Masters Theses

Date of Award

8-2020

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

John S. Schwartz

Committee Members

Jon M. Hathaway, Daniel C. Yoder

Abstract

Stream bank erosion rates are commonly modeled using the excess shear stress equation εr = kd(τ-τc)a, where the exponent a is assumed to be unity. The coefficient of erodibility kd, and the critical shear stress τc [Tau c] are properties of the bank soil and the applied shear stress τ [Tau] is a function of the channel geometry and discharge. This equation appears to overstimate bank erosion likely due to effects of channel morphology and bank vegetation, among other factors. The objective of this study was to determine whether the excess shear stress equation could be parameterized with an α [alpha] coefficient to more accurately estimate stream bank erosion accounting for the effects of channel curvature and bank vegetation. The study included seven stream bank sites at Beaver, Bullrun, and Stock creeks located in Knox County, Tennessee with USGS gaging stations located downstream. Bank erosion pins were placed at four channel morphology/vegetation categories to measure retreat or soil deposition; they were straight and curved channels with and without woody bank vegetation. At each channel cross-section, 18-inch steel pins were installed vertically along the lower, middle, and upper portion of the banks. A mini-jet test device was used to approximate kd and the τc using the Blaisdell method (BM). Flows and stage heights were modeled with HEC-RAS to determine the τ at each pin. Pin erosion averages varied by category and bank position ranging from -0.03 to -0.78 feet during the one-year study period and were used as estimates of site erosion rates εr [Epsilon r]. With all the parameters being known alpha parameters in the modified excess shear stress equation were solved for each of the erosion pins for the different channel categories. The alpha values typically varied between about 0.001 and 0.73 showing that excess shear stress equation over predicts erosion. Due to the wide range of alpha values, further research is needed to better assess the factors that influence bank erosion and improve the predictability of bank retreat rates.

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