addendum to the environmental study report for remedial ......addendum to the environmental study...

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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

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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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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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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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• 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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• 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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• 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.


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.


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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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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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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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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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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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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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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H349588-000-200-230-0003, Rev. C, Page 4-9


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

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Number of Persons Subject to Life Safety Risk

Loss of Life - Probability vs. Consequence

Additional risk control is required

Millbrook Dam Overto

addendum to the environmental study report for remedial ......addendum to the environmental study...

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