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
(
Channel-bend morphodynamics along meandering rivers influence channel morphology, river migration rates, channel–floodplain connectivity, and aquatic habitat. River point bars, fundamental to channel-bend morphology (Blondeaux and Seminara, 1985; Ikeda et al., 1981), steer flow and induce convective accelerations (Dietrich and Smith, 1983) that influence boundary shear stress (Dietrich and Whiting, 1989) and sediment transport fields (Dietrich and Smith, 1983; Legleiter et al., 2011; Nelson and Smith, 1989). Channel migration rates are furthermore controlled by the collective processes of bar accretion and bank erosion. Bars along the inner bends of river meanders, although typically broadly described as point bars, also comprise chute bars, tail bars, and scroll bars that reflect distinct formative conditions (e.g., obstructions and/or stream power variations) and produce distinct morphodynamic feedbacks (Kleinhans and van den Berg, 2011).
Channel dynamics are tightly coupled with the recruitment and succession of riparian vegetation on river bars (Amlin and Rood, 2002; Eke et al., 2014; Karrenberg et al., 2002; Nicholas et al., 2013; Rood et al., 1998). 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). Plant traits including height, frontal area, and stem flexibility vary with elevation above the baseflow channel, influencing both the susceptibility of plants to uprooting during floods and their impact on morphodynamics (Bywater-Reyes et al., 2015, 2017; Diehl et al., 2017a; Kui et al., 2014). Vegetation effects on hydraulics, bank erosion, and channel pattern also depend on the uniformity of vegetation distribution on bars, which can vary depending on wind versus water-based dispersal mechanisms (Van Dijk et al., 2013), and on whether plants occur individually or in patches (Manners et al., 2015).
Experimental work in flumes has shown that vegetation is vital to sustaining meandering in coarse-bedded rivers (Braudrick et al., 2009). Vegetation's effect on stabilizing banks, steering flow, and impacting morphodynamics furthermore depends on seed density and stand age. Uniform vegetation on bars has been shown, experimentally, to decrease bank erosion rates, stabilize banks, and increase sinuosity of meander bends (Van Dijk et al., 2013). Gran and Paola (2001) showed that vegetation, by increasing bank strength, generates secondary currents associated with oblique bank impingement that may be more important than helical flows generated by channel curvature. Other experiments have generally suggested vegetated bars decrease velocities over the bar and push flow toward the outer bank. For example, tests in a constructed, meandering laboratory stream with two reed species planted on a sandy point bar showed that vegetation reduced velocities over the vegetated bar, increased them in the thalweg, strengthened secondary circulation, and directed secondary flow toward the outer bank (Rominger et al., 2010). Another study in the same experimental facility, but using woody seedlings planted on the point bar, also found reduced velocities in the vegetated area of the bar, with the greatest reductions at the upstream end, and the effect varying with vegetation architecture and density (Lightbody et al., 2012). In a flume study where meandering effects were simulated in a straight channel by placing dowels representing vegetation patches in alternating locations along the edges of the flume, vegetation reduced velocity within and at the edges of the vegetation patch and increased velocities near the opposite bank (Bennett et al., 2002). Experiments in a high-curvature meandering flume, in contrast, showed that vegetation inhibited high shear-stress values from reaching the outer bank (Termini, 2016), inconsistent with studies simulating moderate sinuosity channels.
Vegetation's effect on river morphodynamics has also been simulated with computational models. Reduced-complexity models that approximate the physics of flow have successfully reproduced many of the features observed in channels influenced by vegetation, such as the development of a single-thread channel (e.g., Murray and Paola, 2003). Two-dimensional models that use shallow-water equations and, in some cases, sediment transport relations, provide an alternative that may be less dependent on initial conditions and more capable of representing the physics of vegetation–flow interactions (Boothroyd et al., 2016, 2017; Marjoribanks et al., 2017; Nelson et al., 2016; Nicholas et al., 2013; Pasternack, 2011; Tonina and Jorde, 2013). Investigations of channel-bend dynamics influenced by vegetation using two-dimensional models often represent vegetation by increasing bed roughness (see Green, 2005, and Camporeale et al., 2013, for comprehensive reviews). Nicholas et al. (2013) simulated bar and island evolution in large anabranching rivers using a morphodynamic model of sediment transport, bank erosion, and floodplain development on a multi-century timescale, where vegetation was modeled using a Chezy roughness coefficient. Asahi et al. (2013) and Eke et al. (2014) modeled river bend erosional and depositional processes that included a bank-stability model and deposition dictated by an assumed vegetation encroachment rule. Bertoldi and Siviglia (2014) used a morphodynamic model coupled with a vegetation biomass model, which accounted for species variations in nutrient and water needs to simulate the co-evolution of vegetation and bars in gravel-bed rivers. Vegetation was modeled as increased bed roughness via the Strickler–Manning relation that varied linearly with biomass. Their model showed two scenarios: one where flooding completely removed vegetation, and one where vegetation survived floods, resulting in vegetated bars. These two alternative stable states (bare versus vegetated bars) have been found experimentally as well (Wang et al., 2016).
