Technology & Science

Why governments miscalculate your risk of flooding

Think your home is safe from flooding? Unfortunately, you’re probably wrong. And the government’s estimates of your flood risk are likely wrong too, thanks in part to climate change and outdated flood maps that don’t take that into account, researchers say.

Flood maps are often out of date and don’t consider climate change

Floodwaters in August surround a home in Spring, Texas. More than half the flooding that resulted from Hurricane Harvey took place outside known flood zones, a preliminary analysis shows. (David J. Phillip/Associated Press)

Think your home is safe from flooding? Unfortunately, you're probably wrong. And the government's estimates of your flood risk are likely wrong too, thanks in part to climate change and outdated flood maps that don't take it into account, researchers say.

We've seen the extreme flooding in Texas following Hurricane Harvey — houses and semi-trucks half-submerged in swampy brown water, people paddling in boats or wading through rivers that were once streets.

A preliminary estimate of the flooding in Harris County in the Houston area by researchers at the University of California, Davis, suggests that more than half the flooding there happened outside any mapped flood zone.

What used to be a one-in-100 year event might now be more frequent.- Glenn McGillivray ,  Institute for Catastrophic Loss Reduction

Unfortunately, it doesn't take an exceptional hurricane like Harvey to flood areas beyond a known flood plain, as residents of Windsor and Essex County in southern Ontario found out in late August, when more than 5,000 homes suffered flood damage from heavy rain.

Why does so much flooding seem to happen outside known flood zones these days?

Partly, it's because the flood maps used by municipal governments to calculate the risk of flooding are inaccurate.

"A lot of them are quite old," said Glenn McGillivray, managing director of the Institute for Catastrophic Loss Reduction, an independent not-for-profit disaster research institute established by the property insurance industry and affiliated with Western University.

Decades-old flood maps

Many flood maps were made decades ago, when there was a federal cost-sharing program that used to help defray the cost, and haven't been updated since.

Not only were methods for predicting flood risk more primitive then and based on less data than we have now, but other things may have changed since the maps were made.

A city may have paved over vegetation that used to absorb some of the water, changing surface water flows and causing more water to be directed into the sewer system more quickly.

More than 5,000 basements flooded in Windsor, Ont., during heavy rain in August. Heavy rainfall can flood homes that are far from lakes and rivers.

And climate change could lead to bigger storms and heavier rain in many areas — something that some researchers think is already happening.

"We might see more frequent floods because of climate change," McGillivray said. "What used to be a one-in-100 year event might now be more frequent."

Donald Burn, a University of Waterloo civil and environmental engineering researcher, said that's because warmer air can hold more water.

"Think of that as like a big bucket up in the sky. That bucket's getting bigger as the air gets warmer, therefore there's the potential for more of that to be dumped out at any given point in time," said Burn, a researcher with a cross-Canada network of flood researchers called Floodnet.

That could change the size and location of flood plains around lakes and rivers. "If we don't have updated information," Burn said, "we think we're not in the flood plain, but we are."

Political hurdles

McGillivray said there are many reasons why municipal governments are relying on decades-old flood risk estimates. One is the cost of making new flood maps.

But he said he also often hears from politicians who are concerned that designating an area a flood zone will drive down home prices.

"We have to get over that," he said, noting that there's little evidence to support that fear, while it's much clearer that actual floods do drive home prices down.

The only up-to-date flood maps for the Montreal area cover the Lake of Two Mountains and the Mille Îles River. For most of the Montreal Island's shoreline, there is no current data. Across the country, many municipalities rely on decades-old flood maps. (Sources: Centre d'expertise hydrique du Québec and Google Maps)

Of course, whether you're on a flood plain or not only matters if the flooding is from a nearby body of water.

McGillivray said people are mistaken if they think they're safe because they're not near a lake or river.

"Where do you get urban flooding? You get it where it rains most," he said. "Generally, you can get a flood anywhere if it's associated with heavy rainfall.... If you are connected to a municipal sewer system, you are at risk of basement flooding or sewer backup."

Nicholas Santos, a co-author of the recent analysis of flooding in Harris County, Texas, said that sometimes municipal systems are only rated for a certain level of storm and not prepared for extreme storms like Harvey, which itself follows other major storms from the Gulf of Mexico that have caused major flooding in recent years.

"I feel like these storms are at the very least the new normal if we haven't been reclassifying them all along," he said.

Municipalities may need to take more extreme storms into account and upgrade their infrastructure to handle more rainfall in the future.

Data gaps

Burn said there are challenges when it comes to predicting what kind of storms we can expect in different areas with climate change.

Some Floodnet researchers are in the process of improving short-term flood forecasting, while his research looks at estimates for the types of floods that can be expected at different locations, and how municipalities control them.

Cities still allow people to live and build in high-risk, disaster-prone areas, partly because of outdated flood maps and politics, experts say 2:11

"We never have as much data as we would like to have," he said. "The data is maybe not telling us the whole story of what's going to happen in the future."

Canada's network of rainfall and stream flow measurement sites doesn't necessarily cover the places where we want to predict flood risk, and some of them have only been operating for a couple of decades, he said.

Still, methods for mapping flood risk have improved over the years, everyone agrees, and some places are already taking advantage.

For example, Newfoundland and Labrador is one of the few places where municipal flood maps have been updated to take into account climate change.

"That's certainly something which should be happening," said Burn. "And hopefully with some of the results that we come up with, that will better equip other jurisdictions to undertake that kind of analysis."

About the Author

Emily Chung

Science and Technology Writer

Emily Chung covers science and technology for CBC News. She has previously worked as a digital journalist for CBC Ottawa and as an occasional producer at CBC's Quirks & Quarks. She has a PhD in chemistry.


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