00/2015 public information centre addendum to the environmental study report: millbrook dam tuesday,...
TRANSCRIPT
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Public Information Centre Addendum to the Environmental Study Report: Millbrook Dam
Tuesday, October 20, 2015
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Hatch Ltd. • Hatch is an employee-owned firm with headquarters in
Mississauga, Ontario. • Hatch brings significant engineering and environmental
assessment experience in the rehabilitation, development, design and planning of major and minor dam related projects.
• Over the last 15 years we have undertaken over 300 dam safety assessments for various clients including the MNRF, CAs and various municipalities.
• Hatch has served clients for over six decades with corporate roots extending over 100 years and has project experience in more than 150 countries around the world.
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What are the Project Objectives? 1. Update the Preliminary Screening of
Options:Alternative 1 – Do Nothing
Alternative 2 – Full Decommissioning
Alternative 3 – Channel Works
Alternative 4 – Reduce Probability of Failure
Alternative 5 – Attenuation
Alternative 6 – Increase Hydraulic Capacity
2. Update Solutions for Achieving
Increased Hydraulic Capacity:Concept A – Spillway Removal and Channel Restoration
Concept B – Spillway Removal and Off-line Pond
Concept C – Spillway Reconstruction with Headpond Retention
3. Identify and Evaluate Various Options to Achieve the new Preferred Solution (Concept C)
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1. Update the Preliminary Screening of Options
Hatch has updated the preliminary Screening of all broadly defined, generic options as presented in the 2013 draft Environmental Screening Report (ESR) based on the 2015 Dam Safety Assessment Update Report prepared by D.M. Wills Associates Ltd.
The preferred option continues to be Alternative 6 – Increase Hydraulic Capacity as was previously presented in the draft ESR, and aligning best with expressed community values.
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What are the Project Objectives? 1. Update the Preliminary Screening of
Options:Alternative 1 – Do Nothing
Alternative 2 – Full Decommissioning
Alternative 3 – Channel Works
Alternative 4 – Reduce Probability of Failure
Alternative 5 – Attenuation
Alternative 6 – Increase Hydraulic Capacity
2. Update Solutions for Achieving
Increased Hydraulic Capacity:Concept A – Spillway Removal and Channel Restoration
Concept B – Spillway Removal and Off-line Pond
Concept C – Spillway Reconstruction with Headpond Retention
3. Identify and Evaluate Various Options to Achieve the Preferred
Solution (Concept C)
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2. Updated Solutions for Achieving Increased Hydraulic Capacity
The solutions for achieving increased hydraulic capacity remain the same as those presented in the 2013 draft ESR (Concepts A, B and C) and continue to incorporate:
i The community values as presented in the 2013 draft ESR
ii The need to address the problems associated with the condition of the dam
Hatch has updated Concepts A, B and C to also incorporate: iii Alignment with the findings of the 2015 Dam Safety Assessment Update Report.
iv The estimated 50-year life cycle cost of each of the Concepts, including the cost of management and major maintenance over a year 50 operation of the facilities
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i. Community Values The following community values were identified in the 2013 draft ESR for consideration in the evaluation of potential solutions:
• Maintaining the size and water level of Mill Pond • Headpond sediment removal and restoration of Mill Pond• Maintenance of public access across the dam via a foot bridge• Waterfall effect producing sights and sounds similar to existing
conditions
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ii. Dam Condition Problems The evaluation of alternatives has also incorporated the need to address the existing issues and problems associated with the current condition of the dam:
• seepage through the embankments• deteriorating penstock intake structure
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iii. Findings of the 2015 Dam Safety Assessment Update Report
The 2015 Dam Safety Assessment Update Report was undertaken to update the 2008 Dam Safety Review for Millbrook Dam to incorporate the a number of Technical Bulletins released by MNRF in 2011 under the Lakes and Rivers Improvement Act (LRIA).
The 2015 Report concluded that the hydraulic capacity of the dam needed to be increased for the new Inflow Design Flood (IDF) of 181 m3/s.
