The effect of emergent vegetation on bedload transport pathways in a meandering-anabranching channel

The effect of emergent vegetation on flow, bedload transport, and bed morphology in a diffluence-confluence unit on a meandering channel

Constructed wetlands are widely used to reduce nutrient loading to downstream waters, but they can also emit methane, a potent greenhouse gas. This trade-off between water quality benefits and climate impacts is driven by microbial processes that remain poorly understood in winter. We examined microbial community composition and methane-cycling potential in surface water samples from constructed wetlands in two agricultural regions of Sweden during the winter season, focusing on the effects of emergent vegetation and environmental conditions. Western wetlands, characterized by higher total nitrogen and dissolved oxygen, exhibited significantly greater microbial diversity and more complex co-occurrence networks than eastern wetlands. At the phylum level, Actinobacteriota and Firmicutes were more abundant in the west, while Bacteroidota dominated the east. The effects of emergent vegetation were region-specific: in the west, vegetated zones supported higher diversity and enrichment of plant-associated taxa. Several taxa affiliated with methanotrophs showed higher relative abundance in vegetated zones of the western wetlands, suggesting vegetation may enhance methane oxidation potential in surface waters, even though methane concentrations were similar. Overall, winter microbial networks remained structured, emphasizing the need for integrated microbial and biogeochemical studies to guide wetland design features, such as vegetation and nutrient regimes, that support both methane mitigation and nutrient retention in cold-climate agricultural landscapes.

Vegetation is an important agent in fluvial geomorphology and sedimentary processes, through its influence on the local hydraulics that determine sediment transport. Within stands of emergent vegetation, bed shear is substantially reduced through the absorption of momentum by drag on the stems. This stimulates deposition of sediment and reduces capacity for bed load transport. The effect of emergent vegetation on hydraulic parameters (including equilibrium bed gradient, flow depth, and velocity) and on bed load transport rate has been investigated experimentally for one sediment size, stem diameter, and stem spacing. Bed load transport rate was found to be closely related to bed-shear stress, which must be estimated by partitioning total flow resistance between stem drag and bed shear.

The use of helophytes in assessing eutrophication of temperate lowland lakes: Added value?

The effect of field conditions on low Reynolds number flow in a wetland

The present work investigates the combined effects of the upstream sediment mining pit and vegetation on the riverbank using emergent rigid vegetation beyond the toe on the flow structure and morphological changes due to fluvial erosion. A steep gradient of streamwise velocity and other turbulence parameters such as Reynolds shear stress (RSS), transverse RSS, and turbulent kinetic energy (TKE) at the interface of the vegetated and unvegetated part of the test segment was observed. The cross-sectional analysis showed that vegetation increased the velocity of the unvegetated main channel, and the sandpit increased even the near-bed velocity with a similar trend in its longitudinal variation at the center line of the main channel. The abrupt variation in RSS and transverse RSS at the location of the berm induces instability and erodes the berm present at the toe of the riverbank. The combination of the vegetation and sandpit led to increased TKE of the flow at the near-bed and berm locations. The morphological analysis showed complete riverbank erosion in both cases of the unvegetated riverbank, i.e., without or with an upstream pit. The installed stems of rigid vegetation on the riverbank helped decrease the fluvial erosion of the riverbank, and its profile observed minimal changes over the length of the test segment. However, the main channel erosion was amplified due to the vegetation (in no-pit case) at the beginning of the test segment, which eroded the bed of the main channel by about 67% of the bed thickness. Also, in the vegetated riverbank cases, the upstream pit caused an increase in erosion by 7.66% at the center of the main channel. The study helps establish the hypothesis of negating the effects of sediment mining on bank erosion by using the rigid vegetation on the riverbank beyond its toe location, which performed well by maintaining the riverbank profile.

Many wetlands around the world are characterized by shallow water, dense vegetation in the littoral zones, no significant riverine inflow and minimal circulation. Recent research on the hydrodynamics of such wetlands has identified convective circulation as being important for flushing of the littoral zones. To quantify this process, a parameterization of the convective discharge per unit width, which had been previously developed for nonvegetated systems, was extended to include a drag coefficient dependent on Reynolds number and vegetation density. The drag coefficient also included the effect of anisotropic permeability of the vegetation. The effects of relatively dense emergent vegetation (~17% by volume) on convective flushing of shallow wetlands with low‐Reynolds number (~100) flow was then investigated using experiments in a laboratory convection tank (0.5 by 2 by 0.1 m) and in a wetland mesocosm (5 by 15 by 1 m). Bottom convective currents of ~1‐10 mm s21 were measured in both the laboratory and the mesocosm. These currents resulted in the shallow, vegetated regions of the mesocosm being flushed in 4 h. The discharge per unit width (m2 s‐1) predicted by the developed parameterization compared favorably (R2 = 0.7) with the discharge per unit width measured in both the laboratory and the mesocosm. The short timescales of convective flushing, even in the presence of reasonably dense vegetation, indicate the likely significance of this mechanism in sheltered wetlands.

