Date Thesis Awarded

4-2014

Document Type

Honors Thesis

Degree Name

Bachelors of Science (BS)

Department

Geology

Advisor

Gregory S. Hancock

Committee Member

James M. Kaste

Committee Member

Randolph M. Chambers

Abstract

Erosion processes in bedrock-floored river channels play an important and well-documented role in shaping channel cross-sectional geometry as well as the broader landscape. However, the influence of weathering on channel slope and geometry is not well understood. Subaerial weathering can produce variability in rock resistance to erosion within channel cross-sections. Recent numerical modeling results suggest that weathering in some settings may preferentially weaken rock in the channel banks relative to the thalweg. We hypothesize that average cross-section erosion rate is a first-order control on the relative effectiveness of weathering between the banks and the thalweg. Erosion rate, in turn, is influenced by the effectiveness of weathering processes at increasing bedrock erodibility. We test this hypothesis on tributaries entering below the migratory Great Falls knickzone on the Potomac River, Virginia. Erosion rates along tributary profiles vary from well below 0.5 m/ky to over 0.8 m/ky as knickpoints spawned by Great Falls migrate upstream.

We used a Type N SilverSchmidt hammer to measure rock compressive strength at multiple flow heights above the thalweg from sites below, within, and above the migratory knickpoints. At each cross-section, strength was measured as close to the thalweg as possible, immediately below the soil-bedrock interface, and at several heights in between. 60 Schmidt hammer measurements were taken at each height, 20 from each of three 0.06 m2 grids aligned parallel to flow at similar elevation. We used a Fowler contour gage combined with automated photograph analysis to quantify rock surface roughness. At each sample height, six roughness profiles were recorded. These procedures were completed at nine channel cross-sections measured on three tributaries to the Potomac.

Compressive strength values for in-channel bedrock ranged from 18.2 to 69 Q-values. All nine cross-sections measured showed significant decreases in compressive strength with height above the thalweg. Six of nine cross-sections showed significant increases in surface roughness with height. Differences in compressive strength and surface roughness between the thalweg and the top of the channel banks tend to be greater in cross-sections with a low erosion rate, and smaller at high erosion rate cross-sections. Our results indicate that rock erodibility in bedrock channels varies inversely with bedrock inundation frequency, and that the observed variability may be caused by weathering. This work confirms first-order predictions of weathering-inclusive cross-section evolution models. Though our results suggest that variations in erodibility may scale with erosion rate, further work is needed to rigorously test this prediction.

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