Canada: Fortress McMurray: After decades of building on the floodplain, a city moves to protect itself from its capricious rivers

The project is also something of an anachronism. “The era of big structural defence developments was really the 1950s and sixties,” says Jason Thistlethwaite, professor at the University of Waterloo’s School of Environment, Enterprise and Development. “None of these structures offers absolute protection. So there’s been a real move away from building these defenses and a shift toward what the Dutch are doing: ‘Making room for the river’ " – an approach that includes government-funded buyouts of properties in flood-prone areas, and measures to prohibit new development that would sit in harm’s way.

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Fort McMurray’s story illuminates how and why communities of perfectly rational people can build directly in harm’s way – and illustrates, as well, the high costs of addressing the mistake of doing so.

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Fort McMurray is topping its new defences at 250.5 metres above sea level – slightly above the 1-in-100-year flood level. Even then, it’s taking a gamble. During the worst flood on record, in 1875, rivers peaked at an estimated 252 metres. If floodwaters ever did overtop them, the dikes could make matters worse by trapping water inside the city.

But that disaster has been estimated as a 1-in-350-year event. Karl-Erich Lindenschmidt, a professor at the University of Saskatchewan’s Global Institute for Water Security, who specializes in river ice, thinks Fort Mac is building its dikes to a reasonable elevation. He has modelled ice-jam floods in the community and determined that there’s a limit to how high waters could rise before the force would overwhelm an ice jam and flush it downstream: It’s about 250 metres. “I think that’s a very viable and defendable water level,” he says.

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