Building on river floodplains has proven costly and devastating to Canadians. A new Globe analysis reveals which cities are most at risk

When rivers spill over their banks, the resulting damage is not simply the result of nature’s caprice. It is also determined by how many homes, farms, roads and hospitals people built in a flood’s path.

A new analysis by The Globe and Mail has discovered that of 150 Canadian communities with populations greater than 10,000, more than 30 have at least one-tenth of their buildings within river floodplains. This analysis was facilitated by new flood maps covering the entire country released by the University of Western Ontario.

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Gaps in the armour

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Officials fully realize what’s at stake. A 2009 study by engineering firm BGC Engineering estimated potential damages from a major flood of around $1-billion. “The consequences of a breach in the City flood protection system would be extensive,” a 2016 municipal report noted. Once nearly half a city is in a floodplain, though, backing away can be prohibitively difficult and expensive.

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How close is too close?

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Land use planning has come a long way since the 19th century. Today, legal restrictions are imposed on the location, scale, density and type of buildings allowed in flood risk areas. Those areas are often mapped based on estimated “return period” of certain floods, which help determine the margin of safety.

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Despite such rules, some communities nevertheless choose to increase their exposure.

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