Friday, January 05, 2007

Moving a River No Easy Task

If you’ve traveled through E.T. Seton Park recently, you may have noticed a massive construction project. Basically they are moving the course of the Don River a little bit to the left over the course of about 300 metres. This project is being performed by the Toronto Region Conservation Authority. It is not something they are doing on a whim. In fact they have been talking about this project for several years. It is only now that they are getting around to working on it.

E.T. Seton Park: moving the Don River

Over time, a mature river in a wide valley such as the Don will create meanders. These are long gradual bends caused by variations in flow. As the river goes around a curve, erosion will slowly eat away at the outside bank, moving the river in one direction until it reaches the side of the valley or cuts a new channel. If it reaches the side of the valley it will slowly eat away at the side causing it to erode and collapse. This is one method that the river uses to alter the landscape. This process is studied as geomorphology, the science of landforms.

The land adjacent to the Don Valley is heavily urbanized. Before such rules as the ravine bylaw were introduced, property owners were allowed to build right up to the edge of the ravine. It so happens that on the south side of the valley just west of the Ontario Science Centre is a property owned by Tremco, a chemical company that makes sealants and paints. They have a plant very close to the valley edge, indeed their rear parking lot extends right to the edge. By the vagaries in the river’s course it turns out that one of the meanders is cutting into the bank just below the Tremco plant. The bank is beginning to undercut the parking lot. Rather than let the river take its natural course our response is to change the course of the river to conform to our human needs.

I visited the site a couple of months ago and had a chance to talk to the site manager, Afzal Memon, who works for the TRCA. He told me that the project is scheduled to last almost a year. During that time the project will go through three phases. The first phase, underway right now, is to dig a new river channel and create temporary paths for park users to get by the construction zone. Once this is accomplished, the next phase will be to move the construction equipment to the other side of the river and let the water flow through the new channel. They will then fill in the old channel at the bottom of the eroded slope. Once this is done they will truck in loads of fill and dump it on the slope. This will stabilize the slope and improve the grade.

The final phase will be to build a bridge over the new channel, reconnect the original path, and re-landscape the entire area. They will also create a small pond from part of the remnant channel that will improve the natural habitat of the riverine environment. It’s expected that all this will be complete by the end of the summer 2007.

Facing north from existing path: digging the new river channel

Facing south: more excavation.

Eroded slope. At the top you can see piles of material at the very edge of the ravine.

A view of the construction zone from the top of slope (click to enlarge).

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