-
This document contains confidential information intended only
for the person(s) to whom it is addressed. The information in this
document may not be disclosed to, or used by, any other person
without Hatch's prior written consent.
Otonabee Region Conservation Authority
Addendum to the Environmental Study Report for Remedial Flood
Control
WorksFor
Millbrook Dam
H349588-000-200-0003 Rev. C
January 15, 2016
-
This document contains confidential information intended only
for the person(s) to whom it is addressed. The information in this
document may not be disclosed to, or used by, any other person
without Hatch's prior written consent.
Otonabee Region Conservation Authority
Addendum to the Environmental Study Report for Remedial Flood
Control
WorksFor
Millbrook Dam
H349588-000-200-0003 Rev. C
January 15, 2016
-
Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
H349588
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Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
Table of Contents
Executive Summary
..............................................................................................................................
ES-1
1. Introduction
........................................................................................................................................
1-1
1.1 Background
................................................................................................................................
1-1 1.2 Addendum
Process....................................................................................................................
1-3
2. Update of Previous Studies Completed
..........................................................................................
2-1
2.1 Millbrook Dam Bathymetric Survey Technical Brief,
2014......................................................... 2-1
2.2 Millbrook Dam - Dam Safety Assessment Update Report, 2015
.............................................. 2-1
3. Update of Stakeholder Consultation Undertaken
..........................................................................
3-1
3.1 Community Liaison Committee Meetings
..................................................................................
3-1 3.1.1 Community Liaison Committee Meeting - October 8, 2015
......................................... 3-1 3.1.2 Community
Liaison Committee Meeting - November 23, 2015
.................................... 3-2
3.2 Public Consultation
....................................................................................................................
3-3 3.2.1 Project
Stakeholders.....................................................................................................
3-3 3.2.2 Website Updates
..........................................................................................................
3-3 3.2.3 UPDATE: Millbrook Class Environmental Assessment Addendum
- September
3, 2015
..........................................................................................................................
3-3 3.2.4 Notice of Public Information Centre - October 1 and 8,
2015....................................... 3-4 3.2.5 Public
Information Centre - October 20, 2015
..............................................................
3-4
3.3 First Nation and Métis Consultation
...........................................................................................
3-7 3.3.1 First Nation and Métis Communities
.............................................................................
3-8 3.3.2 UPDATE: Millbrook Class Environmental Assessment Addendum
- September
3, 2015
..........................................................................................................................
3-8 3.3.3 Response from Alderville First Nation
..........................................................................
3-8 3.3.4 Notice of Public Information Centre - October 1, 2015
................................................. 3-8 3.3.5 Public
Information Centre - October 20, 2015
..............................................................
3-9
3.4 Agency Consultation
..................................................................................................................
3-9 3.4.1 Federal and Provincial Agencies
..................................................................................
3-9 3.4.2 Local
Municipalities.....................................................................................................
3-10
4. Update of the Preliminary Selection of Options
.............................................................................
4-1
4.1 Preliminary Selection of Options
................................................................................................
4-1 4.1.1 Alternative 1: Do Nothing
..............................................................................................
4-1 4.1.2 Alternative 2: Complete Removal of the Dam (Full
Decommissioning) ....................... 4-1 4.1.3 Alternative 3:
Channel Works
.......................................................................................
4-2 4.1.4 Alternative 4: Reduce Probability of Dam Failure
......................................................... 4-2 4.1.5
Alternative 5: Attenuation of Flood
...............................................................................
4-3 4.1.6 Alternative 6: Increase Hydraulic Capacity
...................................................................
4-3 4.1.7 Dam Safety Risk Assessment Screening of Options
................................................... 4-5
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5. Update of Concepts A, B and C
.......................................................................................................
5-1
5.1 Concept A: Spillway Removal and Channel
Restoration...........................................................
5-1 5.2 Concept B: Spillway Removal with Off-Line Pond
.....................................................................
5-3 5.3 Concept C: Spillway Reconstruction with Headpond Retention
................................................ 5-5
5.3.1 Identification of Alternative Methods for Spillway
Reconstruction with Headpond Retention
.....................................................................................................
5-6
5.3.2 Assessment of Options to Increase Spillway Capacity
.............................................. 5-11 5.3.3
Recommended Option to Increase Spillway Capacity
............................................... 5-12 5.3.4
Resolution of Existing Dam Safety Issues
..................................................................
5-12
6. Evaluation of the “Do Nothing” Alternative and Concepts A, B
and C ........................................ 6-1
6.1 Environmental Effects Evaluation
..............................................................................................
6-1 6.2 Additional Evaluation Criteria
.....................................................................................................
6-1
6.2.1 Additional Permitting and Approval Requirements
....................................................... 6-1 6.2.2
The Effectiveness of the Method to Produce the Desired Result
................................. 6-2 6.2.3 Technical Feasibility
of Undertaking the Method
........................................................ 6-10 6.2.4
Safety and Liability Issues
..........................................................................................
6-10
6.3 Selection of Preferred Alternative Method for Carrying Out
Remedial Project – Concept C: Spillway Reconstruction with Headpond
Retention
............................................................
6-10
7. Description of the Recommended Option to Achieve Preferred
Alternative .............................. 7-1
7.1 Project Description
.....................................................................................................................
7-1 7.1.1 Spillway
.........................................................................................................................
7-1 7.1.2 Hardening of the Embankment Dam
............................................................................
7-1 7.1.3 Bank and Shoreline Protection
.....................................................................................
7-1 7.1.4 Filling of the Abutments
................................................................................................
7-1 7.1.5 Toe Berm Extension
.....................................................................................................
7-1 7.1.6 Public Safety and Boom Installation
.............................................................................
7-2 7.1.7 Pedestrian Bridge
.........................................................................................................
7-2 Public access will be maintained across the dam via a path and
foot bridge along the
crest of the dam that will be designed to meet, or exceed
accessibility requirements.
................................................................................................................
7-2
7.2 Project Implementation
..............................................................................................................
7-2 7.2.1 Construction
..................................................................................................................
7-2 7.2.2 Operation and Maintenance
.........................................................................................
7-5
8. Detailed Environmental Analysis of the Preferred Alternative
..................................................... 8-1
8.1 Assessment Methodologies
.......................................................................................................
8-1 8.1.1 Potential Effects
............................................................................................................
8-1 8.1.2 Mitigation Measures and
Monitoring.............................................................................
8-2 8.1.3 Net Effects
....................................................................................................................
8-3
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9. Update of Cumulative Effects Assessment
....................................................................................
9-1
10. Conclusion
.......................................................................................................................................
10-1
11. References
.......................................................................................................................................
11-1
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Otonabee Region Conservation Authority Addendum to the ESR for
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List of Tables Table 3-1: Comments and Questions Expressed
During the Community Liaison Committee Meeting on October 8, 2015
.........................................................................................
3-1Table 3-2: Comments and Questions Expressed During the Community
Liaison Committee Meeting on November 23, 2015
...................................................................................
3-2Table 3-3: Project Stakeholders Notified during the Addendum
Process ..................................... 3-3Table 3-4:
Comments and Questions Expressed During the Public Information
Centre on ............. October 20, 2015, 1:00 PM Session
.............................................................................
3-5Table 3-5: Comments and Questions Expressed During the Public
Information Centre on ............. October 20, 2015, 7:00 PM
Session
.............................................................................
3-5Table 3-6: Comment Sheet Questions, Stakeholder Responses and
Project Response, Public Information Centre on October 20, 2015
......................................................................
3-7Table 3-7: First Nation and Métis Communities Notified During
the Addendum Process ............. 3-8Table 3-8: Provincial and
Federal Agencies Notified during the Addendum Process
................... 3-9Table 3-9: Provincial and Federal Agencies
Notified during the Addendum Process ................. 3-10Table
4-1: Basis of the Probability Estimates
................................................................................
4-8Table 5-1: Preliminary Rating of Options for Concept C –
Increase Spillway Capacity for IDF * ......
....................................................................................................................................