Bitterroot River, Montana, showing model domain, location of acoustic Doppler current profiler (ADCP) velocity measurement cross sections, downstream boundary, tree crowns mapped from airborne lidar, and the location of the vegetated bar. Inset map shows location in the northwestern US.
Although the aforementioned models produce many of the features of river
morphodynamic evolution, when vegetation drag is dominant over bed friction,
using conventional resistance equations (e.g., Manning's) to model
vegetation's effect on the flow introduces error. Increasing the roughness
within vegetated zones increases the modeled shear stress and therefore
artificially inflates the sediment transport capacity at the local scale
(e.g., vegetation patch or bar), although reach-scale results may be
appropriate (Baptist et al., 2005; James et al., 2004). Vegetation drag can
also be treated in computational models by representing plants explicitly as
cylinders (e.g., Baptist et al., 2007; Vargas-Luna et al., 2015), comparable
to the approach of many flume studies, or by accounting for drag from
foliage, stems, and streamlined vegetation, but such an approach is currently
not widely adopted because of limited ability to specify all parameters
(e.g., Boothroyd et al., 2015, 2017; Jalonen et al., 2013; Västilä
and Järvelä, 2014). Vargas-Luna et al. (2015) showed, through
coupling of numerical modeling and experimental work, that representing
vegetation as cylinders is most appropriate for dense vegetation. Iwasaki et
al. (2015) used a two-dimensional model that accounted for vegetation drag
(as cylinders) to explain morphological change of the Otofuke River, Japan,
caused by a large flood event in 2011 that produced substantial channel
widening and vegetation-influenced bar building. They found that vegetation
allowed bar-induced meandering to maintain moderate sinuosity, whereas in the
absence of vegetation, river planform would switch from single thread to
braided. Marjoribanks et al. (2017) modeled the effects of vegetation mass
blockage and drag, specifying vegetation as cylinders, for a small
(
As the above review suggests, there have been considerable advances in laboratory and computational modeling of vegetation effects on hydraulics that complement understanding of bar and bend morphodynamics and reciprocal interactions between riparian vegetation and river processes (Corenblit et al., 2007; Gurnell, 2014; Osterkamp and Hupp, 2010; Schnauder and Moggridge, 2009). Challenges persist, however, in representing field-scale complexities in a modeling framework that allows for testing field-scale interactions between plants, flow, and channel morphology on vegetated point bars. Here, we tackle key elements of this problem by investigating how the distribution of woody vegetation on a point bar influences bend hydraulics and flow steering across a range of flood magnitudes using a two-dimensional modeling approach informed by high-resolution topography and vegetation morphology data that spatially define vegetation drag. We model a range of vegetation densities and plant morphologies representing different stages of pioneer woody vegetation growth on a point bar. We vary discharge in the model to represent the stage-dependent effects of vegetation on hydraulics, as well as different flood stages that may be important for the recruitment of plants and the erosion or deposition of sediment within the channel bend. We predict that the presence of woody vegetation affects bar and meander dynamics by steering flow, thereby influencing the morphodynamic evolution of vegetated channels. 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. The insights derived from our analysis are relevant for understanding ecogeomorphic feedbacks in meandering rivers and how such feedbacks are mediated by plant traits and flow conditions, and for riparian plant species management along river corridors.
