. Water levels for the Oclober 20 (55 mm) and October 30, 2011 (52 mm) storms.

Gray Brook Hydraulic AssessmentRVA 1123't9

FIl.lALMarcfi 10,2014


5.0 RESULTS

The following paragraphs discuss the results of the hydrologic and hydraulic analyses of

Gray Brook.

5.1 Gray Brook Hydrology

The hydrologic analysis of peak flow rates along Gray Brook was performed using the

PCSWMM.NET model to account for the attenuation affects of the channel and floodplain.

Modeling results indicate that peak flow rates in the lower reaches of Gray Brook are

regulated not only by the natural storage, but also by the ability to discharge through the

aboiteau during high tides.

Modeling was performed for a range of storm durations and frequencies to determine which

conditions were most likely to result in flooding. Runoff from the short duration, high

intensity storms tended to be attenuated by the storage along Gray Brook. The longer

duration, high rainfall depth storms, however, have a tendency to fillthe storage volumes

and result in surcharging and flooding. To account for this response, 24 hour duration

storms were used throughout the analyses.

Table 4 summarizes the peak flows in Gray Brook at Route 114 and at the aboiteau.

The peak flows at Route 114for the 50 and 100 year return period storms are controlled by

conditions downstream of Route 1 14. A review of the overall hydraulics reveals that runoff

5-1


FINALMarch 10,2014

Table 4. Peak Flows along Gray Brook

Rainfall Return Period(Years)

Peak Flow 1m3ls)

Upstream of Route 114 Upstream of the Aboiteau **

2 5.5 14.9

10 11.0 16.5

25 14.4 17.7

50 16.6 * 18.3

100 16.6. 22.6

Peak flow reoulated by the culvert capaciw and 114** Peak flow influenced by filling and draining of channel and marsh storage between Petitcodiac tide cycles.


storage downstream of Route 114 fills and causes a high tailwater elevation that submerges

the culverts before their conveyance capacity is exceeded.

The peak flows at the aboiteau occur as high tides recede and stored runoff is released

through the aboiteau.

5.2 Gray Brook Hydraulics

The hydraulic model provides a detailed picture of the Gray Brook flood dynamics. The

general sequence of a flood event is as follows:

. heaw rainfall and snowmelt result in elevated flows,

o runoff begins to fill the Ducks Unlimited impoundment and the storage in lower

sections of Gray Brook,

. high tides restrict or prevent discharge through the aboiteau (approximately 4 hours

per high tide cycle, 8 hours per day),

o runoff is unable to discharge completely between high tide cycles, resulting in a

cumulatively increasing volume of runoff retained in storage,

o water levels rise in the lower reach of Gray Brook with surcharging that extends

upstream of Route 114,

. the floodplain upstream of Route 114 is at a lower elevation (4.5 m) than that

downstream of Route 114 (5 m - 5.5 m) and becomes surcharged first,

. water levels upstream and downstream of Route 114 continue to rise, with a slightly

higher (approximately 0.1 m) level upstream of the road due to headloss through the

culverts,

. nearby properties at a lower elevation than the Route 114 road surface are affected

by the rising waters,

. Route 114 is overtopped at water surface elevation of 5.9 m,

. runoff reduces to a rate that can be discharged between high tides and the

surcharging slowly subsides,

o the elevated flood waters take two (2) to three (3) days to discharge and subside.

The components that affect this sequence of events are discussed in greater detail in the

following sections.

Gray Brook Hydraulic AssessmentRVA 1 12319

FINALMarch 10, 2014

5-2


5.2.1 Petitcodiac River Tides

The rising and falling Petitcodiac River tides regulate the timing of the discharge from Gray

Brook through the aboiteau. ln order for Gray Brook to discharge through the aboiteau

gates, the upstream water level must be higher than the river water level to create a driving

head. An average high tide at Hillsborough has a 6.0 m peak water level elevation, which

fully submerges the discharge gates of both the lower and upper aboiteau pipes. A higher

high water large tide represents the highest tide expected in an average year, and is

approximately 8.17 m. Both these high tide elevations are above the road surface at the

Route 114 culverts.

ln dry weather conditions, there is sufficient storage along the lower reach of Gray Brook to

store the flow and discharge between high tides cycles. During wet weather conditions, this

storage is filled more quickly. Water levels rise in the marsh when the volume stored during

high tide cycles is unable to discharge fully during low tide. As this continues for multiple

tide cycles, the stored water level projects back to Route 114 and further upstream.

