Conservation Authority may review parts of Ottawa’s 100-year flood map after recent flooding
Flooding on the ground in Constance Bay stretched further than area shown on the 100-year flood plain map
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The flooded area in Constance Bay surpassed what the city’s 100-year floodplain map predicted, even though water levels haven’t surpassed the city’s predicted maximum flows.
The water level drawn on the map is shown at least 50 metres back from Bayview Drive in Constance Bay, but when I visited the area on Sunday, I observed that water had reached sections of Bayview Drive and houses on both sides were flooded.
John Price, director of water resources engineering at the Mississippi Valley Conservation Authority, which has jurisdiction over the shoreline in Constance Bay, said this is because the actual elevations on the ground may be slightly different from the topographical model used in the calculations to draw the map.
So flood waters in some areas may stretch further than the map shows, even if actual water levels did not reach higher than those predicted when the 100-year flood map was created.
Price said that the water level used in the calculations may be right, but in certain areas, “maybe the topography that you’re using to draw that line may be kind of out a little bit so the actual flooded area may be larger than what you're showing” on your map.
“There are some variables in there that could lead to that flood line maybe not being perfectly representative everywhere,” he said.
Price said the best information he had was that the flood in Constance Bay resulted from a 50-year water-level high. This information may change in the coming days, he said, as the conservation authorities in the areas are gathering photos of the flooding, including aerial shots.
"If there's large areas I'll say that are outside of how the flood plain is delineated now that are flooded now, that would potentially result in kind of a direct redrawing of the flood plain line," he said.
Cities use historical data to build flood maps that predict the likelihood a specific area is at risk of flooding. A one in 100-year flood means there is a one per cent chance that flood risk areas will flood in any given year, according to Paul Beckwith, a climate system scientist and University of Ottawa geography professor.
The frequency of extreme weather events is increasing all around the world, making predictive tools like flood maps more difficult to design, according to Beckwith.
“Those terms are all based on a stable climate. So if the climate is stable you can come up with the probability that an event will happen, an extreme weather event will happen,” Beckwith said. “When the climate is changing rapidly, as it is now, then we start seeing some strange things. You can’t really believe those numbers.”
Gail Faveri, a manager with Environment and Climate Change Canada and Canadian Secretary for the International Lake Ontario-St. Lawrence Board said Monday’s peak on the Ottawa River was the highest ever recorded, since tracking began in 1967, she said. “It’s about twice what the average peak is.”