Hydroelectric developments include flood-control structures designed to let excess water escape safely from the reservoir. This "safety valve" prevents water from spilling over the dam crest. It takes the form of a spillway, a weir or sometimes a combination of both.
The spillway at Robert-Bourassa dam is called "the giant's staircase." The structure is named after its unique shape, which features 10 steps, each 10 metres high and 122 metres wide, which makes it look like an enormous staircase. Each step has the area of two Canadian football fields!
The spillway has a discharge capacity of 16,280 cubic metres of water per second, about twice the flow of the Saint-Laurent at Montréal. This phenomenal capacity corresponds to an exceptional flood likely to occur once every 10,000 years.
The spillways at La Grande-3 and La Grande-4 have a curved design that looks like a ski jump. The upward lip is designed to project the jet of water dozens of metres into the air, which dissipates part of its energy and prevents structural damage.
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