5-11Table 5-2: Considered Concept C Options – Relative Capital Cost
Estimate ............................. 5-12Table 6-1: Evaluation of
the “Do Nothing” Alternative and Concepts A, B and C
......................... 6-3Table 6-2: Estimated Cost Comparison
of the Alternatives
...........................................................
6-9Table 8-1: Detailed Environmental Analysis of Preferred
Alternative ............................................ 8-4
List of Figures Figure 4-1: Causes of Dam Failure (Source:
ASCE/USCOLD, 1975) ............................................
4-7Figure 4-2: Life Safety Risks Presented by Millbrook Dam
Overtopping Compared with CDA Dam Life Safety Risk Acceptability
Criteria
...........................................................................
4-9Figure 5-1: Concept C – Dam Reconstruction with Headpond
Retention .................................... 5-15Figure 5-2:
Graphical Rendering of Downstream View
................................................................
5-16
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List of Appendices
Appendix A Stakeholder Consultation
Appendix A 1 Reengagement Letter, September 3, 2015
Appendix A 2 Notice of PIC, October 1, 2015
Appendix A 3 ORCA Presentation, October 20, 2015
Appendix A 4 Hatch Presentation, October 20, 2015
Appendix A 5 PIC Display Panels, October 20, 2015
Appendix A 6 Sample Comment Sheet
Appendix A 7 Response from Alderville First Nation
Appendix A 8 Correspondence with the Ministry of Tourism,
Culture and Sport
Appendix A 9 Correspondence with Transport Canada
Appendix B Detailed Cost Estimates
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rights relating to the use of this document or its contents.
Executive Summary The Millbrook Dam is located in the Village of
Millbrook, Township of Cavan Monaghan, County of Peterborough,
Ontario on an upstream reach of Baxter Creek, approximately 20 km
southwest of the City of Peterborough. The Millbrook Dam is
currently comprised of a 120 m long earthen embankment dam with a
concrete spillway (approximately 7 m wide). A U-shaped overflow
weir controls flow through the dam at the spillway entrance.
The dam was originally constructed in the 1820s with a timber
crib spillway (replaced by a concrete spillway), and purchased by
the Otonabee Region Conservation Authority (ORCA) in 1967. Since
its purchase, assessments of the dam have revealed various issues,
which when combined with deterioration due to age, have raised
serious concerns with regard to the potential for dam failure.
Before works to repair, rehabilitate or replace the dam can be
undertaken, the Ministry of Natural Resources and Forestry (MNRF)
must approve the proposed works under the Lakes and Rivers
Improvement Act (LRIA). LRIA approval will only be granted if it is
demonstrated that the proposed works for the dam and spillway(s)
are in compliance with the LRIA Administrative Guide and Technical
Bulletins (MNRF, 2011).
A dam safety assessment completed in 2008 (Millbrook Dam,
Hydrotechnical, Dam Classification Study, Dam Safety Review,
Feasibility Assessment - IBI Group, 2008; hereinafter referred to
as the “Dam Safety Review”) concluded that an increase in the
hydraulic capacity to a minimum of 100 cubic metres per second
(m3/s), and reinforcement/stabilization of the dam was necessary.
Accordingly, in 2013, the ORCA completed a draft Environmental
Study Report (ESR) for Remedial Flood Control Works - Millbrook Dam
(MMM Group, 2013; herein after referred to as the “draft ESR
(2013)”) in accordance with Conservation Ontario’s (CO’s) Class
Environmental Assessment for Remedial Flood and Erosion Control
Projects (Class EA) to consider a reasonable range of alternatives
to address issues associated with dam design, construction and
deterioration. From this process, the draft ESR (2013) recommended
spillway reconstruction with retention of the headpond (Mill Pond)
upstream of the dam (identified as Concept C in the draft ESR
(2013)). Specifically, the draft ESR (2013) recommended a 36 m
long, sharp-crested weir with a height that would result in the
lowering of the headpond elevation by 0.5 m.
As the Class EA process was nearing completion in 2013, new
technical bulletins for assessing dam safety came into effect. In
response, the ORCA undertook an update to the 2008 dam safety
assessment work (Millbrook Dam – Dam Safety Assessment Update
Report - D.M. Wills Associates Limited, 2015; hereinafter referred
to as the “Dam Safety Assessment Update Report). The completion of
the 2015 Dam Safety Assessment Update Report (including
consideration of bathymetric surveys conducted in 2014 to provide a
more detailed account of the amount of accumulated sediment and the
water storage volume) has necessitated changes to the draft ESR
(2013) with respect to the
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
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Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
• Hazard Potential Classification (HPC) which is a measure of
the greatest incremental losses that could result from an
uncontrolled release due to dam breach. In the case of the
Millbrook Dam, the HPC continues to be “High” for both the normal
(sunny day) and flood scenarios
• Inflow Design Flood (IDF) which is the highest peak flow for
which a dam is designed (all reconstructed dams are required to
meet the 2011 MNRF guidelines respecting IDF). The 2015 Dam Safety
Assessment Update Report revealed an increase in the IDF for the
Millbrook Dam from 100 m3/s to 181 m3/s which represents a 81%
increase
• Design Earthquake (DE) which is the most critical earthquake
for which the dam stability is evaluated. The 2015 Dam Safety
Assessment Update Report determined the DE for the Millbrook Dam is
the 2,500-year event.
In consultation with the Community Liaison Committee (CLC), the
above-noted changes were deemed significant enough as to merit an
addendum to the draft ESR (2013) to reflect the impact on the
preferred solution of increased hydraulic capacity and headpond
retention, but not significant enough as to require identification
and evaluation of a new solution to the issues associated with dam
condition and safety.
The ORCA retained Hatch Ltd. (Hatch) to prepare this Addendum to
the draft ESR for Remedial Flood Control Works - Millbrook Dam
(“Addendum”) for the purpose of aligning the previously prepared
draft ESR (2013) with new information respecting the updated HPC,
IDF and DE standards, as well as the amount of accumulated sediment
and water storage volume within the headpond (Mill Pond). The
Addendum also considered updating the previously proposed Concept C
(Spillway Reconstruction with Headpond Retention) to align with the
new information. The Addendum process undertaken subsequent to the
draft ESR (2013) has involved:
• Updating the preliminary Screening of all broadly defined,
generic options as presented in the draft ESR (2013) based on the
2015 Dam Safety Assessment Update Report. The preferred option
continues to be Alternative 6 – Increase Hydraulic Capacity as was
previously presented in the draft ESR (2013), and which aligns best
with expressed community values (maintaining the size and water
level of the Mill Pond, sediment removal, maintenance of public
access across the dam via a foot bridge and preservation of the
current waterfall effect).
• Updating the solutions for achieving increased hydraulic
capacity, which remain the same as those presented in the draft ESR
(2013), specifically Concepts A (Spillway Removal and Channel
Restoration), B (Spillway Removal with Off-Line Pond) and C
(Spillway Reconstruction with Headpond Retention) which continue to
incorporate the community values as presented in the draft ESR
(2013) as well as the need to address the problems associated with
the condition of the dam.
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• Updating of Concepts A, B and C to align with the findings of
the 2015 Dam Safety Assessment Update Report with consideration of
the estimated 50-year life cycle cost.
• Evaluating new spillway reconstruction options to increase the
spillway discharge capacity such as self-regulating overflow weirs,
gated spillways, fuse plugs/gates, embankment dam hardening, among
others. This evaluation was based on consideration of safety,
reliability of flood prediction, loading conditions, potential for
debris blockage, frequency of spill, bathymetry of the river,
available dam types, operation considerations, effects on the
environment and costs.
• Determining based on Hatch’s evaluation of the various
options, that an up to 30-m long ogee weir spillway and hardened
embankment dam with headpond retention was the most cost-effective
option to comply with ORCA’s requirements under the LRIA, while
aligning best with the community’s expressed values as documented
within the draft ESR (2013). This option also maintains the size
and water level of Mill Pond and preserves the current waterfall
effect by producing sights and sounds similar to existing
conditions. Public access will be maintained across the dam via a
path and foot bridge along the crest of the dam that will be
designed to meet, or exceed accessibility requirements.
