Year of Award

2015

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Geosciences

Other Degree Name/Area of Focus

Fluvial Geomorphology

Department or School/College

Department of Geosciences

Committee Chair

Andrew Wilcox

Commitee Members

Rebecca Bendick, Kelsey Jencso, Johnnie Moore, John Stella

Abstract

Alluvial rivers have morphologies that are shaped to varying degrees by the character of the riparian vegetation they support. Floodplain vegetation produces bank cohesion, for example, which in turn is responsible for inducing river meandering that gives rise to in-channel bars suitable for pioneer vegetation recruitment. Once established, pioneer vegetation is inundated by channel-forming flows, where it interacts with flow and sediment transport processes. This dissertation quantifies interactions between in-channel pioneer vegetation, which is under-studied relative to floodplain vegetation, and river processes across spatial scales.

At the seedling scale, I link field experiments measuring woody riparian seedling uprooting forces to numerical calculations of flow forces. Seedling uprooting sets the trajectory of vegetation-river interactions that may ensue if vegetation survives, becomes established and alters river morphodynamics at the patch, bar, and reach scales. I constrain the differential controls on seedlings’ resisting force, and show that substantial bed scour is required to uproot seedlings. These constraints on seedling uprooting conditions inform management strategies aimed at increasing or decreasing riparian species.

I characterize relationships among topographic features created by vegetation patches on river bars and vegetation morphometric parameters. I show that flume-based hydraulic relationships poorly predict field observations. I also demonstrate that the signature of vegetation alters reach-scale morphology. This analysis, combined with one that characterizes the wavelengths of in-channel river topography, shows that vegetation and the topographic features it creates within a channel have a large influence on the distribution of shear stresses compared to other roughness features.

Lastly, using a high-resolution hydrodynamic model that accounts for vegetation drag, I simulate the impact of vegetation succession on channel-bend and meander processes by changing the size and density of vegetation on an in-channel bar. A global sensitivity analysis shows that vegetation parameters are nearly as influential as channel characteristics in altering bend hydraulics. For a river reach, simulations show that a vegetated bar changes channel hydraulics and forces in a manner that would be expected to alter channel evolution, and explains qualitative observations of vegetation-mediated river morphologies. This research thus quantifies under which conditions pioneer seedlings can persist and alter channel topography, with implications for changing the morphology of rivers.

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© Copyright 2015 Sharon Verdawn Bywater-Reyes