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Following the flames: Preparing for landslides in wildfire country

August 11, 2021

By Kyla Grasso

More intense wildfires are creating a scarred earth that is prone to debris flows. How do we reduce risk and build community resilience?

The 2021 wildfire season across the US West is underway. I have seen smoke plumes from my backyard in Redlands, California. I started seeing big-bellied fire planes flying over my house early this spring.

I have been falling asleep to the sounds of illegal fireworks going off around my city for the past month or so, and I cannot help thinking of the infamous “Gender Reveal Fire” of 2020, which burned 22,744 acres just a few miles from my house after an enthusiastic partygoer lit a “smoke-generating pyrotechnic device" at a local park. That fire forced some of my coworkers to evacuate their homes, four residences were damaged, and 20 structures were destroyed. The black, charred hillsides that resulted from the fire just east of Redlands are now more prone to debris flows and will remain so for some time.

Debris flows occur when intense rainfall runs off a hillslope and gathers debris—soil, rocks, trees, and other items—as they travel downhill with high velocities and often for long distances. The fast-moving landslides threaten lives, infrastructure, and environments around the globe. And they are more likely in fired-scarred areas. Stantec’s Geohazards and Geomorphology team works to identify areas prone to debris flows and other geohazards, helps clients mitigate the risks, and works with communities to build resiliency.

The 2020 California fire season saw 9,917 fire incidents, which burned 4.25 million acres.

The frequency, size, and intensity with which wildfires burn in California has increased significantly over the past decade—and particularly in the past few years. The 2020 fire season saw 9,917 fire incidents, which burned 4.25 million acres (about half the area of Maryland), destroyed or damaged nearly 10,500 structures, and took 33 lives, including first responders working on the front lines. The dollar cost to homeowners, communities, and state and federal taxpayers is estimated to be $10 billion (about $31 per person in the US) for this single fire season.

However, the actual cost can carry on for years, or even decades, into the future.

How wildfires feed debris flows

A wildfire destroys vegetation and root systems that play a key role in stabilizing soil and rocks on steeply sloped ground. Those fires also reduce the release of moisture back into the atmosphere and create water-repellent soils by glazing the remaining soil. Steep slopes, typically greater than 25-30 degrees and already prone to debris flows, become even more susceptible after a wildfire, particularly during the following rainy seasons.

A wildfire destroys vegetation and root systems that play a key role in stabilizing soil and rocks on steeply sloped ground.

Abrupt surges of large boulders, cobbles, and logs form the head of a debris flow and are followed by more water-saturated debris streams. As debris moves downslope, it can become channelized in existing streams, leading to increased channel erosion. Debris can reach speeds of up to 10 meters per second (or 22 miles per hour).

As vegetation begins to reestablish itself after a wildfire, debris-flow risks start to taper off two or three years after the wildfire. However, the risk can persist for many years.

New threats require new approaches

With wildfire frequency and severity increasing, what can we do to better protect people, infrastructure, and communities from debris-flow hazards?

Understanding the conditions leading to debris-flow activity is a key first step. But the increasing magnitude and frequency of the wildfires in California and across much of the globe requires a deeper look at proactive measures and estimates of where debris flows will go and their size.

DebrisFlow Predictor allows our team to inform local communities about real debris-flow hazards that could affect their homes and to design mitigation solutions to reduce the risk to people and infrastructure.

Planning, early warning, and mitigating for debris flows often relies on a combination of climatic, geologic, and community data to predict the location, extent, and volume of material transported. This data is used in numerical and probabilistic models to map potential flow pathways and identify communities and infrastructure at risk.

That is where our DebrisFlow Predictor software and our Geohazards and Geomorphology team comes into play. We combine topographic data, data from documented debris flows, and geologic and climate information to provide models of debris-flow runout pathways, identify areas and infrastructure that may be in harm’s way, and work with clients and our engineers to develop mitigation and monitoring strategies.

As an example, the Cameron Peak Fire in Colorado started on August 13, 2020, and it wasn’t 100% contained until December 2. The fire burned more than 200,000 acres and was the largest in Colorado history. The wildfire burned 469 structures.

This area is not out of danger. The fire left behind a landscape prone to debris flow and floods. We are using DebrisFlow Predictor to inform local communities about real debris-flow hazards that could affect their homes and to design mitigation solutions to reduce the risk to people and infrastructure.

DebrisFlow Predictor operates as an agent-based probabilistic model, with each agent represented by a 5-by-5-metre grid cell over a topographic surface. At each timestep in the model, each individual agent performs a calculation based on data from hundreds of documented debris flows. The agent then records information on the probability of deposition or scour at its location and moves to the next timestep as it progresses down the slope. The data generated by DebrisFlow Predictor provides debris-flow runout paths, depth of scour, and burial data. The data is then used for planning and designing mitigation approaches. 

The devastation from wildfires is followed by greater likelihood of debris flows—or landslides—and an increased risk to life and property.

Growing population, growing risks

As our global population grows and expands into more mountainous areas, more people and infrastructure are at risk from debris flows and other geohazards. Coupled with rainfall-intensity monitoring, numerical models provide us with information on the timing, extent, and degree of inundation that a debris flow may produce.

Studies show that awareness and planning for an approaching threat sharply decreases the odds of loss of life or infrastructure. It’s important to not only focus on the science and engineering factors that play into geohazards but to also understand the social and economic factors that play into community and infrastructure development. Communicating our findings in a manner that is accessible to stakeholders, meaningful to engineers, and helpful to city planners is essential.

Wildfires are increasing and so are the associated post-fire risks of debris flows. Now is the time to use modern tools—like DebrisFlow Predictor—to reduce risk and improve community resiliency through science, engineering, and designing with community in mind. 

  • Kyla Grasso

    As a project geologist, Kyla has industry experience including working with power plants, pipeline owners and operators, mining operations, and municipalities to understand the natural hazards faced by infrastructure.

    Contact Kyla
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