To meet our objectives, we model a point bar-bend sequence on the Bitterroot
River, southwest Montana, United States (Fig. 1). Our field site has a
pool-riffle morphology and a wandering pattern, with channel bends, point
bars, and woody vegetation on bars and floodplains. The study reach is
located on a private reserve (MPG Ranch) with minimal disturbance to the
channel and floodplain, and flow and sediment supply are relatively unaltered
by flow regulation, because the only significant dam in the contributing
watershed is
Modeled vegetated bar
To characterize the influence of a vegetated bar on channel-bend hydraulics,
we used an edited version of FaSTMECH, a hydrostatic, quasi-steady flow model
contained within iRIC (Nelson et al., 2016;
We created the flow model domain in FaSTMECH by characterizing the topography and flow boundary conditions (discharge and water surface elevation at the downstream boundary) of a study reach on the Bitterroot River, Montana (Fig. 1). We surveyed channel topography with a combination of airborne lidar, echosounder and real-time kinematic (RTK) GPS surveys (see the Supplement). The resulting curvilinear orthogonal grid we created had an average cell size of 2.5 by 2.5 m for calibration runs (described below), and 5 by 5 m for the remaining runs. We linked transducer stage measurements at the downstream end of the study reach to discharge derived from USGS gaging station no. 2344000, Bitterroot River near Darby, Montana, corrected by contributing area for our field site. Water surface elevations at the downstream boundary for modeled discharges were extracted from the stage–discharge relationship. Discharge was measured at the field site and compared to the adjusted USGS 12344000 value and found to agree within 10 % (Table 1).
Calibration flows, showing the channel drag (
FaSTMECH uses relaxation coefficients to control changes in a parameter
between iterations (Nelson, 2013). Relaxation coefficients were set to 0.5,
0.3, and 0.1 for ERelax, URelax, and ARelax, respectively, through trial and
error. Convergence was found after 5000 iterations (mean error discharge
To address the stage-dependent nature of the impact of a vegetated bar in
altering bend hydraulics, we modeled flows with magnitudes corresponding to
flows with return periods of 2 (
Region around the vegetated bar, showing cross-section (XS) locations and the conventions of the curvilinear grid to which model output was converted.
We edited FaSTMECH to account for vegetation form drag (
We focused our analyses on a point bar (Fig. 1) that supports woody riparian
vegetation (
To test whether overbank (floodplain) vegetation (i.e., beyond the vegetated bar) contributes to flow steering in the main channel and influences the hydraulics of the cut bank – bar region of interest (Fig. 3), we included runs with and without floodplain vegetation for each of the four flows and seven bar vegetation scenarios, resulting in 56 model runs. We represented floodplain vegetation as was observed from airborne lidar (see the Supplement for details). These analyses showed that the hydraulics of the cut bank – bar region of interest (Fig. 3) were insensitive to whether or not floodplain vegetation (i.e., beyond the vegetated bar) was present across the range of modeled flow conditions. Therefore, the descriptions of hydraulics we present in the results section are based only on scenarios varying bar vegetation conditions.
We considered hydraulic (
The effects of point bar vegetation on modeled hydraulics across our study
reach are presented here in several ways. First, we compare vegetation
results, for different density and growth stages, to the no-vegetation case;
and second, we compare results spatially at different cross sections across
the bar at different discharges. For the no-vegetation case, velocity and
shear stress were generally highest in the thalweg and lower over the bar
(Fig. 4). Downstream velocity (
Plan view comparison of channel-bend hydraulics (velocity,
Effect of the vegetated bar on the streamwise (
Effect of the vegetated bar on the streamwise (
Effect of the vegetated bar on the streamwise (
The manner in which different vegetation densities and growth stages
influenced hydraulics varied spatially around the bend. In general, adding
vegetation increased velocity within the thalweg and at the edge of the
vegetation patch compared to the no-vegetation case, creating concentrated
flow paths adjacent to the patch while reducing velocity and shear stress at
the head of the bar and within the vegetation patch. The effect of the
vegetated bar on channel-bend hydraulics became more pronounced with
discharges increasing from
At the downstream end of the bar (XS1; Fig. 5), vegetation increased the
magnitude of downstream (
At the midstream position (XS2), downstream velocities (
At the upstream end of the bar (XS3; Fig. 7), an opposite trend in changes in
Our results illustrate that vegetation enhances flow steering on bars,
complementing previous work on bend dynamics in the absence of vegetation.
