Why Chilliwack is one of Canada’s most vulnerable cities

The city has a greater share of its buildings on a flood plain than any other medium or large community in Canada. And climate change will dramatically increase the flood risk.

By Tyler Olsen | June 6, 2022 |5:00 am

This is the third look at the annual flood risk in the Fraser Valley’s biggest cities. We also examined the Fraser’s flood danger in Langley and Abbotsford. Find those stories here: Langley | Abbotsford. We also wrote about the threat to Mission’s waterfront here.


There’s a reason new townhomes are built so tall on the north side of Chilliwack.

The Fraser Valley has plenty of communities susceptible to flooding, if the Fraser River rises as high as it has in the past. But no city has so many residents living directly on the floodplain as Chilliwack.

The city’s entire northern urban area, along with Greendale and even precious commercial land south of Highway 1 could all be at risk in the event of a major Fraser flood.

Weather conditions mean the Fraser has the potential to flood this year, but even if it stays below its banks, experts say knowing one’s own local flood risk is key to protecting oneself and preparing for an emergency. In the last of our looks at the flood risks in the Fraser Valley’s three largest cities, we examine Chilliwack and what makes it one of Canada’s most vulnerable communities.


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The Floodplain

 

The majority of Chilliwack lies within the Fraser's historic floodplain. 🗺 Fraser Basin Council
The majority of Chilliwack lies within the Fraser’s historic floodplain. 🗺 City of Chilliwack

 

Chilliwack shares many traits in common with other Fraser Valley communities. It has huge swaths of farmland—and farm homes. Some are protected by dikes; others are not, and are at high risk when it comes to a flood. The area also has First Nations that dike builders ignored when constructing flood defences decades ago.

But what makes Chilliwack unique—even in a valley full of floodable areas—is just how much of its city core sits on a floodplain at risk of a major flood, a dike collapse, or both. See the city’s floodplain

A glance at flood models or the history books illustrates why a recent Globe and Mail analysis found that Chilliwack has one of most vulnerable urban cores to flooding in all of Canada. That paper’s reporters found the city has a higher proportion of buildings on the floodplain—18,000, 46.5% of the city’s total—than any other medium or large city in Canada.

History has also revealed the risk posed to the huge low-lying swath of Chilliwack when the Fraser rises, with devastating floods both in 1894 and 1948.

And while dike improvements mean that it would likely take an even larger flood to submerge large numbers of homes in the 21st Century, a changing climate is increasing the risk each year.

Modelling shows how water would rise if the dikes in Western Chilliwack prove too low to stop the Fraser's rising waters. 🗺 Fraser Basin Council/Bing Maps/Tyler Olsen
Modelling shows how water would rise if the dikes in Western Chilliwack prove too low to stop the Fraser’s rising waters. 🗺 Fraser Basin Council/Bing Maps/Tyler Olsen

Click here to go to the bottom of the story and see the maps individually.

A two-front flood

A rising river would pose a two-front problem to Chilliwack.

Dike assessments and modelling suggest that some of the first flooding could be seen in the Greendale area in the city’s west, on Fairfield Island, and near the Hope River Slough. Early localized flooding in those areas is likely to be the result of rising water tables and could be mitigated. It’s not uncommon for seepage to inundate parts of Royalwood Golf Course, for instance; pumps already exist to move that water out of the flood plain, and some of that equipment has been upgraded in recent years. But the higher the river rises, the more they will also challenge the dikes protecting the city.

And if the floodwaters rise as high as they did in 1894, some of Chilliwack’s dikes would be overtopped. in addition to letting water into supposedly protected areas, overtopping is the chief cause of dike breaches, like what took place on Sumas Prairie last year.

It was the failure of dikes in 1948 that made that flood so devastating.

Dike failiures along the Vedder Canal contributed to the major flooding seen in Chilliwack in 1948. 📷 Air Force Museum of Alberta
Dike failiures along the Vedder Canal contributed to the major flooding seen in Chilliwack in 1948. 📷 Air Force Museum of Alberta

A breach of the dikes protecting Chilliwack would be even more disastrous, given the layout of the city and its infrastructure.

Modelling from 2019 suggested that the Chilliwack side of the Vedder dike just west of Greendale was more susceptible to flooding than even the Sumas Prairie portion of the dike. If river levels rise high enough, water from any breach in that area would move in a northeasterly direction, through Greendale, over its homes, and back towards the Chilliwack city centre.

