Masters Theses

Date of Award

5-2008

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

John S. Schwartz

Committee Members

Larry D. McKay, Randall W. Gentry

Abstract

In order to accurately predict the stability of riverbanks, model input parameters must be reliable bank failure estimators. Currently, bank stability models require two input parameters to predict bank erosion: critical erosion shear stress and erodibility coefficient. The investigation’s purpose was to compare two erosion estimation methods and improve the bank stability models for cohesive soil commonly found on the banks.

To accomplish the objective, critical shear stresses and erodibility coefficients obtained using the in situ submerged jet test device (SJT) were measured against results from the closed-loop laboratory flume method for 12 cohesive bank sites. Additionally, SJT critical shear stress values were compared to values found via empirical relationships found in literature that incorporate plasticity index, median particle diameter, percent siltclay or percent clay content to compute critical shear stress. Particle size analysis and Atterberg limit determinations were run classify the sediment type collected.

The critical shear stress values obtained ranged from 0.09 to 5.84 Pa and SJT erodibility coefficients varied from 0.37 to 10.07 cm3/N·s. From flume observations, cohesive soil erosion was influenced by interparticle forces and occurred in aggregate pieces and particle-by-particle. A few critical shear stress values appeared to be unreliable considering the critical shear stress threshold of 1.83 Pa found using the laboratory flume analysis and the limited erosion witnessed. Study results also indicated that sediment properties did not correlate directly with the SJT critical shear stress values or with each other.

Flume observations and variations among experimental results suggest other influential factors exist besides critical shear stress and the erodibility coefficient when quantifying the cohesive sediment erosivity. When empirical results were lower than the flume’s critical shear stress threshold, it was possible the mechanical soil property could not be transferred to the soil types tested or estimates incorrectly assumed zero physical and chemical influences. Because of its complexities, traditional experimental design may not reliably measure cohesive soil erosion. Only through the continued collaboration of various field and advanced degree professionals and the detailed, high-quality documentation of as many influential parameters as possible per project can the goal of estimating cohesive sediment erosion be accomplished.

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