5.2.2 Grav Brook Aboiteau

The NB Department of Agriculture operates and maintains the Gray Brook aboiteaux.

Conversations with representatives from their Land Management Division confirmed that the

Gray Brook aboiteau was designed to provide protection of the land upstream of the dyke

from the effects of the Petitcodiac River. The discharge pipes were not sized to convey peak

flow from the upstream lands, nor to accommodate increases in runoff due to development

within the watershed.

The Gray Brook aboiteau has four (4) 1.1m diameter discharge pipes. The combined

discharge capacity of the aboiteau is 10.0 m3/s when the upstream water level is at the

crown of the higher pipes (5.2 m). Due to the upstream storage of runoff during high tides

and flushing of the runoff as the tide recedes, the discharge capacity of the culverts is likely

exceeded annually. This makes the storage capacity of the upstream channel and marsh

critical in providing flood protection upstream near Route 114.

5.2.3 Route 114 Culverts

The Route 114 crossing of Gray Brook is comprised on three (3) 1.7 m diameter culverts.

These culverts have a combined full flow discharge capacity of 13.8 m3/s. This capacity is

exceeded during a 'l in 25 year return period storm (peak flow 14.4 m3/s1, however flooding

Gray Brook Hydraulic AssessmentRVA't123't9

FINALMarch 10,2014


of Route 114 is expected to occur more frequently due to the cumulative storage of runoff

between high tides described previously.

The water level downstream of Route 114 is expected to rise to the road surface during a

1 in 10 year return period storm. As the existing hydraulic capacity of the Route 1 14 culverts

are already able to pass the peak flow associated with a 25 year storm event, any increase

in capacity of the Route 114 culverts would not reduce the likelihood of flood waters

overtopping the road at its existing (5.9 m) elevation.

5.2.4 Rankino of Floodino Factors

The relative importance of the above three (3) components as potential causes or

aggravating factors in flooding near Route 114 are as follows:

1. Petitcodiac River tides

o high tide elevations are well above flood levels along Route 114

o tide cycles result in accumulated of storage in the marsh and floodplains

2. Gray Brook aboiteau

. stored runoff volumes cannot discharge quickly enough between tide cycles

(function of aboiteau capacity and extent of storage in marsh and floodplains)

3. Route 114 culverts

. flooding occurs as downstream water levels rise due to accumulated runotf

stored in the floodplains and marsh


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54


5.3 Potential Flood Risk Reduction Options

The following sections present potential runoff control measures that were examined to

determine whether or not the risk of flooding of Gray Brook at Route 114 could be reduced.

5.3.1 lncreasinq Discharqe Capacitv of the Aboiteau

The Gray Brook hydraulic model was used to investigate the potential hydraulic relief that

could be achieved by increasing the discharge capacity. The 1 in 100 year storm was

modelled to compare the peak water levels for existing conditions and with the additional

discharge pipes installed at the aboiteau. Table 5 presents the potential water level

elevation reductions that could be achieved with the increased discharge capacity.

lncreasing the discharge capacity of the aboiteau by adding two (2) high and two (2) low

level discharge pipes provides an optimal reduction in peak water level elevation. Although

the additional aboiteau capacity provides a noticeable peak water level reduction at the

aboiteau (0.25 m), this is not reflected upstream of Route 114, where the reduction is less

significant (0.05 m). Further increases to the capacity do not appreciably reduce the peak

water level elevations.

The Gray Brook channel is very flat (0.04 % slope) and long (1400 m) between Route 114

and the aboiteau. During storms, runoff volume is not only accumulated in the channel, but

also in the floodplain upstream of Route 114. The flat grades and expansive storage restrict

the rate at which the runoff volume can be conveyed to and discharged from the aboiteau

between high tides. This is the main factor that limits the effectiveness of increased

discharge capacity at the aboiteau.