• Presenting this option for consideration by stakeholders
including the CLC, various agencies and members of the general
public during a Public Information Centre (PIC) on October 20,
2015. Given that no significant concerns were expressed by Project
stakeholders, this option was carried forward as the revised
“Concept C” into the environmental effects evaluation.
• Evaluating the “Do Nothing” Alternative, and updated Concepts
A, B and C including an evaluation of environmental effects as
prescribed within Table 3 – Detailed Environmental Analysis as
presented in CO’s Class EA; and evaluation of additional criteria
such as the 50-year life cycle cost to enable a comparison of the
relative advantages and disadvantages of each alternative. Based on
the results of this evaluation of the alternatives, Concept C:
Spillway Reconstruction with Headpond Retention remains the
preferred alternative, with principal differentiators being those
related to the community’s expressed values regarding maintenance
of the water level of the Mill Pond; sediment removal; maintenance
of public access; and continued waterfall effect.
• Evaluating the preferred alternative to confirm and update
potential environmental impacts, and refine methods of mitigation.
The evaluation has considered both temporary impacts during
construction of the undertaking, and permanent impacts due to
long-term operation and maintenance of the reconstructed Millbrook
Dam.
• Updating the Cumulative Effects Assessment to consider the
effects of the Project, in combination with other past, present and
future actions.
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Otonabee Region Conservation Authority Addendum to the ESR for
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• Facilitating public and Agency involvement throughout the
Addendum process via direct mailings, public notifications, a
Public Information Centres (PIC), and regular meetings with the
established CLC, thereby ensuring broad public understanding,
support of the process followed, and traceability of
decision-making.
Based on the results of Hatch’s detailed environmental analysis
of the preferred alternative - Concept C: Spillway Reconstruction
with Headpond Retention, this Addendum has concluded that all
potential adverse effects of the Project can be avoided, mitigated
or compensated for satisfactorily, and ORCA can proceed with the
issuance of the “Notice of Filing”.
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1. Introduction 1.1 Background
The Millbrook Dam is located in the Millbrook Ward of the
Township of Cavan Monaghan, County of Peterborough, Ontario on an
upstream reach of Baxter Creek in the Village of Millbrook,
approximately 20 km southwest of the City of Peterborough.
The dam was constructed in the 1820s for the purpose of
supplying waterpower for a grist mill operation. The dam was
purchased by the ORCA in 1967 and is currently comprised of a 120 m
long earthen embankment with a concrete spillway (approximately 7 m
wide). A U-shaped overflow weir controls flow through the dam at
the spillway entrance.
Assessments, investigations and inspections of the dam
subsequent to the ORCA purchase have revealed various issues
associated with the design of, and construction materials used for,
the dam. This, combined with deterioration due to age, raises
serious concern for dam failure. Before works to repair and
rehabilitate or replace the dam can be undertaken, the MNRF must
approve the proposed works under the LRIA. LRIA approval can only
be granted if it is demonstrated that the proposed works for the
dam and spillway(s) are in compliance with the LRIA Administrative
Guide and Technical Bulletins (MNRF, 2011), thereby reducing the
risks of dam failure.
In 2004 a preliminary assessment of the dam’s safety was
completed by Hatch Ltd., and further assessments of the dam’s
safety were completed by IBI Group in 2008. These assessments
concluded that the Millbrook Dam had inadequate hydraulic capacity
and stability at the IDF. In the event of “Sunny Day” failure of
the Millbrook Dam, or failure during a flood event, there was a
risk of incremental loss of life and property damage in the
residential and commercial core of the village of Millbrook, which
lies immediately downstream of the dam. In addition, the 2008 Dam
Safety Review identified that increasing the spillway capacity to a
minimum discharge of 100 m3/s, and reinforcing/stabilizing the dam
were required to ensure the structure meets dam safety
guidelines.
In 2013, the ORCA completed the draft ESR (2013) in accordance
with CO’s Class EA. The purpose of undertaking the Class EA process
was to consider a reasonable range of alternatives, both
functionally different "alternative methods to" and "alternative
methods of" addressing issues associated with dam design,
construction and deterioration, and that also ensure the dam meets
all requisite safety requirements. From this process, a preferred
remediation method was established. As per the draft ESR (2013),
this assessment recommended spillway reconstruction with retention
of the headpond upstream of the dam. This preferred solution is
described in more detail, as follows:
• Removal of the existing spillway and replacement with a new,
stepped, concrete weir capable of passing the required 100 m3/s IDF
which would include setting the weir crest
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at an approximate elevation of 214.6 m - approximately 0.5 m
below the current weir crest height.
• Excavation of accumulated sediment in the headpond to bring
the bottom elevation in the range of 212.8 and 213.9 m
approximating the “natural bottom” of the headpond.
• Additional repair of the earth embankment dam including
installation of an impermeable barrier to prevent seepage through
and beneath the dam, and placement and extension of
reinforcing/stabilizing berm near the west end of the
embankment.
As the Class EA process was nearing completion (following
issuance of the “Notice of Filing of the Environmental Study Report
for Review”, but prior to issuance of the “Notice of Approval of
the Project”), new technical bulletins for assessing dam safety
came into effect. In response, the ORCA undertook an update to the
dam safety assessment work completed in 2008. This update
considered results from a 2014 bathymetric survey of the headpond
(Mill Pond) undertaken by ORCA to provide a more detailed account
of the amount of accumulated sediment and the water storage
volume.
The 2015 Dam Safety Assessment Update Report, reviewed the dam
safety in accordance with the new LRIA technical bulletins issued
by MNRF, the report concluded
• the HPC for the Millbrook Dam is HIGH for both the normal
(Sunny Day) and flood scenarios based on the predicted incremental
effects associated with dam failure
• the IDF for the Millbrook Dam is 181 m3/s, which is one third
(1/3) of the way between the 1000-year peak flow and the Probable
Maximum Flood peak flow
• the peak water level is 217.04 m upstream of the existing
Millbrook Dam at the IDF (181 m3/s)
• the design earthquake for the Millbrook Dam is the 2,500-year
event.
The Millbrook Dam currently meets the required factors of safety
with respect to the stability of the earth embankment.
The completion of the 2015 Dam Safety Assessment Update Report
has necessitated changes to the draft ESR (2013) with respect to
the
• Hazard Potential Classification (HPC) which is a measure of
the greatest incremental losses that could result from an
uncontrolled release due to dam breach. In the case of the
Millbrook Dam, the HPC continues to be “High” for both the normal
(sunny day) and flood scenarios
• Inflow Design Flood (IDF) which is the peak flow for which a
dam is designed (all reconstructed dams are required to meet the
2011 MNRF guidelines respecting IDF). The 2015 Dam Safety
Assessment Update Report revealed an increase in the IDF for the
Millbrook Dam from 100 m3/s to 181 m3/s
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
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rights relating to the use of this document or its contents.
• Design Earthquake (DE) which is the most critical earthquake
for which the dam stability is evaluated. The 2015 Dam Safety
Assessment Update Report determined the DE for the Millbrook Dam is
the 2,500-year event.
1.2 Addendum Process ORCA has retained Hatch to prepare this
Addendum for the purpose of aligning the previously prepared draft
ESR (2013) with new information respecting the updated HPC, IDF and
DE standards, as well as the amount of accumulated sediment and
water storage volume within the headpond (Mill Pond).
This Addendum documents the following:
• Update of Previous Studies Completed.
• Update of Stakeholder, First Nation and Métis Consultation
Undertaken.
• Update of the Preliminary Selection of Options.
• Update of Concepts A, B and C.
• Identification and Assessment of Alternative Methods to
Increase Spillway Hydraulic Capacity.
• Evaluation of the “Do Nothing” Alternative and Concepts A, B
and C.
• Description of the Recommended Option to Achieve Preferred
Alternative.
• Detailed Environmental Analysis of the Preferred
Alternative.
• Update of Cumulative Effects Assessment.
