Why did the forest fires in Gironde resume so violently?

• Eight fires resumed Tuesday afternoon in Gironde in the Landiras sector, burning more than 6,000 hectares of forest in a few hours.

• The fires in this area had however been brought under control in mid-July.

  The new departures would have taken place outside the charred areas, in particularly dry areas due to the heat and previous fires.

• These are all parameters that have generated the formation of particularly violent fires with very rapid progression.

We thought we had overcome it, but not at all.

Tuesday afternoon, the fire which had however been fixed by the firefighters in mid-July in the Landiras sector in Gironde, started again.

Last night alone, violent flames ravaged 6,000 hectares of forest.

If it was not necessarily expected, this new start was in any case feared.

"Never declared extinguished", this fire which has already devoured 14,000 hectares of forest in July was still under surveillance, recalled this Wednesday the delegate prefect of the Gironde, Martin Guespereau.

In addition, the consecutive heat waves and heat waves have not improved the health of the forest in water stress due to the lack of water, for weeks.

"At the slightest spark, it starts again," warned in an interview with

a few weeks ago, Eglantine Goux-Cottin, forest engineer.

Unfortunately, his fears came true.

Crowns of attacked vegetation

Contrary to what the firefighters feared, "the fire did not start from the peat soil" of the area, which raised fears of a smoldering fire under the ground, assures

Bruno Lafon, president of the DFCI (Forest Defense against fire) in Aquitaine.

And contrary to what one might think (me anyway), the fires did not resume at the exact place where they had taken place, but “outside the perimeter”, he specifies. .

For Anthony Collin, teacher-researcher specializing in fires at the University of Lorraine Lemta/CNRS/Polytech Nancy, the reason is quite logical: "Coal and embers do not provide support for the fire so that it goes back a second time Above ".

If it's already charred, that means the fire has already consumed what it could consume.

On the other hand, the vegetation located right next to the burned area is particularly exposed to a new outbreak of fire.

"These meadows and trees on the edge of the flame front are fields of matches", assures

Anthony Collin.

This vegetation was not directly burned, but it was still attacked from a distance by the immense flames.

“Here, we have trees 25 meters high.

When it ignites, we have flames 80 meters high, ”assures Bruno Laffon.

Around each fire core, therefore, crowns of particularly dry vegetation have formed over tens of meters.

"It's the size of the flame that controls the speed of spread"

But if it's dry, it lights up and is particularly strong.

Indeed, the humidity contained in the vegetation normally delays the spread of a fire, since to consume the fuel, the fire must first dry it.

“This is why his development in the first moments was comparable to what took place three weeks ago”, analyzes Anthony Collin.

And in addition to being strong, this “very vigorous fire”, as described by the delegate prefect of the Gironde, also spread extremely quickly.

Firstly because of the lack of humidity as explained above, then because of "the strong winds this night", assures Bruno Laffon and finally because of the nature of the vegetation.

Their size counts for a lot, a bit like a candle: the height of the flame varies according to that of the wick.

"However, it is the size of the flame that controls the speed of propagation since it attacks the virgin vegetation in the distance and promotes more dynamic propagation", explains Anthony Collin.

Thus, grass fires will progress by a few centimeters per second and brush fires up to approximately 5 km per hour.

As for treetop fires, they are the fastest and can reach up to 8 km/h according to the expert.

The speed of a short jog.

Blame it again on humans?

For the time being, the spread of the fire seems to have slowed down slightly, assures us the President of the DFCI, "due to a less violent wind".

Technical cuts have been made in forests where the fire has not yet arrived, in order to stop its progression.

One more fight to win, but the war is far from over.

“We are already at more than 50,000 hectares of burned forest for the year and there is still a month of forest fire season left with complicated and difficult conditions wherever you are on French territory, even in the North”, deplores Anthony Collin.

On the other hand, if Bruno Laffon assures that for the moment "we do not know how the fire started again", the trigger could well be the return of human activity around the areas where there were fires. .

"All it takes is a little clumsiness for it to start up again," recalls Anthony Collin.

For his part, Darmanin speculated on Wednesday that "the fires are the work of arsonists".

"Eight fires started a few hundred meters apart, which is quite unusual," he said.

The investigation will have to determine it.

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