Council Column: So what has been going on with the Millbrook Dam you ask?

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The simple answer is a lot.

The most obvious activities to date have been centered around strengthening the toe berm and stopping a leak around the penstock intake. The toe berm runs along the back side (dry side) of the dam, reinforcing the structure by giving it added width at the base.  The penstock is the opening in the dam where water was channeled down and used to power the turbine in the mill. Once the mill was no longer operational the penstock was closed off, but water still continue to trickle through. The fact the water was travelling through the penstock on its own was not that much of a concern, but it was discovered that the water was actually travelling along the outside edges of the pipe, and slowly eroding materials away, all the while, saturating and weakening the earthen wall of the dam itself.

The leakage was first observed in routine inspections, in summer of 2016, after the mill had been relocated. The seepage was causing erosion, but with the construction of the new dam imminent, it was determined that it wasn’t significant enough to be of immediate concern, but the situation would continue to be monitored closely going forward. After a few months of monitoring it was obvious that the rate of erosion was significantly increasing, and remedial actions had to be taken to ensure the structural integrity of the dam was not compromised.  No longer was waiting for the reconstruction of the new dam going to be possible, action had to be taken for public safety.

Dan Marinigh, CAO of ORCA worked quickly with Hatch, the firm retained to provide engineering services, and the Ministry of Natural Resources, to come up with a plan to stabilize the dam. After considering different options, it was determined that building a cofferdam in front of the penstock was the most efficient use of resources. The cofferdam would dovetail nicely into the upcoming full reconstruction project that is already going to takes place.

The cofferdam in front of the penstock intake is now complete. A cofferdam is essentially a lot of fill placed in front of the penstock which will make it difficult for the water to penetrate through to the intake. The cofferdam will remain in place until the larger dam reconstruction project is completed and the penstock is removed.

That brings us to the dam reconstruction project itself. Many of you attended the Open House information session held in January where ORCA and Hatch unveiled the plans for the new Millbrook dam. The plans presented varied from the previous drawings, in that the water flow came down the east side of the dam and the west side would take overflow in high water events. In the previous design the water flowed down the center and the over flow went on either side. The present design also reduced the width by 4m, down to 26m across, from the previous design that called for 30m.

As it turns out, while Hatch was unveiling their drawings to show the public in January, their plans and drawings were still going through internal technical review cycles. After detailed analysis by teams of engineers and reviews of geotechnical reports, it was evident that the design had to change. The analysis uncovered that the ground under the dam is not stable enough to sustain the foot print of the structure that was proposed. So back to the drawing board they went. Now we are eagerly anticipating a look at their new design, which will look much like the existing “U” shaped configuration that is there today, only twice as big. ORCA will have another public information session once the new design is ready for presentation.

Notwithstanding the emergency remediation of the penstock, toe berm strengthening, and the change of design, the Millbrook Dam Project is still on budget, and is still pretty much on time, with award of the construction contact in the next few months, and the first steps of construction beginning later in the fall of this year.

It has been a long and twisty road to get to this point in the reconstruction of the Millbrook Dam, but we are almost there, with tangible results on the horizon. Stay tuned!

By Ryan Huntley

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