Verification done: more and more floods?

The assertion “We know that with climate change, these phenomena [the floods] will increase in the future,” said the popular morningman Paul Arcand, FM 98.5. And he is not the only one to think so. “These flood phenomena, it is clear that it will become exponential [because of global warming],” said Suzanne Roy, chair of the Climate Change Committee, on QUEB radio. Union of Quebec Municipalities.
R adio-Canada headlined the weekend that “climate change influence […] intensity of floods.” And even Prime Minister Justin Trudeau said on Wednesday during a visit to disaster victims in the Outaouais, that this kind of natural disaster would happen “more and more often”.

Facts

It is true that it is predicted that global warming will make some extreme weather events more frequent, such as intense rain events – it’s even started already . It is therefore not unreasonable to think that, at least in some parts of the world, floods will occur more often. This is the case in the northeastern United States, where we have more than before – but there are several other sectors of the country where the opposite is happening, so well that the general trend is unclear .

In Quebec, however, “the main flood is spring,” notes Dominique Paquin, a climatologist with the Ouranos research consortium. This means that even though floods can occur in any season, the risk is much greater in the spring: in the Public Safety Canada databases, on the 30 so-called “catastrophic” floods recorded in Quebec between 1970 and 2014, plus half (17) occurred between the end of March and the month of May.

But in the climate and hydrological models, it is far from clear that these floods will become stronger, says M me Paquin. Indeed, if our winters warm up, then melting episodes will become more frequent from December to March, and there will be less snow to inflate the rivers in the spring. Moreover, following the flood of 2017, Ouranos had published a “question-and-answer” stating that “on average, it is not expected in the future that the spring floods become more important in southern Quebec in particular. because of a lower snow accumulation during the winter. ”

Ouranos recently released an update of its “questions and answers” to reflect the latest scientific advances. “For large watersheds like the Ottawa River, the long-term behavior, if we look at 100 years, is to reduce the likelihood of flooding,” it says. It is not impossible that in the medium term, the decrease in average spring freshet is accompanied by an increase in extreme floods, but these results are far from certain.

For smaller watersheds, however, higher flood risks can be expected in summer and fall because intense rain events will be more frequent. However, “for spring floods, trends are the same as for large watersheds”, a decrease, reads the document.

Finally, the Quebec Center for Water Expertise – river flow specialists in the government – periodically publishes a hydroclimatic Atlas of Southern Quebec, the latest version of which does not predict an upward trend for spring floods (recurrence of 20 years ) in the south of the province. In fact, five rivers in the extreme south should see a “probable decrease” in their floods by 2050, and the others will be stable. Half a dozen others will experience a “probable increase”, but they are all located much further north, at Lac-Saint-Jean and on the Lower North Shore.

However, the same document provides for “probable increases” in summer and fall “floods” for southern and eastern Quebec, as extreme precipitation is expected to increase. However, compared to the spring, average flows are much lower during these seasons.

The verdict

Largely false. In southern Quebec, the main flood risk is associated with the spring freshet, and the best available expertise predicts that climate change is not expected to increase by the end of the century – at least not for large watersheds, which will even see their risk decrease. In smaller watersheds, however, the risk should increase.

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