DEBRIS BLOWOUTS - THE FORGOTTEN LANDSLIDE MOVEMENT TYPE, AND THE LIDAR THAT BROUGHT IT BACK

The debris “blowout” is not a term often heard in landslide papers and discussions. However, with the availability of high-resolution lidar, coupled with knowledge of historic high-intensity rainfall events, blowout-style landslide features are becoming easier to map and discuss. The term “blow-out” was first used by Eisenlhor (1952) who described them as an effect of torrential rainfalls, where “The soil from the blow-out holes, mostly in a semiliquid form, had run and slid down to the bottom of the hills but had scarcely disturbed the ground surface...”

Hack and Goodlett (1960) continue to define “blowouts” as typically semicircular in plan with a crescent-shaped scarp and a pile of debris at the down slope side that “has slumped or thrown out of the cut or break.” Both the Eisenlhor and Hack and Goodlet papers attribute these features to hydrostatic pressure at “geologically favorable horizons” where groundwater is concentrated by intersecting fractures or sills in the bedrock.

In 2006, the NC Geological Survey identified 299 debris or earth blowouts in Watauga County, attributed to the August 13-14, 1940 storm. These blowouts had semi-circular shapes and evidence of downhill debris movement, but no channelization. These small features were not visible on the 6m lidar at the time of mapping, and were identified in the post-storm 1940 air photos.

In 2022, Appalachian Landslide Consultants, PLLC began mapping the western portion of Transylvania County, NC for the NC Geological Survey. Multiple large historic storms have impacted Transylvania County (Witt, 2005), with the storm of record dropping over 381 mm (15 inches) of rain between July 14-16, 1916. Literature research, combined with discussions with landowners and information from local historians, was used to tie the 1916 storm event to blowout locations. These blowout features have a unique signature when viewed on the 2017 0.5m resolution lidar. They are often circular with a trail of debris downslope, but no incision below. Most of the 20 debris blowouts identified in Transylvania County to date are within biotite gneiss, muscovite schist, or biotite granite augen gneiss. While a few of these lie near mapped lithologic contacts, and most are not far from the Brevard Fault Zone, more work is needed to identify any potential geologic controls on their locations.