A wildfire can cause extensive damage and potentially devastating loss. While it burns, first responders hope for rain to help contain or extinguish the flames. After a wildfire, however, the same rainstorm could lead to significant flooding.

Mitul Luhar, an expert in fluid dynamics, is constructing a hydraulic model that estimates flood patterns when water rushes through at varying rates. The model is part of a broader, collaborative effort being undertaken by the engineering, architecture and cinematic arts schools at the University of Southern California (USC).

In an article on USC’s website, Luhar explained why floods might follow fires and how to reduce the risk.

Why can wildfires lead to floods?
“Wildfires burn away vegetation and modify the properties of the exposed soil. This contributes to flooding in a few different ways,” Luhar said. “First, the exposed soil tends to become very dry, and this makes it less permeable. As a result, rainfall after wildfires does not infiltrate – soak into – the ground as effectively. This means that a larger fraction of the rainfall flows over the ground as surface runoff. Without vegetation on the ground to slow the water down, the flows tend to be much faster and more damaging.”

How can we avoid floods and landslides?
“Often it is the debris flow that causes the most damage and destruction.” Luhar said. “These effects can potentially be mitigated by rapid cleanup after wildfires, particularly if intense rainfall is expected soon after wildfires.

“Somewhat counter-intuitively, water infiltration into soil improves if it is slightly wet (as opposed to hyper-dry, as is the case after wildfires),” he added. “Lightly spraying the soils over a period of several days would lead to more infiltration during intense rainfall and potentially alleviate flood issues. However, this is a tricky balance to achieve. If the soil becomes too wet and saturated, it can cause mudslides.”

Photo by USC.edu.