Relationship Between Wildfires and Floods
10/24/24 – It may seem counterintuitive, but there is a well-recognized relationship between wildfires and floods. What is it?
Simple. Wildfires create conditions that increase the risk of future flooding. Specifically, they affect infiltration; runoff volume and speed; erosion; sedimentation; and more.
How Wildfires Increase Future Flood Risk
Here’s an outline of the the relationship between wildfires and floods:
- Loss of Vegetation: Forest fires burn vegetation, which normally helps to hold soil in place and absorb rainfall. Fires leave soil exposed and unprotected.
- Water-Repellent Soil: Intense fires can cause soil to repel water. This increases the amount of water running off the surface during rainstorms, rather than soaking into the ground.
- Increased Runoff: Without vegetation, rainwater flows more quickly and in greater volumes over the landscape. This runoff can lead to flash flooding, particularly after heavy rains in areas recently affected by wildfires.
- Erosion and Sediment: Post-fire floods tend to carry a lot of sediment and debris since the soil is no longer stabilized by plant roots. This sediment can settle downstream, reduce the carrying capacity of streams and worsen flooding.
In short, forest fires can create conditions that increase the likelihood of flooding and make flood events more severe in the affected areas. This relationship is particularly common in the period immediately following a wildfire when the landscape is most vulnerable to erosion and rapid runoff.
What the Experts Says
According to FEMA, “While some floods develop over time, flash floods—particularly common after wildfires—can occur within minutes after the onset of a rainstorm. Even areas that are not traditionally flood-prone are at risk, due to changes to the landscape caused by fire. … Flood risk remains significantly higher until vegetation is restored – up to 5 years after a wildfire.”
The National Weather Service says, “Locations that are downhill and downstream from burned areas are highly susceptible to Flash Flooding and Debris Flows.”
The Army Corps of Engineers also warns of the relationship between vegetation loss and flash flooding. They say the risk of debris flows lasts two to five years.
The Corps, FEMA and Weather Service all recommend purchasing flood insurance within their discussions of wildfires.
Current Conditions in Southeast Texas: Growing Fire Risk
So how bad are current conditions?
I already have received an email from one reader who was seriously injured fighting a pasture fire in Liberty County.
No weather monitoring site in southeast Texas has recorded ANY rainfall for the month of October. “That’s impressive because October is usually our second wettest month behind May,” says Jeff Lindner, Harris County’s meteorologist.
October 2024 Rainfall and Departures from Normal
You can see the lack of recent rain in the statistics below:
- College Station: 0.00, -3.49
- BUSH IAH: 0.00, -3.82
- Hobby: 0.00, -4.12
- Galveston: 0.00, -3.66
- Conroe: 0.00, -3.67
- Sugarland: 0.00, -3.29
Drought Worsening
Moderate drought now covers more than half of Texas and severe drought covers more than a quarter of the state. All of Harris and surrounding counties are in moderate or severe drought, except for those along the coast.
Says Lindner, “While the recent dryness has been relatively short thus far compared to other dry spells, it has been intense – accompanied by much warmer than normal late summer and early fall temperatures. This has quickly resulted in drought degradation across the entire region and vegetation health quickly declining.”
Fire Activity Increasing
“The result has also been an increase in wildland fire activity in recent weeks – especially when drier air masses move over the region along with gusty winds,” Lindner continued.
“As the area moves deeper into the fall and winter months, more frequent and stronger cold fronts will bring stronger winds and much drier air masses across the area. Without any significant wetting rainfall, fire-weather conditions will quickly elevate,” he said.
Wildfire Risk Soaring
The Keetch-Byram Drought Index (KDBI) values indicate wildland fire risk. Currently, they show significant risk across the area. The scale is from 0 (completely saturated) to 800 (completely void of moisture to 8 inches deep). Values above 650 indicate an increased risk of wildland fire and values over 700 indicate a significant risk.
Current average KBDI values are:
- Austin: 735
- Brazoria: 630
- Brazos: 744
- Chambers: 693
- Colorado: 705
- Fort Bend: 706
- Galveston: 620
- Grimes: 730
- Harris: 727
- Liberty: 739
- Matagorda: 615
- Montgomery: 715
- San Jacinto: 717
- Walker: 691
- Washington: 738
- Wharton: 689
Looking Ahead: Higher than Average Temps/Lower Than Normal Rainfall
We are currently heading into La Niña conditions. “Such global oscillations tend to support southern plains drought,” says Lindner.
The National Weather Service indicates drought development and persistence across all of Texas into early 2025. See below:
Be Careful with Outdoor Burning
Beryl left a lot of dead vegetative debris lying around last summer. It’s now turning into fuel. So be extremely careful with outdoor fires. It’s probably best not to burn those leaves you rake up this fall.
The fire you prevent now could help prevent flooding next spring.
Posted by Bob Rehak on 10/24/24
2613 Days since Hurricane Harvey