Bankfull Hydraulic Geometry of Streams Draining the Southwestern Appalachians of Tennessee

The purpose of this study was to examine the bankfull recurrence interval for streams draining the Southwestern Appalachians Level III Ecoregion 68 of Tennessee, develop bankfull discharge and hydraulic geometry relationships for streams within the ecoregion and compare those relationships to the Ridge and Valley of Virginia, West Virginia, and Maryland (Keaton et al., 2005) and the Piedmont and Blue Ridge of North Carolina (Harman et al., 1999; Harman et al., 2000). For this investigation, a repeatable, systematic process was developed to locate bankfull stage within the Southwestern Appalachians during the spring and summer of 2005. The intent was to develop regional curves of empirically derived hydraulic relationships for this ecoregion, but first it was necessary to correctly identify bankfull stage in the sample streams. Bankfull discharge was defined as the effective discharge or channel-forming flow. Stream surveys were conducted on 11 study reaches (7 had USGS gages for calibration of bankfull) of various sized drainages across the ecoregion. Recurrence intervals were calculated using log Person Type III flood frequency analysis. Results demonstrated an average bankfull recurrence interval of 1.31 years for the Southwestern Appalachians, which was comparable to other nearby physiographic regions.
Regional curves illustrate hydraulic and geomorphic relationships such as discharge versus watershed area, channel width versus channel cross sectional area and many more such relationships. The principal benefits from regional curves are their assistance in validating channel dimensions, pattern and profile for stream restoration designs. The marked variance in geology, climate, topography, and watershed land-uses across physiographic provinces drives the need for developing regional curves for each specific physiographic province. Stream restoration designs in Tennessee rely on curves from other nearby physiographic regions. A comparison of the Southwestern Appalachians regional curves developed in this study to the Ridge and Valley and the Piedmont and Blue Ridge reveals distinctly different relationships. In the Southwestern Appalachians, bankfull discharge and associated cross sectional area were found to be of much greater magnitude than streams in the other two regions.