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2. Update of Previous Studies Completed 2.1 Millbrook Dam
Bathymetric Survey Technical Brief, 2014
In 2014 ORCA updated the bathymetric survey of Mill Pond to
provide a more detailed account of the amount of accumulated
sediment and the water storage volume. In the Bathymetric Survey
Technical Brief (June 2014), which is found in Appendix E of the
2015 Dam Safety Assessment Update Report, it is concluded that
• the accumulated sediment on top of the natural pond bottom
ranges in depth from 0.00 m to 1.14 m and the mean sediment depth
is approximately 0.66 m
• the natural pond bottom elevation ranged from 213.05 m to
215.00 m (at the shore)
• the average natural pond bottom elevation was 214.03 m
• the estimated volume of sediment contained in the millpond is
approximately 12,800 m3.
2.2 Millbrook Dam - Dam Safety Assessment Update Report, 2015
The 2015 Dam Safety Assessment Update Report provides the following
Executive Summary:
D.M. Wills Associates Limited (Wills) was retained by the
Otonabee Region Conservation Authority (ORCA) to provide an update
to the Dam Safety Review for Millbrook Dam (IBI Group, 2008), based
on the Lakes and Rivers Improvement Act Technical Bulletins (MNR,
2011). The 2008 Dam Safety Review (IBI Group, 2008) was completed
based on the Draft Ontario Dam Safety Guidelines (MNR, 1999). The
purpose of this report is to document Wills’ update of the 2008 Dam
Safety Review (IBI Group, 2008) including the
• determination of the 2-year to 100-year, 1000-year and
Probable Maximum Flood (PMF) peak flows
• selection of the Hazard Potential Classification (HPC), Inflow
Design Flood (IDF) and Design Earthquake (DE)
• evaluation of the geotechnical factors of safety for the earth
embankment
• this report was prepared in accordance with the following
Ministry of Natural Resources (MNR) Technical Bulletins:
Technical Bulletin for Classification and Inflow Design Flood
Criteria (MNR, 2011).
Technical Bulletin for Seismic Hazard Criteria, Assessment and
Considerations (MNR, 2011).
Technical Bulletin for Geotechnical Design and Factors of Safety
(MNR, 2011).
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In addition to the above noted technical bulletins, several
other resources, including the Draft Lakes and Rivers Improvement
Act Technical Guidelines - Criteria and Standards for Approval
(MNR, 2004) and others, were used in the hydrology and PMF study.
Based on the updated peak flows and dam breach model, and with the
noted assumptions, Wills provides the following conclusions:
1. The 1000-year peak flow for the Millbrook Dam is 59 m3/s.
2. The PMF peak flow for the Millbrook Dam is 426 m3/s.
3. The Millbrook Dam is classified as a High hazard dam for both
the normal (sunny day) and flood scenarios.
4. The IDF for the Millbrook Dam is 181 m3/s, one third (1/3) of
the way between the 1000-year peak flow and the PMF peak flow.
5. The design water level for the IDF (181 m3/s) is 217.04
m.
6. The DE for the Millbrook Dam is the 2,500-year event.
7. Based on the Geo-Logic report dated July 9, 2014, the
Millbrook Dam meets the required factors of safety with respect to
the stability of the earth embankment, and the toe berm provides
adequate width for seepage control.
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3. Update of Stakeholder Consultation Undertaken The following
sections present the CLC, Public, First Nation and Métis community,
and Agency consultation activities undertaken during the Addendum
process.
3.1 Community Liaison Committee Meetings
3.1.1 Community Liaison Committee Meeting - October 8, 2015 An
invitation was extended to the CLC via email on September 21, 2015.
The agenda for the meeting included
• an ORCA update regarding the Project since the previous CLC
meeting held in September 2013
• a presentation by Hatch on the identification and evaluation
of the various alternatives to reconstruct the spillway while
retaining the headpond, and
• discussion regarding the upcoming PIC and next steps in the
Addendum process.
The meeting on October 8, 2015 was attended by eight (8) members
of the CLC, in addition to representatives of ORCA and Hatch.
Comments and questions discussed during the CLC Meeting are
summarized in the Table 3-1.
Table 3-1: Comments and Questions Expressed During the Community
Liaison Committee Meeting on October 8, 2015
Topic Question Project Response Project Funding How will the
Project be funded? 66% of the funding for the Project will come
from the Federal and Provincial governments under the Small
Communities Fund (SCF). The remainder will be provided by the local
municipalities (90% from the Township of Cavan Monaghan and 10%
from the remaining municipalities).
Will ORCA apply for Water Erosion Control Infrastructure (WECI)
funding for the remaining 33% that is not covered by the SCF?
No other Federal or Provincial monies can be used for the
Project, however ORCA will apply for WECI funding for those
expenses related to the Project which are ineligible under the
SCF.
Sediment removal from the Mill Pond
Will sediment removal be undertaken as part of the Project?
Sediment removal is not an eligible expense under the Small
Communities Funding, but some (15-25%) sediment removal will be
undertaken during site preparation.
What is the schedule for sediment removal?
It would be advantageous to conduct the work concurrently;
however, due to the lack of allocated funding, the schedule for
sediment removal is uncertain.
Penstock Removal What is the schedule for the removal of the
penstock?
It would ideally be undertaken during site preparation, however
there may be an urgency to remove the penstock given the
surrounding seepage.
Weir Width Could the ogee weir be smaller? Based on the criteria
for identifying and
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Topic Question Project Response evaluating the potential options
for redevelopment of the spillway, an up to 30-m long ogee weir
represents the best match with the expressed community values,
while meeting the requirements of the LRIA related to dam safety
and in consideration of 50-year lifecycle costs and risk assumed by
the ORCA.
Headpond Level What are the implications of lowering the
headpond and reducing the width of the weir?
This option does not align as well with expressed community
values (retaining the current headpond water level). In addition, a
change in the water level may result in adverse effects to the
physical environment (aquatic, riparian, etc.).
3.1.2 Community Liaison Committee Meeting - November 23, 2015 An
invitation was extended to the CLC via email on November 4, 2015.
The agenda for the meeting included
• review of comments received during the October 20, 2015 PIC
and subsequent comment period
• discussion of any changes to the environmental evaluation of
the various alternatives to reconstruct the spillway while
retaining the headpond
• discussion of next steps in the Addendum process.
The meeting on November 23, 2015 was attended by nine (9)
members of the CLC, in addition to representatives of ORCA and
Hatch. Comments and questions discussed during the CLC Meeting are
summarized in the following Table.
Table 3-2: Comments and Questions Expressed During the Community
Liaison Committee Meeting on November 23, 2015
Topic Question Project Response Project Funding What is the
Capital Cost of the
preferred Concept C? The 2015 Construction Cost of Concept C is
estimated to be $3,311,000 which includes a 25% contingency.
Sediment removal from the Mill Pond
What is the cost of sediment removal from the Mill Pond?
The Project will include approximately 15-25% of the required
sediment removal from Mill Pond. The remaining sediment removal
activities are not considered to be part of the Project for the
purposes of this Class EA.
What is the schedule for sediment removal?
It would be advantageous to conduct the work concurrently;
however, due to the lack of allocated funding, the schedule for
sediment removal is uncertain.
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3.2 Public Consultation
3.2.1 Project Stakeholders The following Table provides a
listing of Project stakeholders who have been included in the
stakeholder mailing list and provided copies of all notifications
during the Addendum process. In addition, many of these groups have
been represented as part of the CLC. Consultation with First Nation
and Métis communities, provincial and federal agencies, and local
municipalities are discussed in subsequent sections of this
Addendum.
Table 3-3: Project Stakeholders Notified during the Addendum
Process
Project Stakeholders
Save the Dam Mill Pond Group Peterborough Field Naturalists
Kawartha Heritage Conservancy Millbrook and District Lion’s Club
Millbrook and Cavan Historical Society Millbrook Valley Trails
Committee Cavan Monaghan Municipal Heritage Committee Millbrook and
District Chamber of Commerce Ontario Nature Ontario Federation of
Anglers and Hunters Oak Ridges Moraine Foundation Ducks Unlimited
Ontario Field Ornithologists Ontario Fur Managers Federation
Peterborough and the Kawarthas Tourism Peterborough and the
Kawarthas Association of Realtors Trout Unlimited Greater
Peterborough Chamber of Commerce
3.2.2 Website Updates The ORCA’s website was updated continually
throughout the Addendum process. All notifications, presentation
materials, comment sheets and other pertinent information were
posted for stakeholder review and consideration.
