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Streambank Erosion: The Impact of Streambank Geomorphic Characteristics on Root-Derived Erosion Rates 

Emily D. Brown
Freese and Nichols, Inc./North Carolina State University
Winston-Salem, NC

Increased development across the Southeastern United States coupled with an increase in frequency and intensity of storm events over the past few years has led to an uptick in streambank erosion problems that has been noticed by both citizens and local governments. Many stormwater professionals are finding themselves needing answers about the rate and magnitude of streambank erosion so that they can effectively communicate with their citizens and government officials.  However, to date the erosion rates of streambanks have not been well defined across the Southeast, leading to an inability to provide accurate estimates of erosion risk to infrastructure and downstream systems.   The objective of this presentation is to discuss erosion rate data that has been collected over the past five years across the Southeast using analysis of exposed tree roots (dendrogeomorphology). Using this dataset, we examine trends and patterns in streambank geomorphic characteristics that impact the rates of streambank erosion in the Piedmont ecoregion.  Evaluated streambank geomorphic characteristics included critical shear stress, erodibility coefficient, bulk density, median grain size (D50), porosity, coefficient of curvature, coefficient of uniformity, percent sand, percent silt, percent clay, and the percent of material passing the #200 sieve. When compared to measured data, results suggest that streambank erosion rates estimated using root dendrogeomorphology tend to have less variation for streambank composed of sandy clay loam and sandy loam as compared to banks with loamy sand soils. Additionally, banks characterized with sandy loam soils show less variation in root-derived erosion rates as bulk density increases. Banks composed of loamy sand, on the other hand, have increased variation as bulk density increases. Similar trends are observed when looking at the percent passing #200 sieve. Finally, the bank erosion hazard index (BEHI) was determined for each of the sites, and as the erodibility of the stream banks increased, the variation in root-based erosion rates when compared to measured data also increased across all soil types analyzed.