TOPOGRAPHIC THRESHOLDS FOR HISTORICAL GULLY FORMATION AND THE GEOMETRY OF GULLIES, CULLOWHEE, NC

Increased land development in poorly-understood headwater regions of western North Carolina has made understanding watersheds progressively important. The Gribble Gap watershed (0.4 km²) shares a geomorphic setting and land use history common to the southern Appalachians. Logging and conversion of woodlands to pasture in the early 1900s led to extensive soil erosion through the mid-1900s; selective logging also occurred around 1980. Currently, several studies are examining the Gribble Gap Basin to determine 1) what influence past land uses had on geomorphic and hydrologic processes, and 2) how past land uses influence geomorphic and hydrologic processes today. The purpose of this study was to evaluate the slope and drainage area threshold for gully initiation in Gribble Gap. Large gullies are still present in the landscape today but were mapped in the watershed as early as 1938.

In the fall of 2015, we began investigating soil erosion due to land use as part of a Geomorphology course. This work indicated a loss of an A horizon on steep slopes and a tendency for gullies to form in sets. To address whether there was a topographic threshold for gully formation, gullies were mapped with a GPS. We examined the cross-sectional geometry at the gully head and along the channel reach. The program Rivertools^TM was used to measure contributing area and slope at the gully head from a 1/9 arc-second DEM.

Initial results indicate the relationship between slope and drainage area for gully initiation is weak. Gullies downslope from a paved road have a smaller slope and drainage area threshold than the gullies located downslope from a hiking trail. Pavement or impermeable soil can inhibit infiltration, influence the concentration of storm runoff to the initiation point, and may change the position of gully initiation. Gully head geometry varies and includes U-shape, V-shape and square-shape headwalls. The gully with the largest initiation slope (1.09 m/m) is square-shaped, and the gully with the smallest initiation slope (0.128 m/m) is V-shaped. Future work will include mapping more gullies to constrain the slope-area relationship. An expanded gully survey will help determine how topographic and hydraulic conditions control the amount of erosion in this disturbed landscape.