GEOLOGIC, GEOMORPHIC AND GEOTECHNICAL ASPECTS OF DEBRIS FLOW INITIATION SITES IN THE BLUE RIDGE MOUNTAINS OF WESTERN NORTH CAROLINA AT STUDY LOCATIONS IN BUNCOMBE, HAYWOOD, HENDERSON, JACKSON, MACON, TRANSYLVANIA AND WATAUGA COUNTIES (Invited Presentation)

Debris flows make up ~92% of the 3,295 identified slope movements in western North Carolina. They are the most widespread and damaging type of slope movement in the region, and from 1916 to 2004 debris flows and debris slides killed at least 46 people. Between 2004 and 2011 we studied 23 locations to develop conceptual models for actual and potential debris flow initiation sites, and to obtain parameters for use in county-wide debris flow susceptibility models. Debris flow initiation occurred at 17 of these sites, 12 of which are known to have happened either in 1940, 1977, 2004 or 2009. Studies included field-developed geologic cross sections, Shelby tube sampling (triaxial soil shear strength testing), soil sampling (gradation and Atterberg limits), and in situ testing of saturated soil hydraulic conductivity (k_sat). We collected similar data at six other mountainside sites with convergent topography that were not known to be debris flow initiation sites. Inferred detachment depths at initiation sites range from 0.6 to 3m (2-10ft), so we targeted this zone for soil sampling and testing.

The sites are located in basement gneisses and schists, metasedimentary, metaigneous and metavolcaniclastic cover rocks, and orthogneisses. With respect to bedrock compositional layering and foliation, the sites are located on dip slopes, scarp slopes, and slopes oblique to these discontinuities. All sites are characterized by convergent topography except two debris flow sites with planar topography. Intersecting foliation and fracture planes form wedge- and trough-shaped features that influence the formation of the convergent topography. Debris flows initiated on slopes as low as 22°and high as 40°, with 33° being the average of the 17 sites. Effective soil friction angles for colluvial soil at debris flow sites range from 32-39° (34° avg), and for residual soil range from 22-32° (29° avg). Colluvial soil at debris flow sites is generally cohesionless with values for effective soil cohesion that range from 0-1.17 kPa (0-0.17psi), which are generally lower than those for residual soil with values that range from 0.0-13.6kPa (0-2.0psi). Values for k_sat at colluvial debris flow sites range from 5.56E-05 to 3.92E-03cm/sec (1.83E-03cm/sec avg) and, values for residual soil range from 2.72E-05 to 6.01E-03cm/sec (2.46E-03cm/sec avg).