DEBRIS FLOWS TRIGGERED BY AUGUST 24, 2019 STORM IN THE NANTAHALA GORGE, WESTERN NORTH CAROLINA; DID THE UNDERLYING BEDROCK AND THE 2016 WILDFIRES INCREASE LANDSLIDE SUSCEPTIBILITY?

On August 24^th 2019, approximately 100-130 mm of precipitation in a 5-hour period triggered multiple debris flows in the Nantahala Gorge, in Swain County, western North Carolina. First responders and motorists were trapped by debris flows that blocked a 1.2 km section of US 19/74 in at least six places. Multiple state and federal agencies including the NC Geological Survey (NCGS), NC Department of Transportation (NCDOT), Swain County Emergency Management, US Forest Service (USFS), and the National Weather Service collaborated on the initial response. Woody material and debris blocked the Nantahala River in three places, and the USFS removed 6,894 metric tons from the river at a reported cost of ~$255,000. NCDOT reported a cost of $500,000 for highway cleanup and repair, and the Nantahala Outdoor Center lost a reported ~$300,000 in rafting business while the river was closed over Labor Day weekend. Following the event, NCGS produced a series of progressively updated maps on August 27^th and 29^th, and September 10^th to support responders and stakeholders. We used a combination of unmanned aerial system (UAS) imagery, NDVI (Normalized Difference Vegetation Index) imagery, and fieldwork to map the debris flow tracks. Of the 32 mapped debris flows, 23 initiated in areas of moderate or low burn severity from 2016 Ferebee and Tellico fires. To our knowledge, this is the first documented debris flow swarm that directly correlates with 2016 burned areas in the southeastern US.

Questions remain in this ongoing study as to the relative effects of bedrock geology on landslide susceptibility. Based on existing geologic maps, there is a geomorphic asymmetry in the gorge likely controlled by Paleozoic bedrock structures. Bedding and foliation dip to the SE, and the anti-dip slope (SE side) is steeper than the dip slope (NW side). Despite this bedrock fabric, the 2019 debris flows occurred on the SE side. Future work is needed to: 1) clarify geomorphic and structural contrasts on either side of gorge; 2) monitor areas in the gorge that burned but did not fail during 2019 debris flow event; 3) test whether fire effects only influence shallow-seated landslides initiating in soil and not deep-seated landslides that start in bedrock; and 4) look for undocumented landslides on the NW side of the gorge where bedding and foliation dip downslope.