Dietrich and Smith (1983) showed that bars steer flow in a manner that forces
the high-velocity core toward the concave bank. They additionally found that
flow over the heads of bars resulted in stream-normal components of velocity
(
This modeling effort also contributes to evaluation of the stage dependence
of flow steering by bars. Whiting (1997) hypothesized that convective
accelerations arising from flow steering would be most important at low
flows, whereas Legleiter et al. (2011) showed that steering from bars
continued to be important with increasing discharge. Our results suggest that
flow steering will continue to be important over a range of flows for
vegetated bars; i.e., vegetation effects on flow did not decrease with
increasing discharge, consistent with Abu-Aly et al. (2014). We found that
vegetation began to have a detectable impact on channel-bend hydraulics for
flows greater than
In general, we found the impact of the vegetated bar on channel-bend
hydraulics to vary with both plant density and morphology, and our modeling
illustrated nuances in these relationships. Plant morphology differences
affected hydraulics preferentially for sparse cases, whereas dense cases were
similar. Dense young trees did not always result in the maximum alteration to
channel-bend hydraulics – particularly for
We acknowledge that some of our findings may be influenced by limitations of
our modeling approach, which reflect persistent challenges in characterizing
the complexities of vegetation architecture and flow in a modeling framework.
Simplifications including representing plants as rigid cylinders (after
Vargas-Luna et al., 2016) with a constant drag coefficient of 1 are
consistent with other studies but likely overestimate vegetation drag at
higher discharges, when the canopy is inundated and plants are more
streamlined, reducing
The reduction of velocity and shear stress within the thalweg at the bar head caused by the presence of the vegetated bar would be expected to decrease sediment transport in this region. Van Dijk et al. (2013), in an experimental channel, found bar vegetation to increase fine-sediment deposition upstream of the vegetation patch, analogous to the bar head of our work. This may contribute to bar-head maintenance, such that the head of the bar is not eroded. Maintenance of the bar head would be countered by the potential for chute cutoff (van Dijk et al., 2014) or channel switching that may result because of concentrated flow paths. Along the inside (river right) edge of the vegetated bar, a lower-elevation, chute-channel-like region is present, in which flow was concentrated and velocities increased as vegetation size and density increased. Seedling establishment was not successful in the lower-elevation region during the study period, possibly because higher shear stresses in this region limited fine-sediment deposition conducive to recruitment and/or exceeded uprooting thresholds (Bywater-Reyes et al., 2015). The concentrated flow paths adjacent to the vegetation patch, on the inside of the bar, may be characteristic of conditions on vegetated bars along channel bends more generally, where both ridge and swale topography and chute bars may be present (Kleinhans and van den Berg, 2011), and where chute cutoffs and vegetation roughness and cohesion interact to influence morphodynamics (e.g., Braudrick et al., 2009). Seedlings often recruit along floodlines (Schnauder and Moggridge, 2009), forming rows of trees. Low-velocity areas within the rows induce fine-sediment deposition, steering flow away from the rows, and increasing velocity and shear stress adjacent to the rows such that sediment is transported in these regions. This process has been invoked to explain how vegetation creates vegetated islands (Gurnell et al., 2001), alternating patterns of vegetated ridges and adjacent channels (Tooth and Nanson, 2000), and the evolution of anabranching channels (Tooth et al., 2008). Van Dijk et al. (2013) found flood-dispersed vegetation recruited on bars resulted in island braiding, whereas vegetation distributed uniformly across the floodplain maintained a single-thread meandering channel with increased sinuosity and decreased bend wavelength. Our analysis, more comparable to the flood-dispersed case, shows the potential for development of vegetated islands but also for obstruction of chute cutoff through bar-head maintenance; chute cutoff may be more likely in the absence of vegetation (Constantine et al., 2010).
The production of a low-velocity region over the vegetated bar could increase fine-sediment deposition, consistent with flume and field observations. Elevated sediment deposition within patches of woody seedlings, with variations depending on plant characteristics, has been documented in meandering (Kui et al., 2014) and straight (Diehl et al., 2017b) flumes. Gorrick and Rodríguez (2012), working in a flume in which vegetation patches were simulated with dowels, documented elevated fine-sediment deposition within the patches. Zones of fine-sediment deposition on bars associated with roughness from vegetation or instream wood can in turn create sites for plant germination and seedling growth (e.g., Gurnell and Petts, 2006). If reduced velocities result in increased deposition of sediment on the bar, bar accretion would induce additional topographic steering. This feedback would be expected to accelerate channel migration rates.