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Table 5. Peak Water Level Elevations with lncreased Aboiteau Discharge Capacity

Location

Peak Water Level (m)

ExistingConditions

Add 1 Low and1 High Level Pipe

to Aboiteau

Add 2 Low and2 High LevelPipes

to Aboiteau

Add 3 Low and3 High LevelPipes

to Aboiteau

Upstream of

Route 1146.43 6.40 6.38 6.36

Upstream of

the Aboiteau6.31 6.16 6.06 6.05

. Additional pipes equivalent to existing aboiteau pipes (1 .1 m


It should be noted that increases to the aboiteau discharge capacity were not able to reduce

the predicted peak water levels upstream and downstream of Route 114 below the road

surface elevation (5.91 m).

5.3.2 Upqradino Route 114 Culverts

Major drainage systems, such as culverts under highways and main traffic arteries, are

typically designed to convey a peak 1 in 100 year return period flow. The existing culverts

under Route 114 do not have sufficient capacity (13.8 m3/s) to convey the 1 in 100 year

return period flow (18.6 m3/s), however, this is not the primary cause of flooding at the

roadway. As noted in the above section, the peak water level on the downstream side of

Route 114 (approximately 6.32 m) is already higher than the roadway overtopping elevation

(5.91 m). lncreasing the capacity of the culverts would reduce headloss through the pipes,

but would not reduce the water levels across the overtopped roadway.

ln order prevent overtopping of the roadway, the Department could raise the driving surface

of Route 114 and upgrade the culverts to ensure there is have sufficient capacity to convey

the peak upstream runoff. This option would allow the Department to maintain traffic service

along Route 114 during wet weather conditions, but would not significantly impact or reduce

the flood levels along Gray Brook.

Short span bridge, circular pipe and rectangular box culvert options were evaluated as

potential upgrades to the existing Route 114 culverts. The evaluation criteria were as follow:

o provide sufficient capacity to convey peak runoff rates during the 1 in 100 year

rainfall (with a 10% factor of safety), and

o set the pipe crown or bottom chord of bridge above the peak water level (i.e. allflow

through bridge or culvert with no upstream surcharging or restriction of the flow).

Based on these criteria a list of hydraulically acceptable bridge and culvert options was

prepared. Discussions with NBDOT staff indicated that a 3 m x 3 m box culvert with a

finished road surface elevation of 7.1 m would be most practicaland economically feasible of

the options.

It should be noted that raising the existing road surface would extend the footprint of Route

114 due to the widened embankment side slopes. The Department would need to

investigate how this would impact the properties along the raised road.


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5€


5.3.3 Stormwater Retention in the Lower Grav Brook Watershed

The capacity to store runoff at low elevations in the Gray Brook marsh (before the rising

waters are noticeable at Route 114) is limited by the floodplain elevation. There is relatively

little low-elevation storage downstream of Route 114, as the average floodplain elevation

(5.5 m) is higher than the crown of the Route 114 culverts (elevation 4.8 m).

The potential benefit of providing additional low-elevation storage downstream of Route 114

was examined. A stormwater retention pond was modeled with a bottom elevation of 3.0 m

and the area (footprint) was varied for a range of storage volumes. The resulting peak water

level elevations for a 1 in 100 year return period storm are presented in Table 6.

The results presented in Table 6 indicate the following:

A pond volume of at least 375,OOO m3 is required to lower the predicted peak water

level to an elevation below the Route 114 road surface elevation.

A pond volume of at least 1,5OO,OOO m3 is required to lower the predicted peak water

level downstream of Route 114 to an elevation below the crown of the culverts.

Creating the above storage volumes would be very costly and impractical (in terms of land

required) as a primary option to reduce the risk and frequency of overtopping Route 114.


