EROS Work Called 'Critical' to Wildfire Mapping, Response

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When a wildfire rages across the landscape, the danger seldom ends with a final fading ember.

Dennis Staley understands this. A research physical scientist with the USGS Landslide Hazards Program in Golden, CO, Staley tries to figure out where that danger lurks after the fire dies. Where are debris flows likely to start on charred mountainsides? How much rain would unleash a muddy slurry of water, soil, vegetation, and boulders down steep slopes? How large might such a potential debris flow be?

Post-fire response teams rely on Staley for answers. He, in turn, relies on Landsat imagery and the associated remote-sensing products derived in part at USGS EROS near Sioux Falls, SD, for the projections that help those response teams prepare for the worst.

“The work EROS does,” Staley notes, “is critical to what we do.”

Here’s why. Staff at EROS produces what is called a Differenced Normalized Burn Ratio (dNBR) index shortly after a wildfire ends. The index relies on pre- and post-fire Landsat images of the burned area to capture a range of values signifying either an uptick or decrease in greenness across the fire’s footprint. The more negative the values, the more greenness. “The greater the magnitude of the positive values, you can interpret that as the more severely the fire impacted the surface cover,” Staley says.

Those values are then categorized into four loosely defined classes: unburned, low burn severity, moderate severity, and high severity. That information is transformed into Burned Area Reflectance Classification (BARC) data that are handed off to Burned Area Emergency Response (BAER) teams to use in the field as a guide for more in-depth field measurements and observations of fire damage.

As quickly as possible, the BAER teams turn all that information into Soil Burn Severity Maps, which are used in the effort to stabilize landscapes where needed and hopefully prevent further damage to life, communities, property, or natural resources.

Burn maps to support BAER teams are actually produced by both EROS and the USDA Forest Service’s Geospatial Technology and Applications Center (GTAC), says USGS Geographer Birgit Peterson at EROS. GTAC burn maps cover primarily Forest Service and Department of Agriculture lands. EROS takes care of the Department of Interior-managed lands.

“The fact is, we work hand in hand with GTAC to get this information out there as quickly as we can,” Peterson says.

That’s important for several reasons. Hillside soils, vegetation, and rocks no longer anchored by forested mountainsides can pose erosion and runoff risks to water quality in the area, Peterson says. That in turn can impact fish and other habitat. To help prevent that, post-fire mitigation efforts often include quickly reseeding, or putting up barriers to redirect future mud and debris flows.

“Even if we’re not worried about homes or other structures being destroyed because of huge volumes of earth being displaced, it’s still going to be enough to have impacts down the line,” she says. “Those are the kinds of things the BAER teams are on the ground assessing.”

In the past year, Staley and his colleagues in the Landslide Hazards Program have done hazards modeling on 91 different fire events. Among the most significant was the Thomas Fire in Southern California. The largest wildfire in modern California history, that monster blaze burned more than 280,000 acres over a month—from Dec. 4, 2017, to its full containment on Jan. 12, 2018—in Ventura and Santa Barbara counties.

Strong, persistent Santa Ana winds drove Thomas as it destroyed at least 1,063 structures, including 500 in one night in the city of Ventura, and damaged 280 more. As the flames died, heavy rains fell in January on the burned hills above Montecito. The rapid erosion, mud flows, and debris flows that followed caused catastrophic damage in Montecito Creek and San Ysidro Creek. All told, 21 fatalities, 129 destroyed residences, and 307 damaged residences were attributed to the debris flows in Santa Barbara County.

Afterward, Staley and his Landslide Hazards Program colleagues mapped areas in and around Montecito that were inundated by the deadly mudslide, trying to get a sense of its extent and magnitude. Such forensic analyses serve two purposes, he says. There was an immediate need to identify areas that have been inundated so local officials and emergency responders understand where the hazardous areas are if more events occur over the winter. But there’s a longer term goal as well—to use the information they gather to be able to estimate locations that may be susceptible to inundation in future events.

That idea of looking farther through time at hazards is something Staley and EROS staff have been discussing of late. While the immediate analysis that comes with pre- and post-fire Landsat images and the dNBR index will always be important, Staley says the idea of monitoring hazard sites long term using Landsat images would allow them to tie the dNBR index values to vegetation, soil, and hydrologic recovery.

“This is something that would be a huge advancement in the science from our perspective in terms of the hazards assessment and early warning that we do,” Staley says. “One of the most frequent questions we get from folks on the ground that live below places that produce debris flows is, ‘How long will this hazard last?’ We have data based on our monitoring that says, ‘Oh, it could be anywhere from, say, two to five years.’ But we don’t have a good scientific understanding of the factors that contribute to or are correlated with that time frame.”

As far as he knows, there is no place to access that scientific understanding now. So Staley hopes to continue the conversation with EROS because “I don’t think it will happen without their support.”

Funding, as always, will be a key as to whether that becomes a reality, Peterson says. But she agrees that such a collaboration between the Landslide Hazards Program and EROS “fits in very nicely with what we’re trying to accomplish here.”

What they’re trying to accomplish is about more than just making dNBR indices and BARC maps, Peterson says. It’s about savings lives, about protecting communities and natural resources, long after the last ember has cooled. With EROS’ help, with GTAC and the Landslide Hazards Program and all the work of the BAER and Watershed Emergency Response Teams (WERT), Staley believes they are succeeding at that.

The Thomas Fire proved it to him.

“I think 20 fatalities is 20 too many,” Staley says. “But I definitely think the teams we communicated and worked with, the BAER and WERT teams, succeeded in making a difference. I think those teams were responsible for saving multiple lives.”