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iv. Estimated 50-Year Life Cycle Costs
Solutions for Achieving Increased Hydraulic Capacity and “Do Nothing” Alternative
2013 Construction Costs
2015 Construction Cost
50-Year Life Cycle Cost 1
Concept A: Spillway Removal and Channel Restoration $ 3,090,000 $ 3,077,550 $ 3,637,000
Concept B: Spillway Removal with Off-Line Pond$ 3,249,000 $ 3,214,000 $3,781,000
Concept C: Spillway Reconstruction with Headpond Retention $ 2,948,000 $ 3,311,000 $ 3,973,000 2
Do Nothing N/A N/A $ 4,886,000
Notes:1 – 50 year life cycle cost includes cost of dam safety management program 2 – 50-year life cycle cost for Concept C is based upon a 30m Ogee Crest Weir and Harden Embankment Dam.
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The Preferred Solution Remains Unchanged
Based on Hatch’s updated solutions for achieving increased hydraulic capacity, the preferred solution remains:
• Concept A – Spillway Removal and Channel Restoration• Concept B – Spillway Removal and Off-line Pond• Concept C – Spillway Reconstruction with Headpond Retention
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What are the Project Objectives? 1. Update the Preliminary Screening of
Options:Alternative 1 – Do Nothing
Alternative 2 – Full Decommissioning
Alternative 3 – Channel Works
Alternative 4 – Reduce Probability of Failure
Alternative 5 – Attenuation
Alternative 6 – Increase Hydraulic Capacity
2. Update Solutions for Achieving
Increased Hydraulic Capacity:Concept A – Spillway Removal and Channel Restoration
Concept B – Spillway Removal and Off-line Pond
Concept C – Spillway Reconstruction with Headpond Retention
3. Identify and Evaluate Various Options to Achieve the new Preferred Solution (Concept C)
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3. Identify and Evaluate New Spillway OptionsSeveral options to increase the spillway discharge capacity at Millbrook Dam were considered.
•Self-regulating overflow weirs such as a:•Straight weir•Labyrinth Weir•Piano Keys Weir
•Gated spillways such as:•Obermeyer spillway gates•Vertical lift gates•Radial gates
•Fuse-Plug spillway•Fuse-Gate System•Reducing the probability of dam failure through embankment dam hardening•Raising embankment dam crest to allow for higher IDF reservoir level•Lowering of the weir crest to increase the head and discharge capacity
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3. Identify and Evaluate New Spillway Options
The following criteria were also considered in Hatch’s evaluation:
• Safety and protection downstream• Reliability and accuracy of the flood prediction• Loading conditions including seismicity, ice, and sediment• Potential for debris (i.e. ice) blockage• Duration, amount and frequency of spill• Geology, ground topography and river bathymetry• Various dam types• Operational considerations (i.e., remoteness of the site,
experience of the owner, etc).• Potential effects on the environment • Costs (construction and operations)
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Hatch’s Recommendation: 30m Ogee Weir Spillway and Harden Embankment Dam, with Headpond Retention at Existing Level
Based on Hatch’s evaluation of the various options, this has been determined to be the most cost-effective option to comply with ORCA’s requirements under the LRIA, and align with the community’s values as documented within the 2013 ESR.
Design Parameter Preferred Option 2013 Preferred Option 2015
Width of Weir 36 m 30 m
Spillway Design Sharp Crest Weir &Stepped Spillway
Ogee Weir Chute Spillway & Hardened Embankment Dam
Mill Pond Water Level 0.5 m reduction in water level
Maintain existing pond
Sheet Pile Installation Required Not Required
Design Comparison: 2013 vs 2015 Preferred Options
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Existing Dam
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Graphical Rendering of Recommended Option
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General Arrangement
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Ogee Weir Spillway
On Ogee weir spillway is designed to conform with the profile of a sheet of water as it overflows.
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Hardening of an Embankment Dam
Hardening of an embankment dam involves the application of an erosion control “blanket” comprised of concrete blocks, locked together and embedded with high strength geogrid.