MEASUREMENT OF SHALLOW HYDRAULIC CONDUCTIVITY PROFILES IN POTENTIAL LANDSLIDE INITIATION ZONES OF THE MOUNTAINS OF WESTERN NORTH CAROLINA

Available landslide susceptibility models for the Western North Carolina mountains assume steady state shallow groundwater flow through a porous soil. This study examines the shallow flowpaths into a steeply-dipping hollow to explore what processes may route water into landslide failures. The rate that soil hydraulic conductivity (Ksat) decreases with depth is used as a proxy of dominant flow paths. Measurements of soil Ksat at different slope positions and vegetation types can be used for parameterization of predictive models.

Research in a zero-order watershed at the Coweeta Hydrologic Laboratory, North Carolina, examined: (1) the Ksat decline with depth, (2) the differences in Ksat profiles with varying geomorphic site, (3) the differences in near surface Ksat values between rhododendron (rhododendron maximum) and mixed-hardwood slopes and (4) the relation between grain size and slope position. Five slope transects had measurement sites at ridge, mid-slope, and valley floor sites, with two of these transects on rhododendron-covered slopes. A constant head permeameter measured Ksat at 10-30, 40-60, and 70-90 cm depths, and detailed field soil descriptions accompanied each permeability profile. Rain gages and soil moisture sensor arrays are positioned along the central transect to measure soil moisture response to rain inputs.

The preliminary average Ksat value at 40-60 cm depth (0.0027 cm/sec) is not significantly less (α=.05) than the average Ksat value at 10-30 cm depth (0.0039 cm/sec,) suggesting vertical infiltration of water and deeper flow paths. The preliminary average Ksat value for all ridge and mid-slope measurements at 10-30 cm (0.0046 cm/sec) is significantly higher than the average valley measurement (0.0025 cm/sec) suggesting that valley locations are more likely to have shallow flow paths. The preliminary average Ksat value at 10-30 cm depth on rhododendron slopes (0.0030 cm/sec) is significantly lower than the average 10-30 cm value on mixed-hardwood slopes (0.0056 cm/sec) perhaps reflecting the effect of rhododendron on shallow flow paths. Moisture was retained longer in mid-slope and valley locations due to the presence of finer material at depth relative to ridge locations. Future work will determine if observed soil pipes in rhododendron-covered slopes are active during storms.