The increase in velocity and shift of the high velocity core toward the
cut bank combined with low velocities within the vegetation patch would create
a large velocity gradient across the channel. A larger velocity gradient
within the thalweg compared to over the bar would be expected to alter the
dynamics of bank erosion. Parker et al. (2011) proposed that as a simple rule,
bank erosion rate,
The parameter,
Vegetation “pushing” flow toward the outer bank is analogous to “bar push” (Allmendinger et al., 2005; Parker et al., 2011), whereby a rapidly accreting point bar may cause erosion at the outer bank (Eke et al., 2014; van de Lageweg et al., 2014). This increase in bank erosion would be countered by deposition of fine sediment on the bar resulting from the vegetation-induced reduction in velocity in this region, that may in turn induce additional “push” through bar building (e.g., Eke et al., 2014). Coarse bank roughness counters this effect, pushing the high-velocity core back toward the center of the channel (Gorrick and Rodríguez, 2012; Thorne and Furbish, 1995). The balance between erosion of the bank and deposition on the bar would thus dictate whether net erosion or net deposition within the active channel occurs, inducing changes in channel width (Eke et al., 2014), and altering channel morphology.
The presence of a vegetated bar in a gravel-bed river altered both streamwise and stream-normal components of velocity vectors for overbank flows, with an increasing effect with discharge and both plant density and size. Vegetation steered flow away from the vegetated bar, creating concentrated flow paths in surrounding low-elevation side channels and a low-velocity region over the vegetated patch. Flow was slowed at the apex of the bar and increased within the thalweg around the bend. These changes in hydraulics could increase fine-sediment deposition on the bar, potentially creating hospitable sites for vegetation recruitment, and increasing bank erosion that is dependent on stream-normal velocity gradients. This pattern would tend to reduce stream-normal sediment transport at the bar head but increase it around the remainder of the bend.
Following the pattern of hydraulics, we would expect vegetation to change the morphodynamic evolution of channels with vegetation pushing flow in a manner typically attributed to bars, and increasing bank erosion rates. Bank retreat may induce bar building, which could be accelerated by fine-sediment deposition within the vegetation patch. This feedback would induce additional topographic steering from the presence of the bar. With a numerical model, we have characterized mechanisms by which channels with vegetated bars may evolve different morphologies and migration rates compared to those without, thereby contributing to our understanding of ecogeomorphic feedbacks in river–floodplain systems (Gurnell, 2014) and of how life influences landscapes (Dietrich and Perron, 2006).
Aerial lidar data used here are available from the Missoula County, Montana Geographic Information Systems office.
Ground-based lidar data (Wilcox, 2013) are available at
Bitterroot Site 1, Bitterroot Site 2, Bitterroot Site 3,
FaSTMECH solver files and associated MATLAB extraction code (Bywater-Reyes et
al., 2018) are available at
Supporting experimental procedures and additional result figures can be found in the Supplement.
The supplement related to this article is available online at:
SB-R and ACW designed the modeling experiment. RMD contributed to updating FaSTMECH code to account for vegetation drag. SB-R carried out field work and model construction, calibration, and implementation. SB-R wrote the manuscript with contributions from all co-authors.
The authors declare that they have no conflict of interest.
This research was funded by the National Science Foundation (EAR-1024652, EPS-1101342) and EPA STAR Graduate Fellowship. We thank Mark Reiling, Philip Ramsey, and MPG Ranch for access to the Bitterroot site. We thank Missoula County for providing lidar data. We thank Sarah Doelger and UNAVCO, Austin Maphis, Katie Monaco, April Sawyer, and John Bowes for assistance in the field. We give a special thanks to Carl Legleiter for sharing his scripts, and Richard McDonald, Gregory Pasternack, Daniele Tonina, David Machač, Nicholas Silverman, and Doug Brugger for modeling and scripting tips. Edited by: Jens Turowski Reviewed by: two anonymous referees