Here’s a guest column from Paul Santi that’s running in The Denver Post. Here’s an excerpt:
The ingredients for deadly debris flows are steep mountain slopes, a loose mantle of soil and rocks covering the hillsides and choking the valleys, and occasional severe rainstorms. These things converge surprisingly often, and most canyons around here have an apron of both ancient and fairly recent debris flow deposits at their mouths, revealing a historic legacy of these events. However, the next ingredient is the kicker: wildfire. Suddenly, the tiny bit of stabilization offered by our patchy arid-climate vegetation is gone. On top of that, rainfall on burned areas runs off faster and in much greater volumes, since any sort of interception by plants or infiltration into the ground is drastically reduced. Oh, and by the way, the changing climate means that wildfires have been more frequent and larger, and intense rainstorms are more likely. More water, more tumbling rock and debris, and more danger.
That was the story in California last week, as the fire-flood-mud sequence played out once again. That has also been the story in Colorado many times in the past. In 2012, the Waldo Canyon Fire burned hillsides above Colorado Springs, and a rainstorm the next summer brought muddy debris down Highway 24 near Manitou Springs, tragically sweeping two people to their deaths. Post wildfire debris flows have destroyed houses and damaged roads and bridges above Boulder, Glenwood Springs, Durango, and many other Colorado cities over the last dozen years.
So what can we do about this? First, you need to know if you are personally in danger. The simple answer is that if you live near a canyon mouth, you live in risk from debris flows. If the forest above you burned, you are at even greater risk for the next few years, until the vegetation comes back. The same goes for highways crossing these areas. A more complex picture can be filled out with tools developed by the U.S. Geological Survey and others that predict that probability of debris flows, how big they will be, how much rain is needed to cause them, and where they will go. The USGS now routinely produces maps showing probability and volume for debris flows in areas where there were significant wildfires (over 80 of these maps were created in 2017 alone, including the Thomas Fire in Santa Barbara County that caused the maelstrom last week).
Second, and this is just as important, if you live near a canyon below a burned forest, evacuate when it rains, as even small storms can cause debris flows in this hair-trigger setting. Natural processes are highly variable and unpredictable, so warnings and evacuation orders have to be conservative — and this means that there will be false alarms. Don’t fall prey to “evacuation saturation” and ignore a warning because the last one didn’t pan out. It is heartbreaking to think of how many lives would have been saved in California last week had those families been somewhere else at the time.
Finally, we need to carefully consider where we are willing to build. Mountain canyons concentrate geologic hazards, and increase the likelihood of accidental fires. Therefore, the encroachment of our communities into steep forested terrain must be accompanied by planning and vigilance to protect ourselves. The scientific tools help, and they are getting better all the time, but our own awareness and caution can be life saving.