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Why The Risk of Floods Increases Exponentially After Forest Fires

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Hundreds of fires raged across Greece this past week and a half, charring the landscape and increasing the risk of flooding. Credit: Odysseas Karadis / Greek Reporter

After the onset of forest fires, many pray for rain to come, hoping it will extinguish the flames. Rains after forest fires, however, are a great cause for concern, as blazes increase the risk of subsequent flooding exponentially.

When fires rip through the landscape, they burn not only countless trees and plants, but also their roots, which go deep into the ground.

This complex web of root systems is integral to maintaining the structure of the landscape. Roots embedded deep in the ground help to keep the soil in place, stabilizing it during any potential rainfall, no matter how torrential.

Additionally, healthy forest ecosystems protect the ground against the physical impact of the heavy rainfall, and therefore prevent flooding to a degree.

Roots and vegetation destroyed by fires increase flood risk

As forests are stratified into four layers — the upper canopy, the middle level, the underbrush level, and the forest floor — heavy rains are filtered through at the higher levels, and the soil on the forest floor is protected from the brunt of heavy rainfall.

When these roots are burned and destroyed, the soil is not kept in place, and any rainfall received could destabilize the ground, resulting in mud and landslides.

Additionally, the heat of the fire, if strong enough, can cause a chemical reaction in the soil that makes more water resistant, and less likely to absorb any rainwater, compounding the flood risk after fires, even in areas that had been at very low risk before.

Flash flooding in areas with charred earth can take place just minutes after rainfall begins, and flooding after forest fires can be more destructive than normal floods.

This is also due to the fact that the soil is less absorptive and is not held in place by vegetation, which causes rushing rain waters to pick up debris, ash, and soil, creating mudflows.

These conditions last until vegetation and plant life begin to grow back in fire-stricken regions, which can take over five years.

Greek landscape will “change drastically” after fires

Greek scientist Efthymios Lekkas, a Professor of Dynamic Tectonic Applied Geology and Disaster Management at the National and Kapodistrian University of Athens, warned of the environmental destruction caused by the fires and stressed the increased risk of floods in Greece in the wake of the blazes that have plagued the country for over a week.

Lekkas claimed on Wednesday that the geomorphology of the areas in the Peloponnese and on Evia that are now burned will ”change drastically.

In terms of flooding due to uncontrolled runoff after areas have been deforested, Lekkas noted that the least affected area will be that in the Peloponnese. However, he believes that Attica, and to a greater extent Evia, will suffer significantly in the future by flooding, due to the fire destruction.

”In Attica, the flood risk is high given the human interventions in the hydrographic network,” Lekkas stated.

”However, the greatest flood risk is found in Evia, where basins that discharge water develop in areas of villages along the coastal zone,” the professor warned.

”The risk of landslides in Northern Evia is increasing dramatically and huge consequences are expected. The same applies to the Peloponnese, to a lesser but measurable extent,” Lekkas wrote.

Lekkas claimed that the reforestation in the Peloponnese will be rapid, similar to what happened after the devastating fires of 2007.

”The restoration of ecosystems and especially the flora in the Peloponnese will be fast, according to the data from the fire of 2007, due to the high rainfall, fertile soils, and geological formations,” Lekkas stated.

However, Evia will face a huge problem.

”It is estimated that the 400,000 stremmata will never return to their previous state,” the professor noted.

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