TIME-LAPSE MONITORING OF SIDEWALL MASS-WASTING EVENTS IN A NORTHEAST TENNESSEE GULLY

In the southern Appalachians, the dominant soil order, Ultisols, is highly susceptible to erosion. If left unmanaged these soils can develop into gully systems resulting in land degradation. This study examines gully development through sidewall mass-wasting events at a high temporal resolution using 30 minute time-lapse photography. Prior research at this site found significant mass wasting events occurring between weekly monitoring periods. By shortening the interval of observation to 30-minutes, a more accurate understanding of the frequency and intensity of these mass-wasting events, and their relation to meteorological factors can be determined. Photographs of the gully sidewall were captured from 11/29/17 - 1/5/2018 with a WingScape outdoor time-lapse camera mounted on a plastic stake 3.16 m from the gully, facing northwest and upstream into the gully channel. Meteorological data were collected by a Davis Vantage Pro weather station installed 240 m from the gully site in an open area away from structures. Qualitative visual inspection of erosion in the photos indicates that mass wasting events captured on film occur primarily when temperatures are below freezing, in response to needle ice formation. Antecedent precipitation may also be a factor; two separate precipitation events of 12.7 mm and 12.4 mm recorded on 12/7/17 and 12/23/17, respectively, did not generate mass wasting, yet within 48 – 72 hours, soil creep was observed on both occasions. From 1/1/18 -1/5/18 large amounts of material was observed moving down the northeast-facing sidewall into the channel throughout the day, however no visual evidence of needle ice was captured by the camera. Other explanations for mass wasting during this period include 1) previous needle ice formation on the upper sidewall loosened the soil, 2) needle ice occurred only during nighttime and was not captured in photographs, or 3) lagged erosion response is either longer or shorter than expected. An even finer temporal resolution may be required to determine precisely the processes causing abrupt mass-wasting in gully sidewalls.