Flood plain maps and flood risks

flood Photo by alexmak7/Shutterstock

With out-of-date flood plain maps, it’s hard to know. But what’s at risk is huge

When the Really Big One, the next one-in-100-year flood, hits Mississippi Lake, Ont., Rob Bell knows where the water should stop: about 10 feet away from his place and down a steep slope. Not that he’s worried. “It’s just my luck—good luck in this case,” says Bell, the president of the Mississippi Lakes Association, “that the cottage was built on the highest part of the lot.”

It’s also his luck to be among the minority of waterfront property owners with a good handle on flood risk, thanks to a new flood plain map. These maps graph the extent of a severe flood and offer owners (and prospective buyers) a “what if” glimpse into a potential future. Now, although flooding is an increasing concern, new maps are almost an endangered species in rural Ontario. Many existing ones are 25 to 40 years old.

In 2015, “about three-quarters of Conservation Authority [CA] mapping was in need of some updating,” says Jo-Anne Rzadki, who is responsible for business development and partnerships for Conservation Ontario. Some areas of Ontario don’t have flood mapping at all. A few are even based on aerial photos snapped in the 1960s. In road map terms, it’s as if you’re driving to the cottage using dog-eared charts from when John Robarts was premier. Given the way traffic flows have changed since the ’60s (or even the ’90s), is it any surprise that water flows and waterfront development have changed too?

“Especially since 2009, we’ve seen a rapid escalation in natural disaster events, with flooding being far and away the one that affects Canadians most,” says Craig Stewart, the vice-president of federal affairs for the Insurance Bureau of Canada (IBC). Now, he adds, “flooding accounts for about 75 per cent of losses—for both insurers and governments—to Canadian properties.”

And the trend seems poised to accelerate. When the Muskoka Watershed Council used global climate models to look at the region’s projected 2050 water flows, “there were some surprises,” says Peter Sale, the former council chair and a retired environmental scientist. With winters and springs that are trending wetter, “we were getting estimates of three times as much water flowing in winter and spring as is currently flowing. That’s a hell of a change.”

It’s hard to know exactly how this will all play out. Even if winters are milder on average, there will still be cold spells, with unpredictable results on ice cover. It’s likely, though, that spring and winter flooding will become more frequent and more severe. The Muskoka floods of recent years seemed bad enough, but if the climate projections bear out, Sale says, “we ain’t seen nothing yet.”

If this sounds dire, the silver lining is a modest but growing resurgence in flood mapping. After the 2013 floods in Calgary and southern Alberta (and their $6 billion recovery bill), the federal government launched its own $200 million subsidy for mapping, risk assessments, and other flood mitigation measures. Now, at least 29 new Ontario projects have taken advantage of the federal cost-share, and even municipalities with little or no CA coverage are getting in on the act. Muskoka District has an $800,000-plus project ready to launch this year, while Haliburton County hopes to follow suit later in 2018.

Using aircraft-mounted technology called LidAR (Light Detection And Ranging), these surveys vault flood mapping into the computer age. As the survey plane flies over a watershed, it bounces light beams off the ground, creating a 3-D digital image of the landscape. Because water reflects those beams, crews don waders or take to boats to sound the depths of creek channels and riverbeds. “I try to fill in what the LIDAR can’t capture,” says Galen Yerex, the flood plain GIS mapping technician for the Kawartha CA. Yerex also catalogues bridges and culverts. In a flood, “these structures usually create bottlenecks,” especially if they’re too small for the heavy flow. The resulting washouts and failures could also limit emergency access and threaten public safety.

By combining the surveys and images with calculations of water flows, rainfall, snowmelt, evaporation, and soil moisture levels, engineers assess flood risks along the plain. The most important region is the regulatory flood zone—the area likely to be inundated by the kind of severe flood that the Ontario government requires agencies to plan for. In most of Southern Ontario, the flood used to delineate the flood plain is Hurricane Hazel, the 1954 storm that killed 81 people and devastated a swath from Lake Ontario to Lake Simcoe. Other regions project the impact of the 1961 Timmins downpour, a series of severe thunderstorms that drowned a family of five, or the “one-in-100-year flood.” This last calculation is a statistical one, based on a flood level that has a one per cent chance of happening in any given year. But after a one-in-100-year flood, don’t think you’re in the clear for the next 99 years. “You could have a 100-year flood two years in a row,” says John Price, the director of water resources engineering for the Mississippi Valley CA. Back-to-back floods of that magnitude should be almost freakishly unlikely. Then again, climate change can shift the odds, and shoreline development, new dams, and shoreline alterations can change the extent of the flood plain.

Some maps also designate the floodway: the most hazardous part of the flood plain, with the deepest, fastest-flowing water. These high-tech products aren’t so much “maps” as “dynamic information models,” says Price. “They make it easier to run different scenarios to cover those what-if questions.”

Questions include “ ‘What happens when we store an extra foot of water in Lake X?’ ” says Ted Spence, a Catchacoma Lake cottager and the chair of the Coalition for Equitable Water Flow, representing 35 reservoirs in the Trent-Severn Waterway. For planners, it’s vital to know how much water a lake can store without damaging shoreline properties. When the water rises, how many roads will be submerged? Culverts washed out? Bridges and dams compromised?

Some CAs make newer flood plain maps available online, but, especially before buying shoreline property, it’s best to call the local municipality or CA and ask about development restrictions, says Price. “That way an individual doesn’t buy a piece of property with the idea that they’re going to do X with it, and they find out they can’t.”

Mary Lou Morris, an Ottawa-area sales representative for Royal LePage Team Realty says that, thanks to 2017’s Ottawa River flooding, flood risk “is one of the first questions that comes up.” She routinely pulls flood maps and checks restrictions on waterfront for buyers. In some cases, CA officials have agreed to walk the property and discuss general flood-proofing options with buyers. In the case of the local Mississippi Valley CA, “the regulations they’ve applied are really there to help you,” says Morris.

While flood plain mapping focusses on “fluvial” flooding—the risk from rivers, lakes, and watercourses—the insurance industry is also concerned about another kind of deluge: “pluvial” flooding, triggered by intense rainstorms. “We’re increasingly seeing these slow-moving storm systems, capable of dumping tons of water in one place for extended periods,” says the IBC’s Craig Stewart. If your cottage property funnels water from that kind of downpour into a basement or a crawl space, pluvial flooding can be a threat “even if you’re outside the one-in-100-year flood plain.” To get a handle on this broader risk, the industry is developing its own hazard maps. They are still a work in progress, says Stewart. But he expects some of the data to be publicly available within a few years.

In the meantime, 2017’s floods in Ontario, Quebec, and parts of the Maritimes have made it clearer that flood mapping is just the starting point for a long-term conversation in cottage country—a discussion that will involve everything from upgrading cottage roads and bridges to planning setbacks and flood-proofing cottages and yards, for example, with shoreline restoration. “We’re trying to prepare for the future, so we can avoid these damaging flood impacts to properties and people,” says Charlsey White, the director of planning for Haliburton County. Flood plain mapping is the foundation of those efforts. “It may seem like a small step,” she says. “But we have to start somewhere.”

Ray Ford wrote about lake effect snow in the Winter ’17/’18 issue.

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