Effect Of Emergent Vegetation Research Articles

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

A diffluence-confluence unit on a meandering channel, coupled with the broadening of the divergence zone and meander circulation, promotes sediment deposition on the convex bank, forming shoals to stimulate vegetation growth, which results in water level elevation, velocity reduction, and bank stabilization. Despite these significant impacts, current research on the influence of this channel remains limited. This study conducts a series of flume experiments to quantify the influence of vegetation in a such channel on flow hydraulics, bedload transport, and bed morphology, and their inter-links. The results reveal that increased vegetation cover density subtly influences the flow depth in straight inlet reaches, triggers higher flow depth and fluctuations in the left anabranch, and induces uneven velocity distribution, especially below the crest of the curved channel. Centrifugal forces and vegetation-induced water levels shape the transverse water surface slope, resulting in a downward concave trend and unique slopes within each anabranch. The bedload transport capacity rapidly increases until an armoring layer forms, a process expedited by vegetation, particularly in high discharge conditions. This vegetation effect, amplified with higher density, enhances bedload transport rate and size but is lessened by increased discharge, which also reduces fluctuations during armoring layer formation. The statistical parameters of bed morphology at the grain-scale and structure-form-scale indicate that denser vegetation reshapes erosion dynamics across anabranches, intensifying downstream scouring and propelling the sediment deposition zone, especially under high discharge. Furthermore, the flow bifurcation ratio intensifies with escalating vegetation cover density and is assessed by a modified model consider vegetation effects with heightened precision. The apex scour depth downstream of the unit, induced by the confluence of water from both anabranches, can be characterized as a function of the dimensionless discharge and vegetation density. Concurrently, a refined model for bedload transport rate, influenced by the turbulent kinetic energy arising from emergent vegetation in this intricate reach, is proposed with high accuracy.

Effect of emergent vegetation on riverbank erosion with sediment mining

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.

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

Macrophyte-based methods for lake ecological status assessment universally include hydrophytes. Emergent plants (helophytes) are presumed to respond more directly to soil characteristics, wind exposure or shoreline management, hence are usually not considered as reliable indicators of water nutrient enrichment. The aims of the study were to explore the potential role of helophytes as eutrophication indicators and to test whether including or excluding emergent vegetation affects the indicator value of the Ecological State Macrophyte Index (ESMI) used in lake monitoring in Poland. Data on macrophyte composition and abundance (76 hydrophyte and 48 helophyte communities) and water quality from 490 Polish lowland lakes were analysed. Based on the frequency distribution and non-metric multi-dimensional scaling ordination, helophyte communities that exhibited clear trends of occurrence and abundance along the eutrophication gradient and enabled differentiation between lakes in diverse ecological conditions were identified. The effect of emergent vegetation on assessment metric diagnostic capacity was tested using modified index ESMI calculated with helophytes being included (CompESMITOT) and excluded (CompESMIHYDR) by comparing correlations between both metrics and eutrophication indicators (trophic state indices). Compared to CompESMITOT, CompESMIHYDR correlated slightly, though significantly weaker with most of the water quality indicators, and only for TSITP the difference in metric responses was statistically non-significant. The added diagnostic value of including emergent vegetation was between 0.04 and 0.08 of r2 increase and this benefit was more pronounced in more degraded lakes. The presented results demonstrated that emergent vegetation provides reliable information on ecosystem ecological conditions and can support assessment of the ecological status of lakes under eutrophication pressure.

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

Stormwater runoff has been an environmental concern since the 1980s. Green infrastructure, such as constructed stormwater wetlands (CSWs), is a tool in stormwater management, however, little is known about the hydraulic diffusion processes that impact water quality in low flow, laminar (i.e. baseflow) conditions. Diffusion provides the mechanisms that distribute and mix water through a CSW and therefore how pollutants will be spread through the CSW impacting the water quality. Laboratory experiments were performed by Nepf, H.M., Sullivan, J.A., Zavistoski, R.A. [1997. A model for diffusion within emergent vegetation. Limnology and Oceanography, 42(8), 1735–1745], and Serra, T., Fernando, H.J.S., Rodriquez, R.V. [2004. Effects of emergent vegetation on lateral diffusion in wetland. Water Research, 38(1), 139–147] to examine the effect of plant density on diffusion in laminar flow conditions. Nepf, H.M. [1999. Drag, turbulence, and diffusion in flow through emergent vegetation. Water Resources Research, 35(2), 479–489] proposed a model predicting the diffusion coefficient based upon the plant density for both laminar and turbulent flow conditions. The present study examines the effect of field conditions on diffusion in a laminar flow field and verifies the diffusion model created by Nepf, H.M. [1999. Drag, turbulence, and diffusion in flow through emergent vegetation. Water Resources Research, 35(2), 479–489]. The results from the present study show that the laminar flow model, based solely on mechanical diffusion, is not sufficient for field conditions and the total diffusion model must be used. The variability in flow conditions and stem diameter found in the field produce pockets of turbulence and dead zones that must be considered to predict the diffusion coefficients in low flow CSWs. A sensitivity analysis of the dead zone term shows that the laboratory, field and diffusion models lie within an acceptable theoretical range for the observed or predicted diffusion coefficient. In addition, a model was created using the Danish Hydraulic Institutes Mike 21 software. Model results indicate that non-uniform velocities significantly affect the diffusion coefficient and a range of diffusion coefficients should be considered when designing CSWs.

Experimental Study of Bed Load Transport through Emergent Vegetation

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 effect of emergent vegetation on convective flushing in shallow wetlands: Scaling and experiments

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.