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A Paleoflood Assessment of the Greenbrier River in Southeast, West Virginia, USA

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A Paleoflood Assessment of the Greenbrier River in Southeast, West Virginia, USA A Paleoflood Assessment of the Greenbrier River in Southeast, West Virginia, USA A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Master of Science Sara A. Thurkettle May 2019 © 2019 Sara A. Thurkettle. All Rights Reserved. 2 This thesis titled A Paleoflood Assessment of the Greenbrier River in Southeast, West Virginia, USA by SARA A. THURKETTLE has been approved for the Department of Geological Sciences and the College of Arts and Sciences by Gregory S. Springer Associate Professor of Geological Sciences Joseph Shields Interim Dean, College of Arts and Sciences 3 ABSTRACT THURKETTLE, SARA A., M.S., May 2019, Geological Sciences A Paleoflood Assessment of the Greenbrier River in Southeast, West Virginia, USA Director of Thesis: Gregory S. Springer Slackwater deposits and hydraulic modeling were used to extend the historical record of floods and determine the critical threshold of sediment entrainment in the Greenbrier River of southeastern West Virginia (SE WV). The southward flowing bedrock river incises Paleozoic sandstones, limestones, and shales. The river has experienced three catastrophic floods since 1985: the 1985, 1996, and 2016 floods, which caused extensive damage in communities alongside the river with the most recent flood destroying roughly 1,200 homes in the Greenbrier River watershed. This study better constrains the frequency of floods that have the potential to cause similar damage, which is a matter of urgent need. Paleostage indicators (PSIs) found in Greenbrier River Cave were used as proxies for extending the historical record and reconstructing recurrence intervals of floods. Radiocarbon dating was used to determine ages of pre-historic slackwater deposits (floods) in the cave. Wolman counts were performed in the river channel to determine stream competence and sediment transport thresholds. Known discharges and recoverable paleostages were used to calibrate channel roughness in a 1- dimensional modeling program, HEC-RAS. Channel roughness (Manning’s n) during large floods was determined to be 0.029. Discharges and clast entrainment velocities were calculated using a Shield’s critical shear stress equation and clast size data, then compared against velocities determined in HEC-RAS. HEC-RAS was used to back- 4 calculate discharges for the pre-historic flood deposits which allowed for the 100-year flood frequency to be re-evaluated. Recurrence intervals were assigned to discharges, calculated from clast size data and critical shear stress equation, using the existing flood frequency analysis. The reliability of using paleostage indicators and HEC-RAS to create flood chronologies is discussed. 5 ACKNOWLEDGMENTS I would like to acknowledge the Ohio University Geological Sciences Alumni Association for providing funding for my thesis research through their research grant and summer fellowship. I would also like to acknowledge my field assistant that helped with the collection of field data: Andrew Alder. 6 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Acknowledgments............................................................................................................... 5 List of Tables ...................................................................................................................... 8 List of Figures ..................................................................................................................... 9 Chapter 1: Introduction ..................................................................................................... 10 Chapter 2: Previous Research ........................................................................................... 12 History of Paleoflood Hydrology................................................................................ 12 Hydraulic Models: Uses in Paleoflood Research........................................................ 14 Chapter 3: Study Area ....................................................................................................... 17 Historical Flooding of West Virginia ......................................................................... 17 Greenbrier River ......................................................................................................... 20 Regional Climate ........................................................................................................ 26 Chapter 4: Objectives ........................................................................................................ 28 Chapter 5: Methods ........................................................................................................... 29 Field Techniques ......................................................................................................... 29 Data Processing ........................................................................................................... 35 Hydraulic Modeling .................................................................................................... 35 Estimating Critical q Values and Sediment Transport Velocities .............................. 38 Flood Frequency Analysis .......................................................................................... 39 Chapter 6: Results ............................................................................................................. 41 Grain Size Analysis..................................................................................................... 41 Channel Roughness and Paleodischarge Estimates .................................................... 43 Silt Line and Slackwater Deposits .............................................................................. 50 Flow Competence Analysis ........................................................................................ 51 Flood Frequency Analysis .......................................................................................... 52 Chapter 7: Discussion ....................................................................................................... 55 Confidence in Channel Roughness Estimate .............................................................. 55 Slackwater Deposits and Silt Lines: How comparable are results obtained from them? ..................................................................................................................................... 59 Reliability of the 1996 and 2016 Flood Deposits ....................................................... 60 7 Sediment Transport and Stream Competence ............................................................. 62 Modeled Velocities versus Clast Size-derived Velocities .......................................... 64 Discharges from Clast Size Analysis versus Previous FFA ....................................... 66 RI’s of 1985, 1996, and 2016 Based upon 2010 FFA ................................................ 68 Paleoflood Recurrence Intervals ................................................................................. 68 Reconstructed Flood Frequency Analysis .................................................................. 70 Chapter 8: Conclusions ..................................................................................................... 73 References ......................................................................................................................... 75 Appendix A: Cross Section Data for HEC-RAS .............................................................. 81 Appendix B: Frequency Curve Data for HEC-SSP .......................................................... 93 Appendix C: Wolman Count Data .................................................................................... 94 8 LIST OF TABLES Page Table 1 Summary of selected historical floods in West Virginia from 1877 to 2016. .... 18 Table 2 A summary of each cross section location.......................................................... 33 Table 3 A list of Manning’s n (channel roughness) values used in the paleoflood literature. ........................................................................................................................... 37 Table 4 A summary of clast sizes measured in the field as well as their equivalent sorting classification. .................................................................................................................... 43 Table 5 Calculated water surface elevations are presented in columns 1 and 2 correspond to the 1996 and 2016 floods. ............................................................................................. 44 Table 6 A summary of discharges (Q) calculated using various channel and bank roughnesses (n). ................................................................................................................ 48 Table 7 A summary of discharges calculated using SWD’s from the 1996 and 2016 floods................................................................................................................................. 50 Table 8 A summary of the critical discharges (qc) calculated and parameters used to calculate the values. .......................................................................................................... 51 Table 9 A summary of discharges calculated from qc values and channel width. ........... 51 Table 10 A summary
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