THE SEPTEMBER 18, 2018 SUNNY DAY LANDSLIDE IN WARRENSVILLE, ASHE COUNTY, NORTH CAROLINA: DISASTER NARROWLY AVERTED FOLLOWING A SERIES OF UNFORTUNATE EVENTS (Invited Presentation)

A debris slide and subsequent fire on September 18, 2018 destroyed the Life Store Bank building and damaged adjacent property in Warrensville, North Carolina. Employees in the bank at the time narrowly escaped with minor injuries. Emergency responders determined that the slide pushed a propane tank and an electrical generator against the building. An electrical arc from the generator sparked a propane explosion and the fireball witnesses observed. Slide debris buried part of an unoccupied drive-through lane for the bank.

NCGS geologists responded to a request from the Ashe County Emergency Management Department, and investigated the slide area on September 19^th to assess slope stability regarding public safety. We documented slope and debris slide features, geologic materials, and produced a field-developed geologic cross section through the 1,400m² debris slide. Unmanned aerial system imagery provided by the North Carolina Cooperative Extension agency was an invaluable mapping aid. We concluded that the triggering mechanism for the slide was rainfall from remnants of Hurricane Florence following above normal precipitation in the previous month. National Weather Service data show that Florence passed over the area from September 15^th to the 18^th. The bulk of the 72-hour precipitation ranged from 127mm to 295mm for three locations within a 9km radius of the site. August precipitation was 100mm to 150mm inches above normal in the area.

Our investigation revealed geologic conditions, and hillside modifications that spanned prior decades had predisposed the slope to failure. Colluvial deposits from past slope movements comprising the slide area are derived from sheared amphibolite of the Ashe Metamorphic suite. Curved trees within the 2018 slide mass indicate pre-existing marginally stable conditions. The 2018 slide coincided with the area of a slide reported in 2000, which involved a cut slope likely constructed during site development prior to 1993. A retaining wall built after the 2000 slide allowed for a drive-through lane behind the bank. The propane tank was placed in a vulnerable location on the upslope slide of the retaining wall, which was over-topped by the 2018 slide. A septic leach field involved in the 2018 debris slide was damaged; however, it is unclear if the septic system was a causal factor in the slide.