Improvements and infrastructure

Can you imagine someone strapped to the grill of your car, replacing the engine while you're driving? Similarly, how do you replace a dam that is not only located in the middle of an urban area, but is a crucial part of a 24 hours a day, seven days a week, 365 days a year water management operation? It is the kind of question that the people who operate and maintain the Scotts Mills Dam, adjacent Lock 19 in Peterborough, Ontario, and those who understand its necessity, had been wondering for years.

When the Scotts Mills Dam had finally lived out its useful life – how could this complex task be undertaken?

The original Scotts Mills Dam, at what was then known as Whitlas Rapids, was built between 1837 and 1844. Both the dam and adjacent lock have been rebuilt, repaired, and relocated over the years, with the most recent dam structure constructed in 1904 – one of the first concrete dams ever built in Canada. In the years prior to 2015, engineering inspections identified the declining condition of the Scotts Mills Dam structure.

Scouring at the base of the dam due to the undertow effect had undermined the dam’s structural integrity, and concrete strength inspections showed deterioration beyond what was deemed acceptable. While the dam as it stood did not pose an imminent risk to public safety, these factors indicated that it had neared the end of its useful life. As a result, a project was initiated to replace the Scotts Mills Dam. Preparation of the site began in 2017, with construction advancing in the spring of 2019, and completion of the new dam expected in summer 2022.

The project is easily one of the most complex construction endeavors on the Trent-Severn Waterway in recent memory. During planning, considerations had to be made for in-water work restrictions associated with the timing of fish spawning, and the fact that the dam was attached to Lock 19, a historic masonry structure. As well, several competing construction priorities are at play in this urban setting, like balancing timely construction that reduces operational down-time, with reasonable hours of work, noise, and traffic.

Finally, it's imperative that Parks Canada maintain an ability to utilize the dam for water management to the fullest extent possible throughout construction. This is not only to manage water within the City of Peterborough, but as a part of the larger watershed.

A watershed, or drainage basin, is an area of land where surface water (from rain or melting snow and ice) converges to a single point at a lower elevation. The Trent-Severn watershed in central Ontario includes several smaller watersheds and covers an irregular area from near the southern tip of Algonquin Park, over to Bancroft, south almost to Belleville, just south of Port Perry and Newmarket, and over to Barrie and Georgian Bay at Midland. This area is over three times the size of Prince Edward Island and is a complex system of interconnected rivers and lakes controlled by more than 130 dams. The water levels within this area are managed by Parks Canada (with a few exceptions) for a variety of objectives including navigation, mitigation of flooding, and the protection of the environment.

Just how important is the Scotts Mills Dam, in the scheme of things?

Each individual dam forms a link in the chain of dams across the vast watershed, and is significant for the function of the Trent-Severn watershed and its navigable waterway, including the Scotts Mills Dam. Its role in water level management is also really important for the waterside economy of the City of Peterborough, and the quality of life that Peterborough citizens enjoy.

Within the City specifically, the dam helps to regulate upstream and downstream water levels, including Little Lake at its centre. As a part of a system of dams in Peterborough that work together, the operation of the Scotts Mills Dam has implications on hydro generation in Peterborough, the City’s water intake, and the outflows of creeks within the City limits.

The project to replace the Scotts Mills Dam has been designed to respect the historic look of the site, while replacing the old dam with a new one that will have a life expectancy of more than 80 years. The dam will optimize hydraulic capacity, increase the safety of water management operations, and be constructed in a way that allows for improved access for maintenance activities. In addition, the project is the first step in creating a more public friendly lock site, with pedestrian access onto the dam itself restored.

So how do we replace the engine while we’re driving down the highway?

The new Scotts Mills Dam is being constructed in phases, allowing one part of the dam to remain operable for water to pass through, while construction continues on another part. The portion under construction is enclosed by a cofferdam - a temporary dam constructed across or within a body of water that allows an enclosed work-site to be pumped dry.

But what happens during the spring freshet, when localized snow melt combines with snow melt travelling downstream, in addition to increased precipitation? Anyone who has seen the Scotts Mills Dam operate during the spring melt period will know that this dam frequently operates with all of it’s sluices at full capacity. And while many strategies are undertaken that reduce the requirements for lower capacity dams during the spring melt, the overall water management strategy doesn’t change for construction. One solution has been to remove all materials and machinery during the spring, and plan to intentionally flood the construction site.

Yes, you read correctly. Cofferdams are made of many types of materials, in many styles. The cofferdams at the Scotts Mills Dam project have been constructed in such a way that, not only do they double as work platforms, but they can be partially dismantled more easily than other types. This allows for a planned and costed method of allowing more water flow capacity during the spring melt (if needed).

As long as the project to replace the Scotts Mills Dam remains underway, this major construction site continues to be a front-row, drive-in-style attraction from across the river. There, many people gather safely and unprompted. Those who love watching construction. Local residents who consider the Scotts Mills Dam and Lock 19 a part of their lives. And the many retired Parks Canada employees who have wondered: how can it be done?