By Marlene Cimons, National Science Foundation
Southern Californians who live in areas prone to wildfires often fear catastrophic mudslides as an invariable consequence of the charred landscape. But dust clouds, as it turns out, are a far more likely aftermath—and potentially just as dangerous.
Severe fires, often driven by Santa Ana winds, can burn the so-called biotic mat, a dense network of fine chaparral roots overlain by algae, lichen and moss that overlay and protect the soil underneath. This leaves the soil vulnerable to the high winds, which carry the particles and cause serious dust pollution.
"You can get all this dust flowing into Los Angeles, where the air already may not be so good, and it becomes a health issue," said Manny Gabet, associate professor of geology at San Jose State University, who has been studying the airborne dust that results from landscape fires and wind.
"After fires, we all focus on mud flows because they are so dramatic, but, in fact, they happen only rarely, whereas, the Santa Anas blow every year and bring up dust with them," he added. "To the public, it might not seem like a big deal because it doesn't always show up in the news, but it can be an important contributor to air quality, and to peoples' health."
The project is examining the relationship between fire, the biotic mat, and wind erosion through a program of wind-tunnel experiments. Gabet plans to collect undisturbed soil surface samples from chaparral hill slopes, and subject them to controlled wildfire conditions in the laboratory to simulate fires of varying temperatures and durations.
He also will place the samples in a wind tunnel to see what it takes, in wind speed and biotic mat composition, for the dust to start flying.
Gabet's research is funded by a $96,000 grant from the National Science Foundation as part of the American Recovery and Reinvestment Act of 2009. The money has created jobs for several additional researchers, and enabled the scientist to purchase sediment traps and other equipment from several small businesses.
More importantly, the information gained from the work ultimately could enable experts to predict future "hot spots" of dust pollution, so that people in the region could take preventive measures. This eventually could produce substantial health care savings, possibly even prevent many of the estimated thousands of premature deaths that occur annually in California as a result of poor air quality.
This research is specific to Southern California, "which gets hit by fires all the time," stirred by winds that occur annually, Gabet said. "If the fire isn't very hot, it burns only the leaves and the biotic mat is fine. But if it's hot enough to burn the vegetation all the way to the ground, the biotic mat gets vaporized, and the dust and the soil that was being protected by the biotic mat is exposed. Bring in the Santa Anas and you get huge amounts of dust."
Gabet will measure the wind speed that occurs following serious fires, as well as the size of dust particles that result. He set out a series of dust traps and anemometers, instruments that measure wind speed, after last summer's disastrous Station fire, the largest fire in Los Angeles county history. The blaze blackened 250 square miles of forest, destroyed scores of homes and other structures, and killed two Los Angeles County firefighters.
"We set these out in spots where the fire intensity had varied," he said. "Where the fire intensity was low, the biotic crust was still there, and there was very little dust. But in places where the biotic mat clearly had combusted, there was a lot more dust in the traps, so I could relate the amount of dust being blown to both wind speed and the presence or absence of the biotic mat."
He also is examining the size of the particles. "The EPA [Environmental Protection Agency] has set out specific guidelines on the size of particles, related to health," he said. "When you look at the really fine particles, those are the ones you have to worry about. I can analyze the size of the particles. The idea then, using satellite imagery, is to determine where the hotspots were in the fire."
In the future, armed with these kinds of data, "you can go out to areas hot enough to have combusted the biotic mat, and possibly spread some stuff around to prevent dust from being blown around," he said. "You can map where the biggest sources of dust may be, and take mitigation measures."
Also, "if you know it's going to be a very windy day and the winds will be focused in an area where the biotic mat has been destroyed, you can issue health warnings, or advise people to stay indoors," he added.
As part of his research, he also has built a wind tunnel where he plans to generate Santa Ana-type wind velocities. "I'm going to go to Southern California and get soil samples, which I will burn and put in the wind tunnel," he said. "I'm going to simulate what happens after the biotic mat has been destroyed and the material is hit by wind gusts."
The idea is to calculate the wind speeds necessary to create problematic dust clouds for different levels of the biotic mat.
"When the biotic mat is present, you need really high winds to get anything going," Gabet said. "But as that biotic mat gets more and more destroyed, the wind speeds you need become less and less. If you need 80 mile-per-hour winds to get it going when the mat is there, maybe you need only 20 miles-per-hour once it is destroyed. That all plays into the predictability and our understanding of what can happen."
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