Low landslide risk after tragic Smokies fires

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Great Smoky Mountains National Park is at low risk of one common type of landslide, according to the USGS' first-ever hazard assessment for an Eastern forest fire.

This story has been updated to describe a US Department of Agriculture assessment of the potential for slope failures other than debris flows after the Chimney Tops 2 fire.

After a hard holiday season for the communities around Gatlinburg, Tennessee and the Great Smoky Mountains National Park, a U.S. Geological Survey research team offers some hopeful news. Though it will take time for the fire-scarred mountains to regrow their natural vegetation and capacity to control erosion, the soil and rocks within the national park are at low risk of one type of landslide common to recently burned areas.

The fire known as Chimney Tops 2 started in Great Smoky Mountains National Park on Nov. 23, 2016, during an extreme drought. On Nov. 28 as gale force winds blew, the fire merged with other blazes and spread rapidly, devastating parts of Gatlinburg and nearby Pigeon Forge.

Understanding debris flows

USGS scientists have experience studying and analyzing the common cycle of back-to-back disasters when flooding and a certain type of landsliding follow wildfire, especially in the West. Burned mountainous areas are prone to erosion and flooding, and can often lead to what are called debris flows: a slurry of mud, soil, sand, gravel and rocks traveling down a slope, often with dangerous speed and power.

A new assessment by the USGS Landslide Hazards Program, the first one ever done after an Eastern wildfire, finds the park’s overall risk of a debris flow is “relatively low” in the wake of the devastating Chimney Tops 2 fire, said USGS research physical scientist Dennis Staley.  The assessment predicts that in a few highly susceptible parts of the burned area, it would take an intense rain burst, at the rate of .4 inches of rain in 15 minutes—a rate the area typically sees once every two to five years—to trigger a debris flow. In most of the park, it would take an even more intense rainfall to cause debris flows. Although some Gatlinburg businesses and homes border the national park, the USGS analysis found the most vulnerable areas are within the park, and neighboring areas are less likely to be affected by a debris flow.

However, longer lasting, steady rainfall after a fire can trigger other kinds of slope failures, such as rock falls or the downhill movement of a part of the ground surface. An analysis by the US Department of Agriculture’s Natural Resources Conservation Service found that if the burned area’s soils become saturated, rockslides or other types of slope failures are likely in some of the steeper, more severely burned locations.

The USGS model focuses on hazardous debris flows, but “there are hazards other than those we model,” Staley said. “If it rains for a prolonged amount of time, flooding, slope failure or rock falls could occur. People should listen to emergency managers and weather forecasters in order to be prepared for these hazards.”

Low hazard doesn’t mean no hazard

Though the debris flow hazard in the Chimneys area is low, it’s not non-existent. To help land managers, USGS landslide experts are working with the National Park Service to identify slopes that are most erosion-prone. Land managers can use this information to determine potential mitigation measures to take such as stabilizing hillsides and stream banks, Staley said.

The computer-modeled landslide assessment, pioneered by the USGS in 2002, is widely used throughout the West, where the USGS team applied it to 36 wildfires in 2016. The new Eastern assessment is based on Western fire conditions, which are usually more extreme than in the East, so the "low risk " finding is a conservative risk assessment and errs on the side of caution.

 “We have used these debris flow forecasts throughout the West, where they work very well, but this has never been done in the East before,” Staley said, “because it is somewhat rare to have a large fire on mountainous terrain in the Southeast.”

Start with science

After wildfires, federal land managers often ask USGS scientists to develop detailed debris flow hazard maps of the burned areas. The maps factor in the steepness of the burned slopes, the fire’s effect on the canopy, properties of the local soil such as how easily it erodes, and the severity of the burn at soil level.

The information comes from satellite imagery and other types of remote sensing, and from Burn Area Emergency Response (BAER) teams – groups of fire experts and biologists from the National Park Service, the U.S. Fish and Wildlife Service and other agencies who conduct on-the-ground damage assessments after wildfires on federal lands.

A computer model analyzes the mountains and hillsides to assess how much rainfall over how short a time it would take to cause a debris flow to form and travel downstream. In the case of the Chimney Tops 2 fire, the program estimated that on average, it would take the equivalent of about 1.6 inches of rain per hour to cause a debris flow in the most susceptible areas.

Though the risk of a debris flow in the Chimneys area is low, it’s not non-existent. To help land managers, USGS landslide experts are working with the National Park Service to identify slopes that are most erosion-prone. Land managers can use this information to determine potential mitigation measures to take such as stabilizing hillsides and stream banks, Staley said.

The computer-modeled landslide assessment, pioneered by the USGS in 2002, is widely used throughout the West, where the USGS team applied it to 36 wildfires in 2016. The new Eastern assessment is based on Western fire conditions, which are usually more extreme than in the East, so the "low risk " finding is a conservative risk assessment and errs on the side of caution.

 “We have used these debris flow forecasts throughout the West, where they work very well, but this has never been done in the East before,” Staley said, “because it is somewhat rare to have a large fire on mountainous terrain in the Southeast.”

Start with science

After wildfires, federal land managers often ask USGS scientists to develop detailed maps of the burned areas. The maps factor in the steepness of the burned slopes, the fire’s effect on the canopy, properties of the local soil such as how easily it erodes, and the severity of the burn at soil level.

The information comes from satellite imagery and other types of remote sensing, and from Burn Area Emergency Response (BAER) teams – groups of fire experts and biologists from the National Park Service, the U.S. Fish and Wildlife Service and other agencies who conduct on-the-ground damage assessments after wildfires on federal lands.

A computer model analyzes the mountains and hillsides to assess how much rainfall over how short a time it would take to cause a debris flow to form and travel downstream. In the case of the Chimney Tops 2 fire, the program estimated that on average, it would take the equivalent of about 1.6 inches of rain per hour to cause a debris flow in the most susceptible areas.

Fires and floods

Meanwhile another team of USGS scientists has installed a special streamgage on a pedestrian bridge over the West Prong of the Little Pigeon River, near the park’s visitor center. The solar-powered rapid deployment gauge, designed for quick installation in potential flooding emergencies, uses radar to precisely measure changes in the distance between the gauge and the river’s surface.

That information goes via satellite to the National Weather Service, which will use it to monitor the potential for flooding downstream from the gauge, said Shannon Williams, the Nashville-based USGS hydrologist who oversees the streamgage program in Tennessee. The streamgage readings for the Little Pigeon River are also available to the public in real time.

Though Sevier County, Tennessee has four local streamgages, this is the first USGS-operated gauge in the Gatlinburg area. It lies less than a mile upstream from Gatlinburg, Williams said.

“The geography makes it difficult for the National Weather Service to issue flood warnings based on what the gauge shows,” Williams said. “It’s such a short distance from the headwaters of the park’s mountain streams to the local communities that there isn’t a lot of lead time. But by comparing the amount of rainfall measured in a storm to the water levels recorded on the streamgage, forecasters can get a better understanding of the relationship between rainfall and runoff. And that gives them an idea of what to expect when the next storm is approaching.”

Learn More:

The USGS Landslide Hazards Program

The science behind the debris flow estimates

 Regionwide water information from the Lower Mississippi-Gulf Water Science Center