FINALMarch 10, 2014

Table 6. Peak Water Levels with Potential Storage Pond in Lower Marsh

Storage Pond Area(m')

Storage Pond Volume(mt)

Peak Water Level (m)

At Route 114 At the Aboiteau

50,000 (5 ha) 125,000 6.22 6.03

100,000 (10 ha) 250,000 6.02 5.95

150,000 (15 ha) 375,000 5.85 5.81

200,000 (20 ha) 500,000 5.69 5.65

300,000 (30 ha) 750,000 5.55 5.36

400,000 (40 ha) 1,000,000 5.49 5.12

500,000 (50 ha) 1,250,000 5.45 4.95

600,000 (60 ha) 1,500,000 5.45 4.81


6.0 CONCLUSIONS

The following conclusions are based on the results of the Gray Brook hydraulic assessment

and the information presented in previous sections of this report.

1. Gray Brook is expected to overtop Route 1 14 during a 1 in 10 year return period

rainfall. This flood risk is increased during the following conditions:

preceding wet weather conditions that saturate the ground and infill a portion

of the channel and floodplain storage,

spring freshet conditions when the ground is still frozen and snowmelt

contributes to runoff,

winter conditions which result in snow and ice restriction of the channel

conveyance capacity and reduction of potential storage volume in the

floodplain, and

problems at the aboiteau flood gates (i.e. silt or ice restricting operation).

Flooding of Gray Brook at Route 114 initiates as a result of elevated downstream

water levels (between Route 114 and the aboiteau). This occurs before conveyance

capacity of the three (3) 1.7 m diameter culverts under Route 114 is exceeded.

lncreasing the conveyance capacity of the Route 114 culverts will not prevent

flooding of the roadway or nearby properties. These flood levels are dictated

primarily by flood levels in the downstream marsh.

The risk of runoff overtopping Route 114 can be greatly reduced by raising the

driving surface elevation of Route 114 and increasing the culvert conveyance

capacity. This would help maintain traffic flow, but would not reduce the flood risk for

nearby properties.

lncreasing the capacity of the Gray Brook aboiteau with additional pipes would

reduce the peak water surface elevation at Route 114 by less than 0.1 m during a 1

in 100 year storm and would not prevent overtopping of the driving surface.

6. The stormwater retention pond volumes required to prevent overtopping of the road

surface (375,000 m31 or surcharging of the Route 1 14 culverts (1,500,000 m3; are

excessively large and not considered to be practicalflood risk reduction options.

6-1

c.

d.

2.

3.

5.

Gray Brook Hydraulic AssessmenlRVA 112319

FINALMarch 10, 2014


7.0 RECOMMENDATIONS

The following recommendations are proposed to reduce the risk of overtopping Route 114

and to maintain traffic flow during peak flood conditions.

1. Replace the existing three 1.7 m diameter culverts under Route 1 14 with a single

barrel 3 m x 3 m rectangular box culvert.

2. Raise Route 1141o elevation 7.1 m to accommodate the proposed box culvert and to

prevent overtopping of the road during peak runoff conditions.

The following additional recommendations are presented, although they may not fallwithin

the responsibility or authori$ of the Department of Transportation.

3. Control the hydrologic effects of future development within the Gray Brook watershed

(initiate stormwater management practices) to avoid increased runoff rates and

volumes due to development induced changes to the watershed physiography.

4. Discourage development within the Hillsborough marsh and Gray Brook floodplains.

The proposed upgrades to the Route 114 crossing will reduce the likelihood of

overtopping the road, but will not lower flood levels within the channel, marsh and

floodplains.

5. Encourage periodic inspections of the aboiteau to ensure there are no pipe

blockages and that the outlet discharge gates are not blocked by ice or silt. Proper

operation of the gates is essential to ensure runotf stored within the marsh is able to

discharge effectively between tide cycles.


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7-1


8.0 SIGNATURE

We trust that this information addresses your current needs and will help the Department to

understand the Gray Brook flood dynamics and develop a strategy to maintain service at the

Route 114 watercourse crossing during high flow conditions. lf you have any questions or

require further information please contact us at your convenience.

8-1

R.V. Anderson Associates Limited

Troy Poirier, P.Eng.

Associate


FINALMarch 10, 2014

hydraulic assessment - gray brook - hillsborough, nb - 2014.04.14

Documents

gray brook aboiteau

gray brook hydraulic

gray brook hydrology

gray brook watershed

gray brook hydraulics

hydraulic structures

anderson associates

anderson associates