Meanwhile, some dikes to the city’s north also don’t meet provincial standards and could be overtopped in the event of a major flood. That water would have a short distance to travel to reach downtown Chilliwack and, again, would threaten to create a full breach in the dike.

On its website, the City of Chilliwack stresses that a flood is unlikely.

“The dikes in Chilliwack provide a very high level of protection,” it says. “The likelihood of floods this large occurring in any given year is low, and the City thoroughly checks and monitors the integrity of the dike system and drainage pump stations during freshet to ensure they are operating properly.”

City of Chilliwack maps show where the dikes protecting the city are considered insufficiently high. 🗺 City of Chilliwack
City of Chilliwack maps show where the dikes protecting the city are considered insufficiently high. 🗺 City of Chilliwack

Roll of the dice

But while that’s true, provincial and federal flood officials say the low annual chance of a flood can give people a false sense of security and reduce the impetus for both individuals and communities to take actions before disaster strikes.

A one-in-200-year flood, for instance, carries only a 0.5% chance of happening each year. But those years add up over time. Over a lifetime—80 years—there is an approximately 33% chance of a one-in-200-year flood. That’s the same likelihood of rolling a one or a two on a six-sided die. The chances of a catastrophic one-in-500-year flood is only 0.2% each spring. But over 80 years, there’s a 15% chance of such a flood: roughly the same change as rolling a one on that die.

The definition of a one-in-500-year flood (or one-in-200) is also increasingly uncertain as climate change increases the severity of rain events, changes weather patterns, and causes the sea to rise— all of which has an impact on flooding upriver.

Assuming all Chilliwack’s dikes held, the Fraser Basin Council’s modelling suggests that unless a dike breaks, a one-in-500-year flood in 2019 would create only very minor flooding in protected areas. But as the climate changes, it’s increasingly hard to pin down just how likely a big flood is. Previous modeling has suggested that a one-in-500-year flood (that is, a 0.2% annual chance) under conditions expected in 2050 could inundate all of Chilliwack. Meanwhile, an exceptionally rare flood today will become more likely in the years to come. But those 2050 conditions could also arrive much sooner than expected—or already be present.

Chilliwack, for its part, is doing more than most Fraser Valley cities to proactively boost its flood defences by regularly upgrading pumps and dikes.

The city also has building rules that influence how homes and businesses are constructed on the floodplain. That’s the reason that most new townhomes and houses have their kitchens on the second floors. Those rules exist in order to reduce damage when water submerges the floodplain again.

But thousands of older homes—some of the most affordable housing in the entire Lower Mainland—were built before those building rules were updated and would sustain heavy damage in the event of flooding.

And the scale of the work is itself a challenge. Chilliwack is protected by roughly 50km of dikes, many of which aren’t up to standards meant to protect against a flood as large as that in 1894 (the Fraser did not rise as high in 1948; much of the damage was linked to the failure of the dikes). Municipalities and local politicians have said they don’t have the money to fund those upgrades in a timely manner and the provincial government has agreed, with Premier John Horgan saying the current funding scheme isn’t working and will be overhauled.

There are also major risks for the people who don’t live on a floodplain in Chilliwack. Unlike in Langley, Mission, and Abbotsford, much of Chilliwack’s critical infrastructure is located on the floodplain on the north side of town.

That includes its water treatment plant, its main fire station, its police department, and its hospital. Earlier this year we explored the potential consequences were the valley’s wastewater plants to flood; read that story here.

A major flood could submerge all those facilities, potentially throwing the entire management of the city into disarray. The city’s website notes that “even if your home or business is located on higher ground, a serious flood event would result in a loss of key services like water, sewer, electricity and gas. In that case, a City-wide evacuation could occur.”

There could also be a major hitch for evacuation and rescue efforts: it’s possible few would be able to get into, or out of, Chilliwack in a timely manner. As residents found last November, a flood has the potential of severing the connection west to Abbotsford.

Similarly, a major Fraser flood could submerge highways heading east, including Highway 1 to Hope, Highway 9 to Agassiz, and Highway 7 on the north side of the river.

So being prepared is critical: the city advises all residents, wherever they live, to be prepared to endure at least three days without services.

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Maps

These and other flood scenarios were mapped by the Fraser Basin council in 2019. Click to enlarge each map below. Find more maps here.

Tyler Olsen

Managing Editor at Fraser Valley Current

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