T138 Landslides and Debris Flows: Global Problems, Local Solutions

Tuesday, 6 November 2012
8:00 AM - 12:00 PM
207BC

DEBRIS FLOW AND LANDSLIDE HAZARD STUDIES IN WESTERN NORTH CAROLINA 2004-2012: FINDINGS, LESSONS LEARNED, AND CHALLENGES

Paper number: T138-8
Program: Technical Sessions
Time: 9:45 AM – 10:05 AM

Authors

Abstract

Recurring weather patterns, especially landfalling tropical cyclones, will continue to trigger debris flows and other landslides in mountainous western North Carolina (WNC). At least seven major regional storm events have occurred since 1916: two in 1916, two in 1940, one in 1977, and two in 2004. The most damaging of these storms occurred in August 13-14, 1940 when the remnants of a hurricane tracked over WNC and set off ~2,100 debris flows and debris slides in Watauga County alone. Intense summer thunderstorms can trigger localized debris flow-slide activity. A period of above average rainfall from September 2009 through February 2010 triggered over 40 debris flow and landslide events in WNC, all of which occurred on slopes altered by human activity. Rainfall rate and duration are critical factors in debris flow initiation. Studies indicate that less rainfall is required to trigger debris flows on slopes where human activity has had a destabilizing effect, than on slopes unmodified by human activity.

Landslide hazard studies by the N.C. Geological Survey have identified 3,295 slope movements and 3,195 deposit areas from past debris flow activity (e.g., debris fans). Debris/earth slides, flows and blowouts make up 92% of the identified slope movements, and have resulted in the most fatalities, injuries and property damage. Slides in rock and weathered rock that include major events along I-40 in the Pigeon River Gorge make up the next largest category at 7%. Losses from 57 landslides that have occurred since 1990 include six fatalities, five serious injuries, 40 destroyed or condemned structures, damage to 24 structures and 56 other private properties, and damage to at least 32 roads.

Landslide hazard maps, authorized by the N.C. General Assembly in 2005, were completed for four counties prior to elimination of the program by the General Assembly in 2011. Budget cuts and changing priorities have created challenges in making the completed maps readily accessible to the public in GIS-based web map viewers, and have reduced our capacity to respond to landslide emergency events. Continuing efforts include educational outreach, and cooperation with the National Weather Service, the U.S. Geological Survey and universities to document and anticipate weather conditions that favor debris flow and landslide activity.