Impacts of Vegetation Growth on Reach-scale Flood Hydraulics in a Sand-bed River and the Implications for Vegetation-morphology Coevolution

Author

Samuel Box, University of Montana, Missoula

Year of Award

2018

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Dr. Andrew C. Wilcox

Commitee Members

Dr. W. Payton Gardner, Dr. H. Maurice Valett

Subject Categories

Geomorphology

Abstract

Vegetation affects river morphology via plant effects on the flow field, sediment transport and deposition, and substrate erodibility. Changes in the flow field caused by above-ground biomass mediate geomorphic response to flooding and habitat suitability for vegetation growth. Using numerical hydraulic models that incorporate drag due to spatially heterogeneous vegetation, I quantify how changes in the antecedent characteristics of woody vegetation in the active channel lead to changes in local bed shear stress, reach-average bed shear stress, and the vegetative component of reach-average total shear stress. Flood hydraulics in turn affect channel morphology and vegetation, providing a feedback mechanism by which changes in vegetation can drive channel morphology into a new steady state. Two reaches of a sand-bed river that have experienced extensive vegetation establishment and growth, geomorphic change, and associated responses to flooding in recent years were modeled at three different points in time, representative of differing strengths and directions of biogeomorphic feedbacks. The presence of woody vegetation increased hydraulic variability, increasing the shear stress coefficient of variation between 8-30%, depending on both discharge and vegetation density and extent. Vegetation establishment and growth increased the proportion of reach-average total shear stress accounted for by vegetation, with an effect that became greater at higher flows. As flow blockage due to vegetation increased, the bed shear stress decreased by over 50%. Vegetation alters reach-scale flood hydraulics in a manner that would not be predictable using either a one-dimensional hydraulic model or a two-dimensional hydraulic model without spatially heterogeneous vegetation. These findings indicate that, at a reach scale, vegetation can drive changes in hydraulic behavior, with effects on shear stress partitioning that are similar to those observed in flumes and with large woody debris.

Recommended Citation

Box, Samuel, "Impacts of Vegetation Growth on Reach-scale Flood Hydraulics in a Sand-bed River and the Implications for Vegetation-morphology Coevolution" (2018). Graduate Student Theses, Dissertations, & Professional Papers. 11144.
https://scholarworks.umt.edu/etd/11144