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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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Volume 6, issue 2 | Copyright
Earth Surf. Dynam., 6, 487-503, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Jun 2018

Research article | 14 Jun 2018

The influence of a vegetated bar on channel-bend flow dynamics

Sharon Bywater-Reyes1,2, Rebecca M. Diehl1, and Andrew C. Wilcox1 Sharon Bywater-Reyes et al.
  • 1Department of Geosciences, University of Montana, 32 Campus Drive no. 1296, Missoula, MT 59812, USA
  • 2Department of Earth and Atmospheric Sciences, University of Northern Colorado, 501 20th St., Greeley, CO 80639, USA

Abstract. Point bars influence hydraulics, morphodynamics, and channel geometry in alluvial rivers. Woody riparian vegetation often establishes on point bars and may cause changes in channel-bend hydraulics as a function of vegetation density, morphology, and flow conditions. We used a two-dimensional hydraulic model that accounts for vegetation drag to predict how channel-bend hydraulics are affected by vegetation recruitment on a point bar in a gravel-bed river (Bitterroot River, Montana, United States). The calibrated model shows steep changes in flow hydraulics with vegetation compared to bare-bar conditions for flows greater than bankfull up to a 10-year flow (Q10), with limited additional changes thereafter. Vegetation morphology effects on hydraulics were more pronounced for sparse vegetation compared to dense vegetation. The main effects were (1) reduced flow velocities upstream of the bar, (2) flow steered away from the vegetation patch with up to a 30% increase in thalweg velocity, and (3) a shift of the high-velocity core of flow toward the cut bank, creating a large cross-stream gradient in streamwise velocity. These modeled results are consistent with a feedback in channels whereby vegetation on point bars steers flow towards the opposite bank, potentially increasing bank erosion at the mid- and downstream ends of the bend while simultaneously increasing rates of bar accretion.

Publications Copernicus
Short summary
Channel bends and bars make up an important physical and ecological unit within rivers. Many riparian vegetation species need river bars for their life cycle, colonizing bars after flooding events. Once vegetation is established on bars, they can alter the flow and patterns of erosion and deposition. We used a hydraulic model to quantify the impact such riparian vegetation of various sizes and densities has on flow dynamics and inferred the expected changes in river erosion and deposition.
Channel bends and bars make up an important physical and ecological unit within rivers. Many...