Although spring floods like the one from last May are expected to be more frequent, the Montreal region is too uncoordinated to properly prepare for them.

This was a conclusion in a report by the Montreal Metropolitan Community (MMC), which was tasked with drawing lessons from the floods that damaged thousands of homes in 24 municipalities this spring.

The water levels observed at the height of the flood were far above the predicted levels, and this was caused by faulty data, the report noted.

The current models that forecast water levels didn't account for heavier rain and faster snow melt caused by climate change or the complex interplay between all the waterways that feed into the Montreal archipelago.

Municipalities are also not accounting for changes in land use along waterways and the reduction of natural areas that can absorb water swells.

Rather, the report found municipalities bordering a river studied their waterways in isolation. As a result, there is a mish-mash of flood plans that predict different flood levels along the same river.

"It's not surprising to me that different communities have different parameters to evaluate flood zones," said Jim Beis, borough mayor of Pierrefonds-Roxboro and member of the urban planning committee of the MMC.

"We're evaluating how data is collected so we can standardise across all communities... we're still in preliminary stages."

The report recommends that municipalities better coordinate flood planning and agree on a single flood model that takes into account all possible factors, including climate change. It urges the adoption of high-tech tools that project real-time flood risks according to changing conditions.

The MMC is comprised of 82 municipalities on the Island of Montreal, Laval, and the South and North Shores.

Cars Pierrefonds flood

Disastrous floods like the one that submerged parts of Pierrefonds-Roxoboro are expected to be more frequent, but authorities in the Montreal area don't have accurate prediction models to plan for them, a new report states. (Charles Contant/CBC)

No up-to-date flood maps

As an example of the disarray, Montreal relies on a study done by the province in 2006 to predict high water levels during floods. Laval, which shares a river with Montreal, commissioned its own study in 2014 using a different methodology.

Their predictions of how high the Rivière des Prairies could rise in a major flood differ by 40 centimetres, the report says.

Without accurate flood models, authorities cannot fully plan for rescue and recovery efforts, or manage land use to prevent construction in flood-prone areas.

Montreal also does not have up-to-date flood maps. The last ones were drawn up in the 80s and have been taken out of circulation.

New flood maps are currently in production, but they are based on a 2006 study, which the report notes, does not consider all the factors that can affect high water levels.

The animation below compares these outdated flood zones that were made for the Rivière des Prairies along Pierrefonds with the actual flood extent last May.

Historical water levels of the Lake of Two Mountains compared to today.

(Hélène Simard/CBC)

A 20-year floodplain is a zone that has a 1-in-20 chance of being flooded each year, while the 100-year floodplain has a 1-in-100 chance.

Province must take lead again

The report also faults the provincial government for gradually backing off from flood planning and shifting the responsibility onto municipalities.

It asks the government to re-establish the Centre d'expertise hydrique du Québec, a waterway expertise bureau that was once responsible to researching flood plans for the province. That bureau was shuttered in 2014.

"The province should take a front seat in this analysis, to assist the communities," Beis said. "We can't spot fix these situations and expect them to not occur downriver."

flood ile mercier bizard island

A helicopter image released by the Montreal Metropolitan Community shows the extend of the flooding on Mercier Island, off Île Bizard. (Montreal Metropolitan Community)

Quebec's municipal affairs ministry said it will discuss the report later this year. 

"We can always improve our way of doing things," a spokesperson for Minister Martin Coiteux wrote by email.

The MMC will recommend tools and techniques that can be used by the entire community, as well as reconstruction strategies to better protect buildings from future floods. That report is expected next year.