A perfect storm

As Toronto cleans up from this week's $1-billion flooding, the city turns its eye to what might help when the next extreme weather event hits

- MAHDIS HABIBINIA

The scenes from Tuesday's downpour were harrowing, especially on the Don Valley Parkway, with abandoned vehicles and stranded drivers.

The deluge came just weeks after the city marked a critical step in the $1.4-billion reclamation of the mouth of the Don River, a project meant to flood-proof the Port Lands. But that doesn't mean the DVP, or the rest of the city, won't flood in the future.

In fact, flood-proofing the highway was never the project's goal, despite what some elected officials and the public may believe.

To understand why the Don River and DVP flooded, first it's important to understand that Toronto's infrastructure was not built to handle that much rain.

Several major downpours last week cumulatively wore down the city's aging infrastructure, so when the chaotic weather hit, rainwater systems were already at or beyond capacity.

As well, Toronto still has combined sewer systems in older parts of the city that don't separate stormwater from sewage, which means they are easily overwhelmed by downpours, said Jason Thistlethwaite, an associate professor at the University of Waterloo's school of environment, enterprise and development. (Newer developments have both a sewage and a stormwater system, lowering the burden on city infrastructure.) "This was one where you have a ton of rain that falls. We've built (developments) over a lot of our surfaces that were able to absorb the water. The water has no place to go... then backs up through people's sanitary systems and into their basements," Thistlethwaite said.

When urban flooding overwhelms the whole system, the combined rain and sewage waters then travel through roads and eventually find their way into local rivers, which then start overflowing, too.

The Don River is no exception.