3.2.3 UPDATE: Millbrook Class Environmental Assessment Addendum
- September 3, 2015 A reengagement letter was mailed to all Project
stakeholders on September 3, 2015. The purpose of this letter was
to provide stakeholders with an update on the status of the
Project; welcome comments and or concerns; advise of the upcoming
PIC; and notify stakeholders of the upcoming 30-day public review
period which would be ending on November 19, 2015. A copy of the
reengagement letter is included in Appendix A1.
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3.2.4 Notice of Public Information Centre - October 1 and 8,
2015 The Notice of PIC for the Addendum was published in the
Peterborough Examiner and the Millbrook Times on October 1, 2015,
and in the Millbrook Times again on October 8, 2015. ORCA also
issued a News Release with notification information on October 8,
2015. Appendix A2 contains a copy of the notice. Notification
regarding the PIC was also posted to Otonabee Region Conservation
Authority’s website on October 1, 2015.
3.2.5 Public Information Centre - October 20, 2015 The PIC was
held on October 20, 2015, at the Gymnasium in the Township of Cavan
Monaghan Municipal Office located at 988 County Road 10 in
Millbrook, Ontario. The PIC was held in two sessions; first from
1:00 PM to 4:00 PM and second from 7:00 PM to 10:00 PM.
Representatives from ORCA and Hatch were on hand to present
information and answer questions.
The purpose of the PIC was
• to provide information to all interested parties and answer
questions
• for ORCA Staff and representatives from Hatch to present
information about the purpose of the Addendum, a description of the
Addendum process, the alternatives considered and a description of
the updated preferred solution – Spillway Reconstruction with
Headpond Retention (copies of presentations by ORCA and Hatch are
included as Appendices A3 and A4 respectively)
• to provide an opportunity to ask questions and/or identify any
concerns related to the Project.
In addition, display panels were prepared to present
• a revised Concept C (including a design comparison with the
2013 preferred option and estimated 50-year lifecycle costs)
• graphical renderings of the view of the revised Concept C
design, looking upstream and from overhead.
Copies of the display panels are included as Appendix A5.
A total of 14 people signed in at the two sessions of the PIC
(although an estimated 14 additional people were in attendance but
did not sign in). The following tables provide a summary of the
comments and questions raised during the PIC.
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Table 3-4: Comments and Questions Expressed During the Public
Information Centre on October 20, 2015, 1:00 PM Session
Topic Question Project Response Sediment removal from the Mill
Pond
Will sediment removal be undertaken as part of the Project?
Sediment removal is not an eligible expense under the small
communities funding, but some (15-25%) will be undertaken during
site preparation.
Estimated Costs Are the estimated costs of the options inclusive
of design and contingency?
Yes, the costs include design and a 25% contingency.
Schedule What is the timeframe from detailed design to
construction completion?
Class EA will conclude in Q1 2016; Detailed Design and
Construction Tender to be issued in Q2 2016; Design and Permitting
will take approximately 18-24 months; Construction Completion
anticipated in 2019.
Seepage Is the dam hardening proposed as an alternative to sheet
piling?
No, seepage is only of concern at the current spillway and at
the penstock. Costs for a toe drain system have been included in
the estimates in the case that seepage is identified to be of
concern during detailed design.
Table 3-5: Comments and Questions Expressed During the Public
Information Centre on October 20, 2015, 7:00 PM Session
Topic Question Project Response Project Funding What is the
breakdown of funding for
the Project? 66% of the funding for the Project will come from
the Federal and Provincial governments under the Small Communities
Fund. The remainder will be provided by the local municipalities
(90% from the Township of Cavan Monaghan and 10% from the remaining
municipalities).
If there are huge cost overruns, will there be an opportunity to
compensate in designing ancillary components such as the bridge or
the sediment removal?
Yes.
Will ORCA apply for Water Erosion Control Infrastructure (WECI)
funding for aspects of the Project which are not eligible funding
under the Small Communities Funding program?
Yes.
Estimated Costs
Why is the cost of the “Do Nothing” alternative so high”?
The “Do Nothing” alternative takes into account the cost for
replacement of a failed dam, with an allowance to represent costs
as a result of damages to downstream property.
Is there a contingency built into the estimated costs?
Yes, there is a contingency of 25% built into the estimated
costs.
Surface Water What is the volume of water that flows over the
dam at present?
Currently, the average flow of water over the dam is
approximately 0.6 m3/s.
Sediment removal from the Mill Pond
Is sediment removal included as part of the Project?
Sediment removal is not an eligible expense under the small
communities funding, but some (15-25%) will be undertaken during
site preparation.
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Topic Question Project Response Sediment in Mill Pond Will the
sediment in Mill Pond
continue to accumulate after the project is constructed?
Yes, there is not enough flow velocity to move the sediment
enough to prevent accumulation; however the proposed ogee weir
design will pass limited sediment.
Is there any way to design the spillway to pass sediment
effectively and prevent the accumulation that presently occurs?
Yes, however these options would not align with the expressed
community values such as maintaining the headpond at the current
level and preservation of the existing aesthetic qualities.
Dam Hardening What is the lifespan of the dam hardening
material?
The approximately lifespan of the material is 50 years,
depending on the manufacturer and model.
Is the dam hardening a proven technology?
A significant amount of research has been done to examine the
effectiveness of dam hardening using the proposed technology, and
it is accepted as a proven technology.
Width of the Spillway Why is a 30 m dam required? Based on the
criteria for identifying and evaluating the potential options for
redevelopment of the spillway, an up to 30m ogee weir represents
the best match with the expressed community values, while meeting
the requirements of the LRIA related to dam safety and in
consideration of 50-year lifecycle costs and risk assumed by the
ORCA.
Aesthetic Appearance What you are proposing looks very nice.
Comment Noted.
Effects to Downstream Bridges
Will the downstream bridges be affected by the Project?
No.
Effects to Baxter Creek How will the Project affect Baxter Creek
downstream of the dam?
There will be a larger footprint at the bottom of the spillway
for the stilling basin, but otherwise, there will be a negligible
effect to the creek downstream.
How will Baxter Creek be affected during construction?
Flow will need to be diverted around the work area during
construction, but will resume its normal flow path over the
spillway following construction completion.
What is the maximum amount of topsoil that can be placed on the
top of the dam hardening material?
The intent is that the layer of topsoil would be thin enough to
encourage the roots of the grass to secure to the hardening
material and provide some stability of the grassed layer.
Seepage Is the present embankment dam leaking?
Only normal seepage is presently occurring.
Comment sheets were offered to those present at the PIC and
online as a means of providing comments and/or identifying
concerns. The following Table provides a summary of the two (2)
comment sheets that were completed during and following the PIC.
The 30-day comment period concluded on November 19, 2015; no
additional comment sheets were received during this time. A sample
Comment Sheet is included as Appendix A6.
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Table 3-6: Comment Sheet Questions, Stakeholder Responses and
Project Response, Public Information Centre on October 20, 2015
Comment Sheet Question Stakeholder Response Project Response
Please use the space provided below to provide your comments on the
updated preferred solution – Spillway Reconstruction with Headpond
Retention:
I feel like the spillway is still very wide compared to
existing. It’s better than some earlier options/rumors.
Comment Noted. Final spillway width will be determined during
detailed design.
I still feel the spillway is too large but am pleased that a
compromise has been reached to meet the ridiculous MNRF
requirements.
On the matter of cost, I offer the following comments:
I’m not concerned with costs – within reason. I wish the money
spent so far on this very prolonged process could be spent on
construction.
Comment Noted.
Good to hear that ORCA is looking at other funding sources to
dredge the complete pond. This has been a very large issue for the
community since before this process has begun so we would like to
see it resolved at the same time as the dam construction.
