HELL AND HIGH WATER

THE RISING RISK OF FLASH FLOODS

ON JULY 8, 2013, a thunderstorm blew into Toronto from the west at 4:30 p.m., and in just 90 minutes, 100 millimetres of water— more than we usually get in a month— inundated the city. The Don River overflowed its banks; power lines failed, plummeting 300,000 people into darkness; and surging sewers flooded nearly 5,000 basements. That storm caused $1 billion worth of damage, making it the costliest disaster in Ontario’s history. And we should brace ourselves for more of the same: the city’s annual precipitation is projected to increase 17 per cent by 2080, with instances of heavy rain becoming 60 per cent more common.

David Sills is an expert in extreme weather at Western University. He says the problem isn’t so much the quantity of rain as the quantity of concrete: the 2013 storm happened in a place that was paved over. Despite our ravines and parks, Toronto is a concrete city, and it needs to become a so-called sponge city. Depaving areas to build water-absorbing parks and bioswales can be expensive and disruptive, but it promises benefits over time. The city’s redevelopment of the Port Lands is one example: by designing a meandering course for the Don, flanked with greenery and a spillway for high-water events, the project will safeguard 290 hectares of the eastern waterfront.

Until then, homeowners can defend themselves against rising tides with backwater valves and sump pumps—the city’s basement flooding protection subsidy program can provide up to $3,400 per property to help out.

Toronto is a concrete city. It needs to become a sponge city