What we know about the dam at risk of failing in western Quebec

Authorities in Quebec are keeping a close eye on a decommissioned hydroelectric dam on the Rouge River, a tributary of the Ottawa River west of Montreal, that's at risk of failing.

The Bell Falls, or Chute-Bell, dam has reached "millennial'' water levels, meaning there is a .1 per cent chance of such a flood occurring in a given year. 

Staff at Hydro-Québec are confident the dam will hold, even if water levels continue to rise with the rain expected over the next few days.

Residents in the area at risk have been told to leave as a precautionary measure.

Here's what we know about the dam, located about 23 kilometres northwest of Grenville-sur-la-Rouge, Que.

A brief history

• The Bell Falls is a hydroelectric dam built in 1915.
• The last major upgrade was done in the 1990s.
• The dam's operations were suspended in the 1980s, in order to complete the work. Operations restarted in 1999.
• In 1999, the dam was once again updated: a floating structure was installed, which adapts to the flow of water, in order to regulate levels.
• The dam has not been producing electricity for about 10 years.
• There are regular inspections, as required by law. The last one was conducted less than a year ago.

How it works

• It's an overflow spillway dam, which means it has a concrete retaining wall, topped with a spillway crest.
• The concrete wall is approximately 60 metres and prevents water from the reservoir from overflowing.
• When water levels are too high, the crest at the top of the wall enables the overflow water to pass through.

How much it can handle

• The dam's reservoir, which is considered small, has a withholding capacity of four million cubic metres.
• It can only withhold water for a few hours before overflowing.
• The normal flow over this dam is 103 cubic metres per second, and it is currently at about 1000 cubic metres per second — nearly 10 times over the normal rate.
• The reservoir is at full capacity, and Hydro-Québec is predicting a 30 per cent increase of water flow in the coming days.