Climate impacts: After the devastating fires, come dangerous mudslides

After the three largest wildfires in Colorado’s history in 2020, we are experiencing an unfortunately well-known consequence: frequent and dramatic mudslides.

One of our two interstate highways, I-70, has been closed repeatedly over the past two months due to about 20 different mudslides — scientifically known as “debris flows.” These debris flows and closures are expensive, inconvenient and dangerous.

Over 100 people were trapped between flow paths in the canyon late last month and luckily were safely evacuated. There is a sense of helplessness in Colorado and there doesn’t seem to be any solution in sight. And it isn’t just happening to us. All over the West wildfires are raging — and it seems like every time it rains someone is in danger or something is being cleaned up after debris flows.

So, how did we get to this place? And more importantly, how can we get out of it?

In what I will call “the good old days,” say, in the 1980s, debris flows were much less common. The first detailed study I did on debris flows analyzed a May 1984 torrent in Davis County Utah. That moment in time had rare and “perfect storm” hydrologic conditions to cause debris flows: late autumn rains that saturated the soil, winter snowpack well above normal that persisted far into the spring, then a warm spell with several sunny days accompanied by nights with above-freezing temperatures. While Utah had dozens of debris flows in 1983 and 1984, those were viewed as exceptional water years, and when the rain slowed down in 1985, we gave a collective sigh of relief and went back to business as usual.

But something has changed over the ensuing years. Wildfires started getting larger and more common — and so did debris flows. While the link between the two has been known for decades, it quickly became something we needed to pay more attention to. Fast forward to today: We see that research in post-wildfire debris flows has skyrocketed, with dozens of peer-reviewed scientific papers published every year. It is so important a topic that the United States Geological Survey has a program dedicated to this line of research, and they provide hazard maps after every major fire in our country.

So, why are debris flows worse after a wildfire? Because both ingredients — loose soil and water runoff — are in much greater supply. In the arid West, the patchy vegetation scattered across the hillsides is normally enough to hold most soil in place and reduce erosion. But after a fire, the soil and loose rock is free to move into gullies and canyons, choking them with the sediment that can later be mobilized by water flow. With vegetation burned, more water is available to mix with the soil and rock, because less of it is intercepted by plants, and less of it is slowed down enough to infiltrate into the ground.

As a result, even small rainstorms can cause debris flows in burned areas, like those we’ve seen wreaking havoc on I-70. So, while Utah in 1984 needed months of moisture to create the right conditions, Colorado in 2021 only needed an inch or less of rainfall in most places to cause the hillside to let go.

If wildfire is the devil that brought us to this place, then climate change is the sorcerer that let him out of the bag. There is ample research linking a warming climate to our increasingly smoky skies and connecting it to more frequent and intense rainstorms that cause debris flows. To prove to ourselves how bad this could be, my team projected climate-induced changes on the 2017 Thomas Fire in California and the ensuing 2018 debris flows, which caused 23 deaths and over $200 million in damage and clean costs. We’ve found that if that event were to occur in the year 2075, between increased fire size in the future and the increased probability of more intense rainstorms, we would almost double the number of watersheds that would have debris flows. The total mud volume emanating from these canyons would be even more than doubled. Sobering thoughts, even for a single example.

So, what can we do now? First of all, add debris flows to the list of reasons we need to reduce carbon emissions and the temperature changes they cause.

We can also provide better local controls on where we build: Our recent expansion into steep forested terrain, known as the wildland-urban interface, exposes us more to wildfire and the subsequent debris flows. Communities can also use USGS debris flow hazard maps to raise awareness of risk and help avoid dangerous areas. Engineered methods to mitigate debris flow risk are numerous, with retention basins and channelization of flow paths being among the most effective.

Finally, as a scientist, I urge us to dedicate funds for research into wildfire management, climate issues and post-wildfire debris-flow analysis. We need to unravel these intertwined problems to provide hope and solutions to seemingly intractable and worsening situations. Our tools are getting better all the time, but we are racing against a foe that has a long head start.

Paul Santi, Ph.D., is professor of geology and geological engineering at the Colorado School of Mines, a fellow of the Geological Society of America and past president of the Association of Environmental and Engineering Geologists.