Effects of emergent vegetation on bed surface texture in a meandering-anabranching channel

Effects of emergent vegetation on wetland microbial processes

Solute flow and particle transport in aquatic ecosystems: A review on the effect of emergent and rigid vegetation

Vegetation present in the water streams, on the banks and in the floodplain areas largely affects the river hydraulics. Indeed, river vegetation significantly influences hydrodynamics, sediment transport, bedforms, and pollutant transport. Environmental management of rivers requires an understanding of the various processes and predictive capabilities of models. In the past, many studies were conducted, especially in laboratory settings, in order to quantify flow resistance due to vegetation. It is only recently that the effects of vegetation on sediment transport came to the attention of researchers. In particular, both suspended and bedload transport were considered. This paper reviews recent works conducted on the effect of vegetation on incipient sediment motion and bedload transport. With regard to the incipient sediment motion, methods based on critical velocity, turbulence, vegetation drag, and velocity in the bed roughness boundary layer have been discussed. For bedload transport, methods based on bed shear stress, turbulent kinetic energy, a revisiting of classical formulas for estimating bedload transport in non-vegetated channels, and estimation from erosion around a single vegetation stem are analyzed. Finally, indications on further research and new development are provided.

Modern sand transport pathways in a multiple‐sand‐ridge system are elusive and have rarely been studied in recent years. We report herein a field with four en echelon linear sand ridges offshore of Hainan Island in the Beibu Gulf and describe the distribution and morphology of these sand ridges in detail for the first time. Dune crest comparisons and seismic profiles are also interpreted to assess sand transport over the dunes. Based on a Delft 3D model, regional tidal currents and tide‐induced bedload transport in the multiple‐sand‐ridge system were simulated to provide insights into the related bedload transport paths. The results show that bedload transport and residual flows are mostly directed to the south on the east side of the sand ridges and to the north on the west side, and these differences coincide with dune asymmetries and migrations. Cross‐ridge transport is weak and mostly converges on the crests of sand ridges from the two flanks. The bedload transport is unbalanced on either flank of the sand ridges, thus leading to the asymmetry and crest kinks of the sand ridges. A distinct correlation is not observed between the net cross‐ridge sand transport and sand ridge asymmetry. In this system, cross‐swale transport is significant between the neighboring sand ridges and helps construct the bedload transport circulation in the swales. Sand accretion also occurs in the swales and benefits from the southwestward bedload transport from the north end of sand ridge 3. Distinct evidence has not been found for closed bedload transport circulation around sand ridges in this multiple‐sand‐ridge system, although limited clockwise current vortices develop on several sand ridges. Additional field observations and simulations are required to characterize the suspended load transport in a multiple‐sand‐ridge system, and the effects of local sand supply conditions on sand transport also need further evaluation. Copyright © 2018 John Wiley & Sons, Ltd.

Grain shear stress and bed-load transport in open channel flow with emergent vegetation

Extended Einstein's parameters to include vegetation in existing bedload predictors

Vegetation is ubiquitous in river channels and floodplains and alters mean flow conditions and turbulence. However, the effects of vegetation patches on near‐bed turbulence, bed load transport rates, and sedimentation are not well understood. To elucidate the influence of emergent vegetation on local and patch‐averaged bed load transport, we conducted a set of experiments in which we varied the mean flow velocity (U), total boundary shear stress (τ), or vegetation density between runs. We measured 2D velocity fields using Particle Imaging Velocimetry and bed load fluxes using high‐speed video. Simulated rigid vegetation caused bed load fluxes to vary spatially by an order of magnitude, causing distinct scour zones adjacent to, and depositional bed forms between stems. These local patterns of sedimentation could impact recruitment and survival of other plants. Large bed load fluxes were collocated with high near‐bed turbulence intensities that were three to four times larger than spatially averaged values. Higher vegetation densities increased the importance of inward and outward interactions, particularly downstream of vegetation. At the patch scale, greater stem densities caused either an increase or decrease in run‐averaged bed load fluxes, depending on whether U or τ was held constant between runs. This implies that sedimentation in vegetation patches is not only a function of bed grain size, sediment supply, and vegetation density and species, but whether vegetation significantly impacts mean and local flow properties, which could depend on vegetation location. Commonly used bed load transport equations did not accurately predict average sediment fluxes in our experiments unless they accounted for the spatial variability in the near‐bed Reynolds stress.