Millbrook Dam – Spillway Reconstruction with Headpond Retention

The Millbrook Dam was constructed in 1822-24, and is a 120-metre long earth embankment dam with a concrete spillway controlled by a U-shaped weir. The weir creates a headpond of approximately 5.5 hectares (13.7 acres) in size. The dam suffers from issues and problems common of dams built almost 200 years ago, including inadequate design, inappropriate construction materials and deterioration due to age. The Millbrook Dam is located on Baxter Creek immediately upstream of the downtown residential and business core of the village of Millbrook. The Millbrook Dam is owned by the Otonabee Region Conservation Authority (ORCA) and is a non-operational dam.

 

Millbrook Dam

Millbrook Dam

Safety of the Millbrook Dam

In accordance with Ministry of Natural Resources and Forestry, Lakes and Rivers Improvement Act and Ontario Dam Safety Guidelines, the Millbrook Dam is assigned a “high” hazard potential classification. In the event of dam failure, the resultant quick release of headpond water is projected to cause incremental flooding in the downstream developed area that could lead to incremental loss of life, property damage, social disruption and environmental harm under both dry and wet weather conditions. The existing overflow weir and spillway are incapable of discharging flood flows and the dam is inadequate to prevent toppling and/or sliding.

Chronology of Activities

February 2, 2017: The Otonabee Region Conservation Authority shares a letter to the community respecting the comments heard during the Open House of January 26,2017. Read the letter here

January 26, 2017: The Otonabee Region Conservation Authority is hosting an Open House to present the design of the new Millbrook Dam. The Open House will be held in the Gymnasium (Cavan Monaghan Municipal Office) from 4:00 p.m. to 7:00p.m.

January 13, 2017: The Otonabee Region Conservation Authority is undertaking remedial actions at the Millbrook Dam to ensure the structural integrity of the dam. Read the details at Millbrook Dam Remedial Actions.

August 29, 2016: The Board of Directors of the Otonabee Region Conservation Authority has contracted the engineering firm, HATCH, for the design and procurement of construction services for the reconstruction of the Millbrook Dam. The proposed timelines for this project are: January 31, 2017 – Design for the reconstructed dam is presented for public viewing; July 31, 2017 – Contract for the reconstruction is awarded and the necessary permits are obtained; and, After July 31, 2017 the reconstruction of the Millbrook Dam begins – anticipate 15 months to completion.

February 25, 2016: The Otonabee Region Conservation Authority has now completed the planning and design process approved under the Environmental Assessment Act in the Class Environmental Assessment for Remedial Flood and Erosion Control Projects for undertaking a remedial project regarding the Millbrook Dam located in Millbrook Ward in the Township of Cavan Monaghan, Peterborough County. The Authority thanks all who expressed and interest and participated in the planning of this project. The Notice of Project Approval was issued today.

The preferred option for the Millbrook Dam is the reconstruction of the spillway and enhancement of the earthen embankment to ensure integrity of the dam during periods of high flood events. Public access across the dam via a foot bridge and other amenities are also incorporated in this option and reflect the community values for the site as expressed throughout the consultation process.

The Authority will now proceed to undertaking the preferred option with an estimated date of project implementation of 2018-2019.

January 21, 2016: The Otonabee Region Conservation Authority opens a 15-day public comment period for the Addendum to the Environmental Study Report for Remedial Flood Control Works for the Millbrook Dam. The Addendum Report may be viewed online or may be downloaded (if downloading, please be aware that the file is large and may take a few minutes to complete the download).

For details on how to submit written comments, please read the “Notice of Filing” of an Addendum for Review (Jan. 21, 2016).

October 20, 2015: The Otonabee Region Conservation Authority will host a Public Information Centre to provide information to all interested parties and answer questions. ORCA Staff and representatives from the consulting firm, Hatch Ltd., will 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. For details, please read the Notice of Public Information Centres

July 2015: The Otonabee Region Conservation Authority receives funding for upgrades to the Millbrook Dam thanks to joint financial support from the Governments of Canada and Ontario through the Small Communities Fund. The Government of Canada will provide up to $1,119,363 through the Fund; the Province of Ontario will contribute up to $1,199,362 to this project; and the Conservation Authority will be responsible for all remaining costs.

