After the three largest wildfires In the history of Colorado in 2020, we are experiencing a woefully well-known consequence: frequent and dramatic landslides.
One of our two interstate highways, I-70, has been repeatedly closed over the past two months due to approximately 20 different landslides, scientifically known as "debris flows." These debris flows and closures are costly, inconvenient, and dangerous.
More than 100 people were trapped between flow paths in the canyon late last month and thankfully they were safely evacuated. There is a sense of helplessness in Colorado and there seems to be no solution in sight. And it's not just happening to us. There are wildfires all over the West, and it seems like every time it rains someone is in danger or something is cleaned up after the debris flows.
So how do we get to this place? And more importantly, how can we get out of it?
In what I'll call "the good old days," say, in the 1980s, debris flows were much less common. The first detailed debris flow study I did looked at a May 1984 torrent in Davis County, Utah. That time had rare "perfect storm" hydrologic conditions to cause debris flows: late fall rains that saturated the ground, well above normal winter snow cover that persisted well into spring, then a warm wave with several sunny days accompanied by nights with temperatures above freezing. While Utah had dozens of debris flows in 1983 and 1984, those were seen as exceptional water years, and when the rain subsided in 1985, we breathed a collective sigh of relief and returned to normal.
But something has changed in the following years. Wildfires started to get bigger and more common - and so did the debris. 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 research into post-wildfire debris flows exploding, with dozens of peer-reviewed scientific papers published each year. It is such an important issue that the United States Geological Survey has a program dedicated to this line of researchand 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 irregular vegetation scattered along the slopes is usually sufficient to keep most of the soil in place and reduce erosion. But after a fire, soil and loose rock can move freely into ravines and canyons, suffocating them with sediment that can then be mobilized by the flow of water. With burned vegetation, there is more water available to mix with the soil and rock, because less plants intercept and less slows down enough to infiltrate the soil.
As a result, even small rain storms can cause debris flows in burned areas, like the ones we've seen wreak havoc on I-70. So while Utah in 1984 needed months of humidity to create the right conditions, Colorado in 2021 only needed an inch or less of rain in most places for the hillside to break loose.
If wild fire is the devil that brought us to this place, then climate change is the sorcerer that let it out of the bag. There is extensive research link a warm climate to our increasingly smoky skies and connect it to more frequent and intense storms causing debris flows. To show ourselves how bad this could be, my team projected climate-induced changes in 2017. Thomas Fire in California and the ensuing 2018 debris flows, which caused 23 deaths and more than $ 200 million in damage and cleanup costs. We have found that if that event occurred in 2075, between the increase in the size of the fire in the future and the greater likelihood of more intense storms, we would almost double the number of watersheds that would have debris flows. The total volume of mud emanating from these canyons would even more than double. Cautious thoughts, even from just one example.
So what can we do now? First, 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 over where we build - our recent expansion into steep wooded terrain, known as the urban-forest interface, exposes us more to wildfires and subsequent debris flows. Communities can also use USGS debris flow hazard maps to raise awareness of the risk and help avoid hazardous areas. Methods designed to mitigate the risk of debris flow are numerous, and retention basins and channeling of flow paths are among the most effective.
Finally, as a scientist, I urge us to dedicate funds to research on 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 improve all the time, but we are competing against an enemy who has a great advantage.
Pablo Santi, Ph.D., is a professor of geology and geological engineering at the Colorado School of Mines, a member of the Geological Society of America, and a past president of the Association of Engineering and Environmental Geologists.