Comment Noted.
I offer the following general comments:
I appreciate the dedication and commitment of the CLC and
appreciate the great amount of work that ORCA has put into this
process.
Comment Noted.
On the sediment issue, I feel it is imperative to find a
solution in order to remove the sediment in the pond on an annual
basis; otherwise we will be in the same situation in the next
decade with another large buildup of sediment in the pond. This
issue must be addressed in the design phase.
Sediment removal is not an eligible expense under the Small
Communities Funding, but some sediment removal will be undertaken
during site preparation. It would be advantageous to conduct the
work concurrently; however, due to the lack of allocated funding,
the schedule for sediment removal is uncertain.
3.3 First Nation and Métis Consultation First Nation and Métis
Consultation is an important part of the Class EA process to ensure
that each community has an opportunity to provide input and express
any concern with the Project.
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3.3.1 First Nation and Métis Communities The following Table
provides a list of First Nation and Métis communities that have
been included in the stakeholder mailing list for the Addendum
process:
Table 3-7: First Nation and Métis Communities Notified During
the Addendum Process
First Nation and Métis Communities
Alderville First Nation Curve Lake First Nation Hiawatha First
Nation Kawartha Nishnawbe First Nation Mississaugas of Scugog
Island First Nation Métis Nation of Ontario Oshawa and Durham
Region Métis Council Peterborough and District Wapiti Métis Council
Williams Treaty First Nations Sacred Water Circle
3.3.2 UPDATE: Millbrook Class Environmental Assessment Addendum
- September 3, 2015 A reengagement letter was mailed to all Project
stakeholders on September 3, 2015. The purpose of this letter was
to provide stakeholders with an update on the status of the
Project; welcome comments and or concerns; advise of the upcoming
PIC and notify stakeholders of the upcoming 30-day public review
period which would be ending on November 19, 2105. A copy of the
reengagement letter is included in Appendix A1.
3.3.3 Response from Alderville First Nation Written
correspondence from Alderville First Nation was received on
September 22, 2015 and stated that the Alderville First Nation does
not have any comments or questions regarding the Millbrook Dam
Class Environmental Assessment. A copy of this correspondence is
included in Appendix A7.
3.3.4 Notice of Public Information Centre - October 1, 2015 The
Notice of PIC for the Addendum was distributed to the First Nation
and Métis communities identified in Section 3.3.1 on October 1,
2015.
Following the publication and distribution of the notice, no
responses were received from representatives of the First Nation or
Métis communities.
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3.3.5 Public Information Centre - October 20, 2015 A PIC was
held on October 20, 2015 as discussed in Section 3.2.5. No First
Nation or Métis community members identified themselves as such
during the PIC.
3.4 Agency Consultation
3.4.1 Federal and Provincial Agencies The following Table
provides a listing of provincial and federal agencies that have
been included in the stakeholder mailing list and provided copies
of all notifications during the Addendum process.
Table 3-8: Provincial and Federal Agencies Notified during the
Addendum Process
Provincial Agencies
Conservation Ontario Ministry of Aboriginal Affairs Ministry of
Energy and Infrastructure Ministry of Municipal Affairs and Housing
Ministry of Natural Resources and Forestry Ministry of Tourism,
Culture and Sport Ministry of the Environment and Climate Change
Ministry of Agriculture, Food and Rural Affairs Ministry of
Aboriginal Affairs Ministry of Energy
Federal Agencies
Aboriginal Affairs and Northern Development Canada Canadian
Environmental Assessment Agency Environment Canada Fisheries and
Oceans Canada Transport Canada
The following sections summarize the responses received from
provincial and federal agencies during the Addendum process.
3.4.1.1 Ministry of Tourism Culture and Sport The Ministry of
Tourism, Culture and Sport (MTCS) provided a response to the Notice
of PIC on November 19, 2015 which stated that MTCS generally
supports the option of a 30m Ogee Weir Spillway and Hardened
Embankment Dam, with Headpond Retention at existing level, as it
appears to achieve its structural goals while also aligning with
the community’s concerns. A copy of this correspondence is included
in Appendix A8.
3.4.1.2 Transport Canada Transport Canada (TC) responded to the
October 1, 2015 Notice of Public Information Centre on November 17,
2015 providing direction to provide no further correspondence
provided the Project will not interact with a federal property and
does not require approval/authorization
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under any of the Transport Canada Acts. Accordingly, no further
consultation with TC is required. A copy of this correspondence is
included in Appendix A9.
3.4.2 Local Municipalities The following Table provides a
listing of local municipalities that have been included in the
stakeholder mailing list and provided copies of all notifications
during the Addendum process.
Table 3-9: Provincial and Federal Agencies Notified during the
Addendum Process
Local Municipalities Township of Cavan Monaghan County of
Peterborough
No formal responses from local municipalities were received
during the Addendum process, although Township of Cavan Monaghan
staff members, the Mayor, Deputy Mayor and two (2) Councilors, as
well as an ORCA Director were in attendance during the October 20,
2015 PIC.
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4. Update of the Preliminary Selection of Options 4.1
Preliminary Selection of Options
The Class EA process requires the identification and evaluation
of all reasonable options which would satisfy specific project
objectives, based upon biological, physical, social, cultural,
engineering and technical, economic, and regulatory
determinants.
For the Addendum, the selection process for the most appropriate
option involved four steps; (i) review of new studies and
information related to the safety of the dam, (ii) review of
previous options, broadly defining all generic options, (iii)
refining those generic options and (iv) selecting a preferred
option.
The generic alternatives presented in the draft ESR (2013) have
been updated and are presented as follows.
4.1.1 Alternative 1: Do Nothing This option is self-evident, but
it is not considered feasible because the potential for dam failure
and associated incremental flooding, produce risks to human life
and property, and the ensuing costs of rehabilitating the ageing
dam require attention. The serious concern for dam failure must be
addressed as soon as possible. The “do nothing” option is included
to provide a background condition or, point of reference for
comparison of other options.
4.1.2 Alternative 2: Complete Removal of the Dam (Full
Decommissioning) Complete removal of the dam would eliminate the
incremental flood hazard associated with dam failure. That is,
complete removal of the overflow weir, spillway and the earthen
embankment would eliminate the responsibilities of dam ownership,
and there would be no concern for incremental flooding due to dam
failure. The headpond and dam footprint would be replaced with a
flat, open area that could be developed into parkland favourable to
terrestrial wildlife. As well, Baxter Creek could be restored to
its pre-dam form and function thereby providing a riverine natural
aquatic habitat. Thus, from public health and property and
environmental safety perspectives, dam removal / decommissioning is
considered to be an acceptable alternative.
Dam removal / decommissioning would, however, remove the
heritage value of the dam itself. As well, loss of the headpond
would result in the loss of associated recreational, cultural,
social and economic values. Dam removal is, therefore, viewed as
unacceptable from heritage, recreational, cultural, and social
perspectives.
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4.1.3 Alternative 3: Channel Works In some situations, flood
mitigation may be achieved through channel works aimed at either
the prevention of entry of flood water, or containing flood flows
to control incremental flooding. This could potentially be achieved
by
• increasing channel capacity downstream of the dam by
excavation and construction of berms
• diversion of water (e.g. high flows) from the area to an
adjacent watershed, and/or
• diversion of flow around flood susceptible areas in and around
Millbrook.
Examination of peak flow rates associated with the IDF (181
m3/s) and the Probable Maximum Flood indicates that these methods
do not appear to be technically or economically feasible, since
channelization works, to either divert the flood flows or convey
them downstream, would be massive - in the order of the size of the
Otonabee River valley. As well, the impact to the environment would
be quite extensive. Consequently, these structural methods to
control incremental flooding due to dam failure are not
feasible.
4.1.4 Alternative 4: Reduce Probability of Dam Failure This
option would involve applying remedial measures to the existing
spillway to re-establish structural integrity or, repairing the
earthen embankment and rebuilding the spillway having the same
conveyance capacity, all with an aim to keeping the dam in its
current configuration while reducing the likelihood of failure due
to over-topping and/or toppling or sliding. For example, it would
be possible to line the earth embankments with erosion protection
materials to increase the resistance of the earth embankments to
erosive forces of water flowing overtop them.
