ORCA Presents New Design for Millbrook Dam

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Photo Karen Graham.  The new dam design was initiated due to hydrogeological issues with the site and brings the scale closer to the existing structure width.

At a public consultation conducted last January, Otonabee Conservation shared a design for the reconstruction of the Millbrook Dam spillway that incorporated a 26 metre wide weir, with a multi-level spillway where water that spilled constantly flowed over the lower, eastern side of the spillway and during times of high water it would spread over the entire width of the spillway.    Attractive finishes were incorporated into the dam, including textured concrete retaining walls, railings of weathered steel and a two metre wide walkway over the dam consisting of an open grate to allow the rain and snow to fall away with a solid strip in the centre for pet comfort.

The public was satisfied with the plan for the most part, but expressed concern over the width of the dam and the visual impact of the western side of the spillway which would be dry most of the time and could invite unwanted graffiti artists.

ORCA has continued to explore ways to address these issues and move forward to address the practical issues presented by the site.  During further design work, the costs of the plan began to escalate.   Based on some geo-technical issues with the site, Otonabee Conservation’s consultants undertook a complete review of the design and concluded that a different design may be warranted and asked the consultants to go back to the drawing board and prepare a new design.

A rendering of the new design for the spillway can be seen below, and includes some significant changes to the last proposal outlined in the January.

The most noticeable change is the size of the dam.  The weir has been changed from a straight 26-meter-wide weir to a u-shaped weir with outside wall dimensions of 5 meters by 16 meters by 5 meters, which is only 5 meters wider than the existing structure.  The floor of the spillway has changed completely to deal with the geo-technical issues on site. These changes are below ground and underwater and are not visible.

There is now a new approach for the flow of the water. A single coffer dam will direct water through a channel to be built where the steel penstock is presently situated. Construction of the spillway will occur in a single phase. In the earlier design sheet piling was to be used to divert the water during construction and construction would have been undertaken in two phases.

The plan for hardening the earthen berm remains unchanged as do the proposed finishing features for the public walkway bridge and walls of the spillway.

These changes come with many advantages.  First, the overall this design is very similar in scale to what exists today. The new 12-meter-wide spillway will replace the existing 7-meter-wide spillway and the weir will be the same u-shape. The size and water level of the mill pond will remain the same and the waterfall sights and sounds will be very similar to existing conditions.

Because the structure is smaller, construction will be less complicated and is expected to fall within the budget and original timeline.  Tenders for the project are expected to be issued by the end of August with construction anticipated to begin in the fall and the project completion the following year.

At the Open House last week, ORCA Chief Administrative Officer Dan Marinigh was pleased with the new design, as it addresses the community’s concerns and falls within budget.  In his experience, large engineering projects like this one often go back to the drawing board for major revisions as new information comes to light.  In this instance, the perseverance of the project management is clearly paying off. KG

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