After a heavy downpour caused flash flooding in and around Toronto streets, Mayor John Tory says he wants to get to the bottom of why a relatively new stretch of road in the city has been repeatedly washed out by pooling during severe thunderstorms.

The Lower Simcoe Street CN underpass – better known to some Torontonians as the tunnel where the silver Ferrari got stuck in the 2013 storm – only opened to traffic in 2009. According to a 2014 city report, it was flooded five times between 2010 and 2013.

Now, after Wednesday's storm, Tory says he wants to know why.

"While it was a bad storm last night, it wasn't the worst we've had and I think that's a warning sign that tells you we better do something about it," Tory said Thursday. "We've got to fix it."

The report identifies two reasons for the repeat flood events at that underpass.

Cause of Wednesday's flooding unknown, city says

The first is that the road is lower than the pre-existing pipes in the area. That's because in order to accommodate a road with a high enough clearance for vehicles of different sizes, existing pipes had to be lowered, causing more strain on the system.

"A persistent severe thunderstorm" dropped heavy rain over Toronto Wednesday evening, causing some flooding along some major roadways. 0:31

That's made worse by the second reason: the fact that the road isn't much higher than lake level, meaning drainage to the lake can be slow, especially during heavy rainfall. Without a steep-enough incline, water can easily back up onto the road.

The exact cause of Wednesday's flooding isn't yet known. Staff are investigating, city representative Randall Meier told CBC News on Thursday.

"It was an extremely severe and intense rainfall that occurred in a short period of time, and this can be part of the reason for the overland flooding," Meier said.

For Tory though, future planning needs to account for more frequent severe-weather events.

'We have to figure out what happened'

"I think nowadays we have the obligation to plan on the basis not of a storm once in a century," Tory said. "We have to figure out what happened and why."

So what led to the flooding that made a Toronto lawyer decide to abandon his car during the 2013 storms? The city report said the culprit was a manhole that was missing bolts – something Jenn Drake, a water management specialist at the University of Toronto's Department for Civil Engineering, said can play a bigger role than you might think.

A pair of Beck taxi cabs carrying several passengers became trapped in a downtown Toronto underpass after heavy rain in August 2013. (CBC)

"That might seem very, very small. But if you don't have the bolts you have four small holes that if you have a pressurized combined sewer, water can squirt up and out," she told CBC.

Drake, an associate professor at the university, says she's not surprised to see flooding in the underpass given the elevation of the pipes.

Blame downtown Toronto, expert says

"If they were a little bit higher up there would be probably better drainage," Drake said.

Enlarging and raising the pipes would mean better drainage, but the costs might outweigh the benefits, she said.

"The ultimate answer is to make the pipes bigger or to make the tank bigger and that would get rid of the water," Drake said. "The challenge with that is that it's actually really expensive."

Her advice to people on heavy rain days is to simply stay away from low-elevation areas.

As for why only a recently constructed road in the city has been washed out time and time again?

"It just means in downtown Toronto, it's really hard to fit in new infrastructure," Drake said.