Why flooding has been such a problem in Ontario this week

Flooding has become Canada's biggest natural disaster problem, in terms of insurance claims. Severe weather costs billions a year, according to the Insurance Board of Canada, growing five-fold since the 1980s.

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Barbara Robinson, infrastructure expert, former chief engineer for the City of Kitchener and current president of Norton Engineering, explains how it got this way.

The following has been edited for length and clarity.

Why has flooding become the biggest natural disaster problem in Canada?

Robinson: First of all, I think we can all agree that climate change is upon us, no question. In terms of the flooding we've seen in Ontario and Quebec over the last couple of weeks, there's actually a structural issue in Canada; we've been building homes in the flood plain.

If you think of the Grand River, we could draw a line around either side of it, that line would represent how high the water will go during a 100 year storm that's a storm that happens once, on average, every 100 years. So people are building homes on those flood plains, so they're pretty much designed to flood.

If you think about it, most cities were built along waterways, so many downtown cities have a river running through them and we channelized a lot of those rivers, we buried them. And so when there's a lot of water in the system, it has no where to go, so it ends up flooding.

In Ontario alone, 300,000 residents live in flood plains. The most obvious solution to this is not to own a home in the flood plain.

Why can't the system handle excessive water?

Robinson: There are a number of different types of flooding and the flooding we saw in the last few weeks is actually riverine or lake-based flooding. Just so much water has come into the watershed, there's nowhere for it to go.

Most of our waterways in Ontario are controlled by dams and structuresand everything was full. The dams were full, the structures were full, Lake Ontario was full, they didn't want to release water down to Quebec because they had flooding, there was just too much water normally it rains on average once every threedays in Ontario.

More common is routine flooding, associated with basements, water on your property, backup, sewer that's actually more common than what we've seen over the last few weeks.

What are cities doing to cope with this?

Robinson: We design our storm systems for a particular return period that's typically one in five yearsand that means the pipe under the ground can convey the flow we expect to see about once every five years. After that, it spills onto the roadwayand that's fine.

As far as sanitary systems goand that's where basement flooding usually results fromone thing cities can do is they can work harder to get the clean water out of the sewers. Most sewer systems in Ontario have 20, 30, 40 per cent of the flow in there is clean water.

And when we build new sewers, we're not getting them as water tight as they should be so they're already letting water in when they're first built, and that's something we're looking at right now. We've got to change that, because that's not good engineering.

Beyond sandbagging, is it really possible to protect your home from flooding?

Robinson: There are lots of things people can do to protect their house.

You want the water away from your house. The downspout from your roof needs to have an elbow on it and it needs to have a three or five foot extension. If that downspout, even with an elbow, is right beside the wall of your house, it has the opportunity to go down the wall of your house and that's a recipe for flooding in your basement.

The grading around your house should be sloped away from your house. You don't want the water coming towards your house.

If you have a catch basin in front of your house, make sure it's not blocked. If it's blocked the water can back up.

Then we have protective plumbing equipment. You need to maintain and back up your sump pump; if you think about what we just saw in Ontario and Quebec, the firefighters are going in and turning off the power in a lot of these homes, for safety reasons. Once your power is off, and you don't have a backup on your sump pump, you don't have a sump pump anymore.

Another thing I'll mention is a backflow prevention valve, and that's a valve that's in your sanitary lateral and it's a one-way valve. It allows sewage to leave your house but it doesn't allow sewage from the sewer system to back up into your house.

A lot of municipalities are funding these things now, they'll cover most, if not all, of putting these things in.