Maintaining the existing over-flow weir and spillway in their
current configuration is prevented by regulatory requirements.
Owing to their age and state/condition, there is a near future need
to address problems of deterioration (e.g., deteriorating concrete
spillway structure, erosion at the abutments and under the
spillway, and deteriorating wood material left from the original
control structure), which will require MNRF approval under the LRIA
to rehabilitate, repair and/or replace. LRIA approval will,
however, only be granted if it is demonstrated that the proposed
works to address deterioration of the Millbrook Dam due to age also
reduce the dam safety risks by addressing the risk of incremental
loss of life and property damage due to dam failure.
It is well-documented that the current conveyance capacity of
the outlet structure is not sufficient to reduce the incremental
flood hazard and associated risks to acceptable levels. Dam safety
studies indicate that a six fold increase in spillway capacity is
required to meet dam safety guidelines. Consequently,
re-constructing the spillway with the same existing capacity would
not be approved under the LRIA, and therefore, is deemed
impractical.
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
H349588
H349588-000-200-230-0003, Rev. C, Page 4-3
Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
4.1.5 Alternative 5: Attenuation of Flood In some cases, flood
mitigation may be achieved by attenuating flood waters upstream of
a developed area to prevent entry of flood waters. That is,
increasing upstream storage by
• construction of upstream dams/impoundments to store flood
flows during high volume runoff events, then gradually releasing
the detained storage at a lower flow rate, over an extended period
of time; and/or
• raising the elevation of an existing dam to increase the
volume of storage in the dam’s headpond.
Creating storage capacity well upstream of the Millbrook Dam is
not a reasonable solution for technical, economic and environmental
reasons, since required storage volumes are too large, estimated to
be 3.01 million cubic meters (MMM, 2013); this would require the
construction of an upstream dam larger than the Millbrook Dam.
Hence, increasing storage capacity upstream of the existing
Millbrook Dam and headpond is not a feasible solution to reducing
the incremental flood hazard and associated risks to acceptable
levels.
Increasing the magnitude of storage capacity of the Millbrook
Dam’s headpond itself was also reviewed as a possible remedial
measure. Increasing the storage capacity of the headpond would
involve raising the elevation of the existing Millbrook Dam earth
embankments and spillway concrete wingwalls to increase the maximum
water level that can be contained in the Millbrook Dam’s headpond.
The minimum volume of storage that would be required would need to
be sufficient to attenuate the regulatory flood standard (i.e.,
Timmins Regional Storm), and has been estimated to be 3.01 million
cubic meters (MMM, 2013). This is in the order of 6500 times the
current storage capacity of the Millbrook Dam headpond. Clearly,
raising the Millbrook dam to contain such a large volume of run-off
could only be achieved with a significant increase in height and
increase in potential incremental flooding. Owing to the regulatory
requirements of the LRIA, the MNRF is unlikely to approve a project
proposal to significantly increase the height of the Millbrook Dam
due to environmental impacts and increased potential for flooding
of upstream and downstream residents. Hence, increasing the volume
of storage in the Millbrook Dam’s headpond, in itself, is not a
reasonable solution.
4.1.6 Alternative 6: Increase Hydraulic Capacity The 2015 Dam
Safety Assessment Update Report included: hydrologic analysis to
update flood estimates and the PMF, dam break study and inundation
mapping, dam classification and selection of the IDF and DE, and
embankment dam geotechnical stability analysis.
The evidence contained in the 2015 Dam Safety Assessment Update
Report has necessitated an Addendum in order to re-evaluate the
options for increasing the hydraulic capacity to accommodate the
new IDF peak flow of 181 m3/s, which is an 81% increase from the
2008 IDF estimate. As well, existing problems with erosion and
seepage at the spillway indicate that the abutment and foundation
cutoffs are no longer intact and will need to be replaced to
prevent a piping failure due to internal erosion.
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
H349588
H349588-000-200-230-0003, Rev. C, Page 4-4
Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
The proposed solution remains, as previously concluded, to
increase the capacity of the dam’s outflow structures to pass the
IDF in a manner compliant with the LRIA, and the subsequent
determination of the type and design of outflow / control
structures which would be best suited to increasing the hydraulic
capacity while also best preserving the biological, physical,
recreational, social, cultural and heritage characteristics and
values of the Millbrook Dam site.
The next step was to explore how best to increase hydraulic
capacity through review of the available engineering studies
provided by ORCA, and assessment of feasible upgrades typically
employed to increase hydraulic capacity at existing dams. The
possible options which were considered are as follows.
4.1.6.1 Increase Water Level in Headpond Public suggestions to
raise the crest of the dam by 0.5 m to its original height prior to
settling are acknowledged. This would result in an increased
hydraulic capacity of the weir but, this alone would not provide
the full amount required. However, this option could be considered
in combination with enlargement of the weir hydraulic capacity. The
previous concerns with stability of the embankment dam have been
addressed in the 2015 Dam Safety Assessment Update Report, which
indicates that the dam meets the required factors of safety with
respect to structural stability under the present IDF and DE
conditions. However, this is only viable if the seepage issues at
the spillway are properly addressed to prevent erosion of the
embankment, spillway abutments and foundation.
4.1.6.2 Stoplog or Manual Actuation Water Control Gates The size
of the watershed above Millbrook is relatively small. As a result,
when heavy rainfall occurs, water level and flow conditions in
Baxter Creek change relatively quickly. As a highly dynamic
watercourse, there is very little time to operate either stoplogs
or manual actuation gates. This scenario occurred in 1980 when the
stoplogs could not be removed before flood waters over-topped the
Millbrook Dam. Accordingly, neither stoplogs or manual actuation
gates are an effective means of controlling water levels in the
Millbrook head pond due to a requirement for manual monitoring, or
meteorological and hydrological conditions monitoring, and
subsequent time for decisions regarding when to increase and
decrease outflows (and by how much), as well as the time to
implement manual operation of the structure.
4.1.6.3 Automatic Actuation Water Control Gates Fully automatic
actuated gates employing computers, data logging, sensory
equipment, electric motors and pneumatic systems (“smart gates”) in
theory do not require a human decision-maker or human operation. In
practice, however, operational functionality of automated systems
cannot be guaranteed even with regular and on-going intensive
maintenance and attention. Provisions for a manual system are,
therefore, necessary as back-up. Accordingly, automated water
control gates do not entirely eliminate the need for human
interaction. The requirement for full time staff who are
knowledgeable in the operation and maintenance of the gate system
makes the use of automated control gates to provide
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
H349588
H349588-000-200-230-0003, Rev. C, Page 4-5
Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
spill capacity one of the most expensive options (in terms of
both capital cost and operating costs).
Typically, because of the high costs, control gates are only
used in cases where other means are not technically viable, and
there is an economic benefit to maintaining full control of flows
and headpond levels. In the case of the Millbrook Dam, an automated
water control structure cannot be relied on to control the risks of
dam failure, and therefore would not be economically viable where
other means of providing spill capacity will be less costly over a
50-year life cycle.
4.1.6.4 Remove Existing Outlet Structure and Widen Opening
Removal of the existing over-flow weir and spillway, including a
portion of the adjoining earthen embankment, can be considered.
Under this option, the spillway would be replaced and the opening
in the adjacent embankment widened and lowered sufficiently to
accommodate the required flows. In doing so, the new spillway would
meet all current dam safety standards.
4.1.6.5 Rebuild Outlet Structure using Different Configuration
This alternative would involve reconstruction of the spillway with
a new, larger overflow weir, sized to convey the required IDF. The
new structure would be inoperable (i.e.no stop logs, gates, valves
or other control mechanisms) and flows would be controlled solely
by the elevation of the weir crest. Concurrent with spillway
reconstruction, the abutment wall cut-offs and the foundation would
be replaced to address the seepage and erosion problems. Spillway
reconstruction in the form of a new weir, along with abutment and
foundation repairs represents a viable solution.
