1. USING A GEOGRAPHIC INFORMATION SYSTEM (GIS) TO MODEL SLOPE INSTABILITY AND DEBRIS FLOW HAZARDS IN THE FRENCH BROAD RIVER WATERSHED, NORTH CAROLINA Anne Carter Witt Dept. of Marine, Earth and Atmospheric Sciences, Box 8208, North Carolina State University, Raleigh, N. C. 27695, awitt48662@aol.com Introduction Catastrophic, storm-generated mass wasting is a destructive erosional process in the portion of the Southern Appalachians that extends through North Carolina. Steep slopes, a thin soil mantle, and extreme precipitation events all increase the risk of slope instability and failure. Since the early 1900’s, several intense storms and hurricanes have tracked through the French Broad watershed initiating hundreds of debris flows and causing severe flooding. In the Appalachian Mountain chain, it has been estimated that as many as 1,700 debris flows occurred in the 20 th century, killing at least 200 people and destroying thousands of acres of farm and forested land. Watershed Area: ~7,000 km² Counties: Transylvania, Henderson, Buncombe, Madison, and Haywood and portions of Avery, Mitchell, and Yancey Major Rivers: French Broad, Nolichucky, Toe, and Pigeon Rivers Population: ~426,000 Research Objectives: 1.To investigate and predict the spatial distribution of regional slope instability within the French Broad watershed by analyzing the results of the GIS-based modeling application: SINMAP (Stability INdex MAPping) (Pack, et al., 1998); 2.To identify triggering mechanisms particular to the area that influence instability and failure; 3.To determine which soil and geologic units are the most prone to instability and those that are the most stable, in general; and 4.To study the effect of parameter variation on the modeling program. The final output of a SINMAP session is a series of maps that define areas of potential terrain instability; shaded green areas are considered stable while dark red areas have a high probability of failure based on parameter inputs. History The mountains of North Carolina have a long history of producing destructive debris flows through the 20 th century. The prior record is incomplete, as the evidence and scarring of previous debris flows has been quickly removed by vegetation and weathering. By reviewing both historical rainfall events that have triggered debris flows, and the locations of these debris flows, predictions can be made about future debris-flow locations and the triggering mechanisms that are common to the region. SINMAP As an extension to ArcView ® 3.x, SINMAP computes and maps a slope-stability index by calculating factors of safety using a modified form of the infinite slope equation. Topographic slope is derived from digital-elevation data (30-meter and 10-meter DEMs) while parameters for soil and climate are considered more variable and are entered as upper and lower bounded values. Actual debris flow and slide locations are used to verify the model results. Debris flows are swift-moving mass-wasting events that occur predominantly in shallow, coarse- grained soil on steep slopes during periods of exceptionally heavy precipitation. They are particularly dangerous to life and property because they travel quickly, often strike without warning, and may be of sufficient density to move large boulders, trees, and cars (Cruden and Varnes, 1996). SINMAP Stability Index map for 125mm recharge and using a 30-meter DEM DatePrecipitation (mm)Debris Flow Occurrences May 1901205Buncombe and Henderson Counties July 1916315South and East of Asheville August 14-15, 1940380Northern part of the watershed August 28-31, 1940200-300Western portion of the watershed November 4-6, 1977200-300Buncombe and Yancey Counties May 2003100-150Smoky Mtn. NP and Cherokee County