PALEOFLOODS IN THE TENNESSEE RIVER BASIN: RECORDS OF MEGAFLOODS USING TRACTIVE-LOAD DEPOSITS

Paleoflood investigations rely on direct sedimentary evidence of floods preserved on floodplains and terraces, in caves and rock shelters, as well as indirect evidence preserved in tree rings and speleothems to estimate the flood stage, discharge, and frequency of past extreme floods. Large tractive-load boulders commonly occur throughout the Blue Ridge, Cumberland Plateau, and Ridge and Valley provinces of the Tennessee basin, can be easily located and measured, and thus could provide a comprehensive regional database to significantly aid estimates of non-exceedance thresholds for any given river system. The overarching objective of this study is to demonstrate the existence of paleoflood indicators of extreme floods in the Tennessee River basin that can be used to estimate peak stage and discharge from tractive-load boulders in bedrock gorges.

Field sites were located using available aerial photographs, USGS maps and digital elevation models, including LiDAR. Tractive-load boulders in bedrock gorges were measured at three sites on the Tuckasegee and Pigeon Rivers. We compiled and processed remote sensing datasets (e.g., LiDAR, DEMs) using ENVI and ArcGIS. Datasets were acquired from the USGS National Map at various resolutions (LiDAR: 3m, point cloud; DEM: 3m) to corroborate field-derived estimates of slope, construct cross-sections, measure channel parameters, and estimate paleoflood stage and discharge. Flow velocities and flow depths of floods were calculated using the adjusted Manning’s equation, unit stream power, and adjusted Shield’s function.

Our key findings and implications from examination of paleofloods in the Tennessee River basin include the following: Tractive-load boulders indicate floods much larger than anything within historical records (e.g., Pigeon River: ~12m depth vs. 5.5m at Hepco USGS gage) and they inform and refine non-exceedance flood thresholds. Sophisticated 1D and 2D computer modeling would greatly improve estimates of extreme floods derived from tractive-load boulders. LiDAR data is a valuable tool for targeting tractive-load deposits within the Tennessee River Basin, estimating slope, and obtaining cross-sectional areas and important hydraulic parameters, and future efforts should focus on rivers that already have LiDAR data.