Weather Whys: What causes burn scar flooding?

Local

Burn scar flooding is something we hear about a lot in southern Colorado. We see our fair share of bad fire seasons, but the fire isn’t the end of the trouble. The burn scar that is left behind can be prone to flooding and cause property loss and death years after the fire has been put out. We’ll be taking a look at why that happens.

In normal circumstances, a forested area has good soil and is covered in vegetation. The vegetation can include larger plants like trees and smaller underbrush like grass and shrubs. When a storm forms over this area, the soil absorbs much of the rain, and what runoff is left travels over the underbrush, which keeps the top layer of soil in place.

When a forest fire starts, it not only burns away all the plant life, but also leaves the soil damaged. The soil can no longer effectively absorb the rain. It actually repels it. The underbrush is gone and can’t hold soil in place. This causes the rain to pick up the soil and take it down the slope, creating mudslides and debris flows, and larger pieces of burned plant life get carried away with it. This travels into low-lying areas, and can impact roads and homes and cause further erosion. 

There’s a lot that can be done to minimize the impacts of flooding in burn scar areas. Flood mitigation techniques include creating basins to catch water, channeling water and debris flows away for flood-prone areas, and removing the debris in the area. Over time, the area can heal itself, but it can take decades for significant regrowth to occur. 

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