Michael Lamb on Post-Fire Debris Flows
How are fires and debris flows connected?
Fires burn vegetation, but they also change the surface of the earth itself by moving sediment and soil. That's where I come at this scientific problem: trying to understand this movement of material and how that shapes the surface of the earth.
Movement events called debris flows— fast-moving torrents of water, mud, and rocks—can be devastating to people and buildings. Sometimes, people call these mudslides, but they're not mudslides; they're full of boulders and rock that can move up to 15 miles per hour. Even though there's water involved, it is mostly rock moving, not water, which makes debris flows particularly destructive.
In Southern California, we live where post-fire debris flows are very common. This is because of three overlapping factors: fire that creates loose soil, steep hills, and high-intensity rainfall events.
One of the reasons we're able to build and live in the region is because we have engineering infrastructure that provides protection from debris flows. If you live up against the mountain front, you probably live near a debris basin. There are about 100 of them that go across the front of the San Gabriel Mountains and, for the most part, they capture all the sediment that comes off the mountains, so that it doesn't go into our neighborhoods. Your tax dollars are funding government employees from local to federal levels who are keeping us safe.
In the first year or two after a fire, there's a much, much higher rate of sediment and soil that's coming off the mountains and caught in the debris basins. The fire mobilizes about 20 years' worth of erosion in a single event.
What can be done to forecast these hazards?
The California Watershed Emergency Response Teams, or WERTs, put out the most authoritative report on where the hazards are, and residents can look up their exact neighborhood. This report results from a collaboration of soil and fire experts at local, state, and federal levels. That 's where I go for information about what's happening in my neighborhood and to find out if I am in an area that's at risk. WERTs make predictions using a model that is trained on historical information. They have found that there are four main variables that control how big debris flows get:
- How much of the landscape burns
- How steep the topography is
- How steep the hills are
- How hard it rains (not how much it rains)
In the wake of the Eaton fire, we can measure these four variables and try to predict what might happen. What we see, for the most part, is that the capacity of the basins is larger than the amount of material that we expect to come off the mountains. That's good—that's what we want, to be able to capture all that material, so that it's not coming down into neighborhoods.
But there are some areas of concern. For example, areas that don't have debris basins. People who live right in front of those areas will receive alerts and need to be vigilant.
Now, if we adjust predictions to account for a bigger rain event—a heavy downpour where we have 0.4 inches falling in 15 minutes—things get a little bit scarier. These heavy downpours occur in our mountains about once per year on average. In this case, the models show us the same problem areas, but now the capacity of the basins might not be large enough to capture all of the material we'd expect to come down.
As with any model, we try to calculate the uncertainty of the predictions. For this model, there is a lot of uncertainty because we don't have a ton of data to feed into the model—there are only about 40 historical events we have data for. Also, the four variables that are used to make predictions don't capture everything that is happening physically. As a scientist, I am working with others on figuring out the physics of these processes so that we can reduce the uncertainty and make these predictions more certain.
—Michael Lamb, Professor of Geology
Dive Deeper
For more information on debris flow hazards, please see the California Watershed Emergency Response Teams (WERTs) reports. Reports on the Palisades and Eaton fires are available.
Download Post-Wildfire Flash Flood and Debris Flow [Survival] Guide, a booklet from the National Weather Service.