Why Do Wildfires Increase Flood Risk?

| January 29, 2025 

An expert on fluid dynamics shares how engineers can help mitigate flooding after major wildfires.

View of the LA River

View of the L.A. River

In the wake of traumatic urban wildfires, a rainstorm might feel like a relief.

However, with extensive fire damage comes the risk of floods, with the potential to cause still more devastation. In Los Angeles, we saw this after the 1933 Griffith Park Fire, after the 2009 Station Fire in the San Gabriel Mountains and in Malibu after the 2018 Woolsey Fire – to name just a few examples.

Mitul Luhar, Henry Salvatori Early Career Chair and associate professor of aerospace and mechanical engineering and civil and environmental engineering, is an expert in fluid dynamics. One of his current research projects involves engineering a 1:120 scale hydraulic model of a section of the L.A. River, housed in the City of Los Angeles’s Hydraulic Research Laboratory in Frogtown.

The model, which has been developed as part of a collaborative effort that spans the schools of engineering, architecture and cinematic arts at USC, replicates potential flooding outcomes when running water is passed through at fluctuating rate and volume. Given his expertise in environmental fluid mechanics, Luhar is keenly aware of the potential ripple effects of the January 2025 wildfires.

Here, he explains why flood often follows fire – and what engineers can do to mitigate risk.

What are the factors that cause potential flooding after large-scale fires?

Mitul Luhar, associate professor in AME and CEE

Mitul Luhar, associate professor in AME and CEE

Wildfires burn away vegetation and modify the properties of the exposed soil. This contributes to flooding in a few different ways. 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.

Wildfires also tend to leave a lot of debris. This is picked up and transported by fast-moving water, leading to more destructive floods and debris flows.

What are some of the ways that flooding and landslides can be averted?

Often it is the debris flow that causes the most damage and destruction. 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). 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.

Where do you see the major risks for LA when it comes to potential flooding if fires persist in the coming years?

Mitul Luhar, alongside the hydraulic model of a section of the LA River

Mitul Luhar, alongside the hydraulic model of a section of the LA River

Unfortunately, climate change is a double-edged sword.

Warmer air can hold more water than cooler air. This means that global warming is likely to lead to an intensification of rainfall as well as wildfires. This can lead to what is now being termed “hydroclimate whiplash,” whereby L.A. is likely to alternate between periods of extremely wet winters (as we experienced over the past couple of years) and very dry winters (as is the case this year). The wet winters lead to significant vegetation growth, which then turns into more fuel for wildfires during dry years. Conversely, wildfires denude hillsides and enhance flood risk in the wet years.

How does your research inform potential solutions?

My collaborators and I are committed to making L.A. more resilient. One of our major efforts focuses on improving water management and flood control practices via revitalization of the L.A. river and surrounding spaces. We are trying to find solutions that simultaneously reduce flood risk in wet years and capture water for dry years. Other colleagues at USC are directly working to address wildfire risk.

An emerging area of research involves the evaluation of aerial fire suppression approaches that are effective in high winds. The Palisades and Eaton fires were catastrophic primarily because of extreme winds prevalent during the first day or two. Windy conditions accelerated the growth of the fire and also grounded aerial firefighting aircraft. This work is very much exploratory, but we are trying to find solutions that are safe and effective in unpredictable, windy conditions.

Are you involved in any committees to share your knowledge about how to address the impact of large-scale disasters?

The L.A. River work is being conducted in partnership with several local, county and state governmental agencies.

In addition, I have been serving as co-chair for the New Voices program at the National Academies of Sciences, Engineering and Medicine, which aims to bring leading early- and mid-career professionals into science policy and advisory work. Infrastructure and community resilience in the face of accidents, epidemics and pandemics, and natural disasters are a major focus area for the New Voices. Some of our major discussion areas include: improved science communication, elevating public trust in science, greater community engagement and coordination, incentivizing translational and applied research, and creating better research-to-operation-to-research pipelines.

Luhar is one of the many USC Viterbi researchers who have shared their expertise on engineering for wildfire resilience. Read more here.

Published on January 29th, 2025

Last updated on January 29th, 2025

Share this Post