June 2015: The ORCA hosted a Public Information Session at the municipal office of the Township of Cavan Monaghan. The formal presentation provided an update on the project work undertaken to date, the approach moving forward to ensure completion of the Class EA for the Millbrook Dam and Authority staff answered questions of those in attendance. A Fact Sheet was made available, summarizing the presentation.

June 2015: The Authority Board of Directors approves the awarding of the contract to Hatch, and the process to revise the Environmental Assessment for the Millbrook Dam is initiated.

May 2015: The Otonabee Region Conservation Authority issued a Request for Proposals to undertake the revision of the draft Environmental Assessment Report for the Millbrook Dam, with a closing date of June 12, 2015.

April 2015: The Board of Directors approved a staff recommendation to revise the draft Environmental Assessment Report for the Millbrook Dam. The dam has to safely pass a volume of water equal to 181 cubic meters per second in order to meet regulatory requirements. In preparing the EA study, a design value of 100 cubic meters per second was used. This change requires that alternative means of increasing the hydraulic capacity of the dam be investigated.

February 2015: The completed Millbrook Dam – Dam Safety Assessment Update Report was presented by D.M. Wills to the Board of Directors of the Otonabee Region Conservation Authority and the key findings of the report were adopted.

September 2014: ORCA staff determined that there was a need to reconfirm the hydrological analysis that the IBI Group had undertaken, and that the results of this analysis and the earlier work undertaken by D.M. Wills be consolidated into a single report, whereby one author could validate all inputs, assumptions and findings/conclusions.

May 2014: D.M. Wills Associates was contracted by ORCA to re-calculate the incremental analysis of the Inflow Design Flood. The resulting calculation doubled the Inflow Design Flood valve from 100 cms to 201 cms.

April 2014: Owing to subsurface seepage erosion, a sinkhole developed in the Millbrook earth embankment dam. ORCA undertook immediate remedial measures to mitigate the threat to the structural integrity of the Millbrook Dam to protect the public against possible harm. The site restoration and the installation of public safety measures resulted in public access to the site being restored in December 2014.

December 2013: ORCA contracted D.M. Wills to review and update the dam safety assessment previously completed by the IBI Group in 2008 to ensure that the dam safety assessment satisfied the requirements of the new dam safety guidelines issued by the OMNRF. In addition, D.M. Wills was contracted to update and improve the accuracy of downstream inundation mapping, which also involved detailed topographical surveys to support the mapping. The findings of D.M. Wills Associates was that the dam breach parameters used by the IBI group in 2008 were both too shallow and too narrow and therefore would not meet with OMNRF approval.

April 2013: New dam safety guidelines established by the Ontario Ministry of Natural Resources and Forestry (OMNRF) came into effect, requiring an applicant to demonstrate that the design of a dam meets the new safety guidelines. The guidelines also changed the formula for determining the Inflow Design Flood value.

2012-2013: ORCA undertook a Class Environmental Assessment for the Millbrook Dam to determine a preferred solution to the problem of incremental flooding caused by the potential failure of the dam. The preferred solution documented in the draft Class EA study was to construct a new weir capable of passing an Inflow Design Flood value of 100 cms. It was not in the scope of the Class EA contract to re-assess the hydrology or the incremental analysis.

2008: The IBI Group was contracted to complete a dam safety assessment for the Millbrook Dam in accordance with the 1999 Draft Ontario Dam Safety Guideline. In the final analysis, the IBI Group undertook the incremental analysis and concluded that the Inflow Design Flood value of 100 cms was sufficient to control the risk due to dam failure.

Stakeholder Participation

The Class Environmental Assessment for the Millbrook Dam is an inclusive process. Various stakeholders have been involved throughout various stages of the process and participation on the Community Liaison Committee supported by ORCA.

 

Environmental Study Report Millbrook Dam
Environmental Study Report: Appendix A to D

(Note to the Reader: Given the size of the Environmental Study Report, it is posted on this website in two parts. Be sure to review both the Report and Appendix A to D.)

Millbrook Dam – Dam Safety Assessment Update Report 2015