Low Landslide Risk After Tragic Smokies Fires, Officials Say

Great Smoky Mountains National Park and nearby communities are at low risk of a mudslide or landslide after recent fires, according to the USGS' first-ever hazard assessment for an Eastern forest fire.

In the midst of a hard holiday season for the communities around Gatlinburg, 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 burned area of the national park are at low risk of a mudslide or landslide.

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 mudslides follow fire, especially in the West. Burned mountainous areas are prone to flooding and erosion, and can 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 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 the most susceptible parts of the burned area, it would take at least .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 the burned-over zone.

And 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 considerably less likely to be affected in the event of a debris flow.

Low risk doesn’t mean no risk

Though the risk of a debris flow in the Chimneys area is low, it is 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, Mr. 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,” Mr. 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 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.”