USING LAHARZ TO MODEL POTENTIAL DEBRIS FLOW INUNDATION ZONES IN AN EMERGENCY: THE GHOST TOWN DEBRIS FLOW, MAGGIE VALLEY, N.C

On the evening of February 5, 2010, North Carolina Geological Survey (NCGS) personnel were contacted by the State Emergency Operations Center to respond to a retaining wall failure and debris flow originating at the "Ghost Town in the Sky" amusement park in Maggie Valley, N.C. The debris flow damaged three homes, buried a state road, and caused the temporary evacuation of 40 people. Because tension cracks and scarps remained in material at the top of the landslide area, the NCGS advised local officials that the potential for future slope failure posed an imminent threat to residents and property downslope. The NCGS prepared potential debris flow inundation hazard maps using a modified form of the GIS-based LAHARZ model to help identify at risk homes and infrastructure. The initial volume and area (21,000 m³, 26,000 m²) of the debris flow determined from field mapping were used to modify LAHARZ model coefficients, thereby calibrating the predictive model equations to local conditions. Next, estimates of the remaining volumes of unstable material at the top of the slide were calculated and used to create four inundation scenarios. Modeled volume estimates ranged from 11,000 m³ to a worst case scenario of 83,000 m³. The resulting inundation hazard map was used to advise local emergency officials where evacuations would be needed should a major rain event occur before the remaining unstable material could be removed and the slope was stabilized. NCGS volume calculations and field mapping were also later used to prepare a cost-estimate for repairs in support of an application for an Emergency Watershed Protection (EWP) grant from the Natural Resource Conservation Service. The EWP project cost was $1.4 million and stabilization was completed in early 2011.