4.1.6.6 Harden the Embankment and/or Install a Fuse Plug The use
of fuse-plugs and hardening of embankments has become a viable
option to increase spillway capacity at existing dams. Several
options are available for engineered erosion protection to reduce
the risk of embankment failure during an overtopping event. As
well, a fuse-plug designed to open in a controlled manner has been
accepted by the MNRF to provide additional hydraulic capacity when
necessary.
A smaller reliable overflow spillway, such as one of the already
proposed weirs, to pass flows in the order of the 1:1000-year to
1:10,000-year flood, and use of a fuse-plug or embankment hardening
(i.e., less reliable methods to safely pass floods) to provide the
additional capacity required for the IDF would be an acceptable
solution that would be approved by the MNRF.
4.1.7 Dam Safety Risk Assessment Screening of Options Hatch has
performed an overall risk assessment of probability of dam failure
as one of the criteria for preliminary screening of the
alternatives. This high level preliminary analysis followed a
process that is currently under review by MNRF as part of the
Province’s interest in including risk-informed decision making
basis to complement the standards-based approach that exists now
for the LRIA.
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
H349588
H349588-000-200-230-0003, Rev. C, Page 4-6
Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
The analysis assesses the risks for dam failure and compares
those risks to recognized allowable risks suggested by Canadian Dam
Association (CDA). This provides ORCA with the information needed
to determine to what extent the dam safety issues need to be
addressed, and allows discounting of Options that will not meet
ORCA’s obligations under the LRIA dam safety guidelines.
4.1.7.1 Methodology for Risk Assessment Risk-informed decision
making has been gaining increased attention as a tool for
understanding the level of safety associated with an existing dam
and determining what level of deviation from the normal performance
condition can be considered tolerable (CDA, 2007).
The MNRF, working in partnership with Ontario Power Generation
(OPG), Grand River Conservation Authority (GRCA), and Hatch have
developed a conceptual framework for regulating the use of
risk-informed decision making. As part of this initiative Hatch
developed a unique risk assessment tool that has been tested by the
MNRF and OPG and found to provide realistic, repeatable results for
assessing the probability of occurrence of Key Dam Safety failure
modes. The methodology for performing a risk assessment consists of
the following:
• Step 1: Selection of Project Failure Modes
• Step 2: Determination of Failure Probabilities
• Step 3: Estimate of Consequences
• Step 4: Risk Evaluation.
Hatch applied this methodology to perform a preliminary risk
assessment of the Millbrook Dam with respect to the potential for
failure due to overtopping and seepage.
4.1.7.2 Preliminary Risk Assessment Hatch has performed a
preliminary risk assessment for the overtopping and seepage failure
modes. Further analysis could be applied for stability, but was not
done since the dam meets stability requirements.
As presented below in our risk analysis, the required IDF on a
risk basis could be as low as 100 m3/s for the 1 in 10,000-year
flood rather than the standards based IDF of 1 in 100,000-year
flood that the peak flow of 181 m3/s represents.
This assessment provides a strong argument for selection of a
smaller reliable spillway, such as the proposed weirs, to pass 65
m3/s to 100 m3/s and use of embankment hardening (i.e., less
reliable method to safely pass floods) to provide the additional
capacity required for the IDF.
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
H349588
H349588-000-200-230-0003, Rev. C, Page 4-7
Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
4.1.7.2.1 STEP 1: Selection of Project Failure Modes The reasons
for a dam to fail have been studied by numerous authors. In 1975 a
study performed by the American Society of Civil Engineers (ASCE)
and the United States Committee on Large Dams (USCOLD) showed that
there were four general causes of dam failure as depicted in Figure
4-1.
Figure 4-1: Causes of Dam Failure (Source: ASCE/USCOLD,
1975)
4.1.7.2.2 STEP 2: Determination of Failure Probabilities As
described in Table 4-1, the risk assessment tool evaluates the
probability of failure for the key failure modes using a variety of
proven methodologies.
In the case of the Millbrook Dam, the key failure modes that
need to be considered for assessment of the overall probability of
failure are
• overtopping risks associated with an extreme flood event are
high
• the potential for internal erosion around and under the
spillway are high
• the potential for internal erosion around the old penstock are
high
• stability risks are low.
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
H349588
H349588-000-200-230-0003, Rev. C, Page 4-8
Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
Table 4-1: Basis of the Probability Estimates
Module Description Basis of Estimate Remarks 1 Failure of Gate
to Open Expert Judgment 38% of Failures
Potential for Overtopping Standard Hydrotechnical Statistical
Analysis
PMF Analysis Statistical Assessments 2 Embankment Dam Piping
Empirical Analysis 33% of Failures 3 Embankment Dam Slope Stability
Empirical Analysis Part of 23% of Failures 4 Concrete Dam Sliding
Mathematical analysis
using the Capacity-Demand methodology
5 Gate Failure Mathematical analysis using the Capacity-Demand
methodology
Part of 8% of all Failures 6 Penstock Failure
In the case of the Millbrook Dam the potential for overtopping
and the possibility of piping failure associated with the ongoing
seepage under the spillway and the old penstock represents the key
dam failure modes. The history of flooding at Millbrook Dam is
dominated by significant rainfall events. It is known that the
existing spillway capacity can only pass the 25-year flood event.
More significant floods will cause overtopping of the dam and this
is known to have happened twice in the 200-year history of the dam,
although it did not cause failure of the dam. This would indicate
the probability of failure due to overtopping ranges from 1.0E-02
to 5.0E-3. Assuming the probability of evacuating persons is low
following the current practices due to the flash flood
characteristics of the small basin, the annual probability of
occurrence of a dam failure with persons in the town of Millbrook
is in the order of 1:100; adding the piping (internal seepage and
erosion) risks at the spillway and old penstock increases the
overall probability of dam failure.
Probability of Millbrook Dam Failure
4.1.7.2.3 STEP 3: Estimate of Consequences Based on the results
of the assessments of risks to life safety in the 2015 Dam Safety
Assessment Update Report, the number of persons subjected to Life
Safety Risk due to dam failure ranges from 1 to 10; for this
assessment we have assumed five (5) persons.
4.1.7.2.4 STEP 4: Risk Evaluation With probabilities of failure
calculated for the potential failure mode and the consequences of
those failures determined, the risk of overtopping failure of the
embankment dam was evaluated.
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Otonabee Region Conservation Authority Addendum to the ESR for
Remedial Flood Control Works - Millbrook Dam
H349588
H349588-000-200-230-0003, Rev. C, Page 4-9
Ver: 04.02 © Hatch 2016 All rights reserved, including all
rights relating to the use of this document or its contents.
With an estimate of the probability of failure of 1:100 years
and five persons at risk under existing conditions, as is shown in
Figure 4-2, the risks associated with overtopping of the Millbrook
Dam require action to reduce the risk in accordance with the CDA
Dam Safety Guidelines and would be required by MNRF to gain LRIA
approvals. Risk assessment for dam safety should consider the
approach as shown in Figure 4-2, which presents life safety risk
guidelines that are consistent with values used in other hazardous
industries and with the principle that risks should be made as low
as reasonably practicable (ALARP).
Figure 4-2: Life Safety Risks Presented by Millbrook Dam
Overtopping Compared with CDA Dam Life Safety Risk Acceptability
Criteria
For a HIGH consequence dam like the Millbrook Dam the CDA would
specify an annual flood frequency of not more likely than 1:5,000
years and potentially in excess of 1:100,000 years.
Further analysis of the existing internal erosion problems at
Millbrook Dam spillway and old penstock would, based on Hatch’s
experience, indicate that piping is also a source of risk,
providing further evidence that the Do Nothing option is not a
viable option and that the selected options moving forward must not
only address the flood capacity but must also address any potential
for internal erosion.
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0.1 1 10 100
Prob
abili
ty o
f Los
s of
Life
mor
e th
an (p
er y
ear)
Number of Persons Subject to Life Safety Risk
Loss of Life - Probability vs. Consequence
Additional risk control is required
Millbrook Dam Overto