Abstract
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.
Highlights
Channel-bend morphodynamics along meandering rivers influence channel morphology, river migration rates, channel– floodplain connectivity, and aquatic habitat
Plants change local hydraulics (Nepf, 2012; Rominger et al, 2010) and sediment transport conditions (Curran and Hession, 2013; Manners et al, 2015; Yager and Schmeeckle, 2013), resulting in strong feedbacks between the recruitment and growth of woody riparian vegetation and bar building (Bendix and Hupp, 2000; Dean and Schmidt, 2011) that can influence the morphology of rivers at multiple scales (Bywater-Reyes et al, 2017; Osterkamp et al, 2012)
Our objectives are to (1) determine which vegetation morphology and flow conditions result in the greatest changes to channel-bend hydraulics, and (2) infer how these changes in hydraulics would impact channel-bend morphodynamics and evolution
Summary
Channel-bend morphodynamics along meandering rivers influence channel morphology, river migration rates, channel– floodplain connectivity, and aquatic habitat. Plants change local hydraulics (Nepf, 2012; Rominger et al, 2010) and sediment transport conditions (Curran and Hession, 2013; Manners et al, 2015; Yager and Schmeeckle, 2013), resulting in strong feedbacks between the recruitment and growth of woody riparian vegetation and bar building (Bendix and Hupp, 2000; Dean and Schmidt, 2011) that can influence the morphology of rivers at multiple scales (Bywater-Reyes et al, 2017; Osterkamp et al, 2012). Pioneer vegetation can occur on all bar types but is most likely to survive on nonmigrating bars, such as forced alternating point bars (Wintenberger et al, 2015)
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