MODELING THE IMPACTS OF STREAM INCISION ON WATER LEVELS IN A SOUTHERN APPALACHIAN FEN

Phinneas Fen is a 9.3-hectare (23-acre) mountain wetland located in Cherokee National Forest (Eastern Tennessee, USA). Previous stormwater concentration caused a headcut that has since migrated upstream into the fen. This project was designed to better understand the effects of the incised stream channel on water levels in the surrounding wetland. Field activities included installing a monitoring-well transect perpendicular to the stream channel; stream gauging; and infiltration, slug, and permeameter tests. Field data indicated a two-layer system, with a 2m thick clayey unit (K=0.0049m/d) above a more permeable sandy unit (K=1.1m/d). The stream channel width is about 0.5-m and baseflow is typically around 3.4E-3 m3/s (0.12 cfs). These data were then used to construct and calibrate a two-dimensional (cross-sectional) groundwater flow model using MODFLOW and Groundwater Vistas. Parameters including hydraulic conductivity and drain dimensions were then varied to estimate the relative sensitivity of each. For example, a deeper incision depth allowed more water to leave the groundwater system, further drying out the wetland. This was particularly apparent when the drain stage dropped from the upper clayey unit into the deeper higher-K unit.