Prepared in cooperation with the Nuclear Regulatory Commission Improving Flood-Frequency Analysis with a 4,000-Year Record of Flooding on the Tennessee River near Chattanooga, Tennessee Scientific Investigations Report 2020–5138 U.S. Department of the Interior U.S. Geological Survey Cover. View looking upstream near river mile 445, Tennessee River Gorge of the Tennessee River, Tennessee. Photograph by Tessa Harden, U.S. Geological Survey, March 22, 2016. Improving Flood-Frequency Analysis with a 4,000-Year Record of Flooding on the Tennessee River near Chattanooga, Tennessee By Tessa M. Harden, Jim E. O’Connor, Meredith L. Carr, and Mackenzie Keith Prepared in cooperation with the Nuclear Regulatory Commission Scientific Investigations Report 2020–5138 U.S. Department of the Interior U.S. Geological Survey U.S. Geological Survey, Reston, Virginia: 2021 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Harden, T.M., O’Connor, J.E., Carr, M.L., and Keith, M., 2021, Improving flood-frequency analysis with a 4,000-year record of flooding on the Tennessee River near Chattanooga, Tennessee: U.S. Geological Survey Scientific Investigations Report 2020–5138, 64 p., https://doi.org/ 10.3133/ sir20205138. Associated data for this publication: Harden, T.M., 2021, Hydraulic modeling and flood-frequency analyses using paleoflood hydrology for the Tennessee River near Chattanooga, Tennessee: U.S. Geological Survey data release, https://doi.org/10.5066/P914SLVM. ISSN 2328-0328 (online) iii Acknowledgments Thomas Aird of the Nuclear Regulatory Commission contributed to the initial stages of the hydraulic modeling effort. v Contents Acknowledgments ........................................................................................................................................iii Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Paleoflood Hydrology ������������������������������������������������������������������������������������������������������������������������������������3 Description of Study Area ............................................................................................................................4 Tennessee River ������������������������������������������������������������������������������������������������������������������������������������4 Hydroclimate.................................................................................................................................4 Tennessee River Gorge ........................................................................................................................4 Historical and Modern Streamflow Record .....................................................................................6 Methods of Investigation ..............................................................................................................................7 Stratigraphic Analysis ..........................................................................................................................8 Geochronology ��������������������������������������������������������������������������������������������������������������������������������������9 Hydraulic Modeling ............................................................................................................................10 Compiling the Overall Flood Chronology .........................................................................................13 Flood-Frequency Analysis .................................................................................................................13 Stratigraphic Analysis and Site Paleoflood Chronologies ....................................................................14 Site Descriptions, Stratigraphy, and Geochronology ...................................................................14 Jeff-n-Steph ................................................................................................................................14 Red Flower ..................................................................................................................................17 Congo ...........................................................................................................................................21 Broken Stem ...............................................................................................................................21 Last Survey..................................................................................................................................21 TBM4 ............................................................................................................................................23 Overall Paleoflood Chronology—Number, Magnitude, and Timing ....................................................28 Flood-Frequency Analysis ��������������������������������������������������������������������������������������������������������������������������31 Summary........................................................................................................................................................40 References Cited..........................................................................................................................................41 Appendix 1. Stratigraphic Descriptions of Supporting Paleoflood Sites ........................................46 Figures 1. Map of the Tennessee River Basin showing the study reach and three nuclear power plants along the Tennessee River, southeastern United States ...............................3 2. Map showing the study area and paleoflood study sites just downstream from Chattanooga and within the Tennessee River Gorge, Tennessee ........................................5 3. Photograph showing the sandstone cliffs of the Tennessee River Gorge, Tennessee ��������������������������������������������������������������������������������������������������������������������������������������5 4. Photograph showing large colluvial boulders common to the hillslope in the Tennessee River Gorge, Tennessee ..........................................................................................6 5. Photograph showing the Tennessee River Gorge occupied by the now-impounded Tennessee River (Nickajack Lake), Tennessee .........................................6 6. Photograph showing boulder alcoves where sediments providing evidence of paleofloods are deposited and preserved for hundreds to thousands of years, in the Tennessee River Gorge, Tennessee ...............................................................................8 vi 7. Photograph showing a laminated flood deposit likely from the 1867 historical flood on the Tennessee River, Tennessee ................................................................................9 8. Schematic showing main reach and sub-reaches of the hydraulic model used in this study of the Tennessee River Basin, Tennessee .......................................................12 9. Photographs showing large boulder creating the rock overhang that helps to preserve the site and stratigraphy of the pit, at the Jeff-n-Steph site, Tennessee River, Tennessee .........................................................................................................................14 10. Diagrams showing location of paleoflood sites in study relative to elevation and surface water profiles from the 1867 historical flood as well as other large floods in the gaged record, Tennessee River Basin, Tennessee ........................................15 11. Schematic diagram showing the four flood deposits, intervening angular gravel clasts, and deposit of local material at the base of the pit, at the Jeff-n-Steph site, Tennessee River Gorge, Tennessee ................................................................................16 12. Photographs showing location of the pit adjacent to a large sandstone boulder and stratigraphy of the pit that includes one paleoflood deposit between deposits of local material, at the Red Flower site, Tennessee River, Tennessee ............20 13. Schematic diagram showing one flood deposit between deposits of local material, at the Red Flower site, Tennessee River, Tennessee �������������������������������������������20 14. Photographs showing location of the pit and stratigraphy of the pit including evidence of two floods, at the Congo site, Tennessee River, Tennessee ..........................22 15. Schematic diagram showing exposed stratigraphy at the Congo site, Tennessee River, Tennessee .....................................................................................................22