Vegetated Bed Research Articles

Characteristics of Shallow Flows in a Vegetated Pool—An Experimental Study

Pools are often observed in gravel-bed rivers, together with the presence of vegetation patches. In the present study, a conceptual model of a gradual varied flow with both convective deceleration and acceleration flow sections has been constructed in a flume to study turbulent flow structures. Vegetation patches with extended canopies were planted in the pool sections in order to increase the thickness of the boundary layer inside the inner zone. The effects of different flows (namely decelerating, uniform and accelerating flows) along an artificial pool on flow velocity, shear stress and bursting events have been investigated. In addition, due to the occurrence of secondary currents in shallow streams, the characteristics of turbulent shallow flow have been investigated along two axes that are parallel to the sidewall of the flume. The results showed that the application of the log law should be used with care to estimate shear velocity along a pool with a vegetated bed. The presence of a vegetation patch causes an increase in Reynolds shear stress, especially along the entrance section of the pool where the flow decelerates. The results of the quadrant analysis reveal that the sweep and ejection events have the most dominant influence over the vegetation patch in the pool; however, the contributions of outward and inward events increase near the bed, especially in the entrance section of the pool where the flow is decelerating. The distribution of stream-wise RMS of turbulence intensity along the pool generally presents a convex shape.

Turbulent Kinetic Energy and Budget of Heterogeneous Open Channel with Gravel and Vegetated Beds

Turbulent kinetic energy (TKE) and budget are indispensable hydraulic parameters to determine turbulent scales and processes resulting from various and different natural hydraulic features in open channels. This paper focuses on experimental investigation of turbulent kinetic energy and budget in a heterogeneous open channel flow with gravel and vegetated beds. Results indicate the turbulent kinetic energy (TKE) value over gravel region of the heterogeneous bed remains approximately constant with flow depth. The highest turbulent kinetic energy was calculated for flexible vegetation arrangement compared to the rigid vegetation. The estimation of the turbulent kinetic energy budget shows the higher values of turbulence production recorded over the flexible vegetated bed, consequently, the dissipation rate exhibits faster decay of turbulence kinetic energy over the vegetated bed in comparison to the gravel bed.

Proposing a constructed wetland within the branch drains network to treat degraded drainage water in Tina Plain, North Sinai, Egypt

ABSTRACT To treat degraded agricultural wastewater due to non-point pollution source in cultivated area (Tina Plain), Egypt, a wetland system within branch drain network was proposed and designed, which consists of a sedimentation pond followed by four vegetated bed cells of reed (Phragmites australis), and an open water disinfection zone. The relaxed tank in series model was used to design the wetland. The model input data were influent flow 600 m3 d−1, winter temperature 18 °C, hydraulic retention time 5.8 d, influent pollutant concentrations 500, 300, 480, and 20 mg L−1 for TSS, BOD, COD, and TN, respectively, and 100,000 MPN 100 mL−1 for FC. Results show a wetland system area of 0.078, 0.24, and 0.3 ha for sedimentation pond, vegetated reed bed cells, and disinfection zone, respectively. The expected removal efficiencies for TSS, BOD, COD, TN, and FC were 96.4%, 96.2%, 95%, 75%, and 97%, respectively. The wetland hydrology was computed using local climate data and FAO-CROPWAT 8 model. The proposed wetland area was compared with literature to show its feasibility for wetland operation conditions. The proposed system met the allowable limits of Egyptian environmental regulations, simple construction, operation, and has a low construction cost of one UDS m−2

Impacts of vegetation over bedforms on flow characteristics in gravel-bed rivers

In the present study, among six gravel-bed river reaches, two natural gravel-bed river reaches with approximately similar bedforms and flow conditions have been found, one river reach with vegetation on bedforms and the other without vegetation on bedforms. Based on field measurements, the impacts of vegetation over bedforms on flow characteristics have been investigated. Flow velocity, Reynolds stress and turbulence intensity distributions over bedforms with vegetation have been compared with those over gravel bedforms without vegetation. By using the quadrant analysis, the dominant events of turbulent flow structures have been assessed for flows over gravel bedforms with vegetation and without vegetation. Results show that the effect of vegetation enhances the anisotropy in turbulence and generates strong secondary currents, which lead to the occurrence of dip phenomenon at the stoss section and the crest of vegetated bedforms. The change of bed roughness from gravel bed to vegetated bed causes a development of a new boundary layer. This resulted in the different shape of velocity distributions over bedforms with vegetation from those over bedforms without vegetation, which can affect the lives of aquatic animals. Reynolds shear stress at downstream section of gravel bedform without vegetation is much higher than that of vegetated bedform, which shows that the erosion takes place from downstream of vegetated bedforms, while in the bare beds; the deposition is at downstream of bedforms. It is found that the presence of vegetation causes a chaotic distribution pattern of Reynolds shear stress, while it has more orderly pattern at the downstream section of gravel bedform without vegetation. Besides, the distribution of turbulence intensity in the lee side of vegetated bedform has a concave shape and in the stoss section, turbulence is generated in ejections and sweeps near the vegetation cover. These findings are significant to the practice of river ecological restoration.

Flow over Flexible Vegetated Bed: Evaluation of Analytical Models

The development of vegetation in the river bed and in the banks can affect the hydrodynamic conditions and the flow behavior of a watercourse. This can increase the risk of flooding and sediment transport. Therefore, it is important to develop analytical approaches to predict the resistance caused by vegetation and model its effect on the flow. This is the objective of this work which investigates the ability of different analytical models to predict the vertical velocity profile as well as the resistance induced by flexible submerged vegetation in open channels. Then it is possible to select the appropriate model that will be applied in the real case of rivers. The model validation is determined after a comparison between the data measured in the different experiments carried out and those from literature. For dense vegetation, the role of the Reynolds number is emphasized in particular with a model using the Darcy-Brinkman equation in the canopy. With a simple permeability, this model is relevant to estimate friction. However, for larger Reynolds number, models based on the fully turbulent flow assumption provide better results.

Secondary Flow Structures in Open Channel with Patchy Gravel and Vegetated Beds

Secondary flows are present in open channel due to the complex three-dimensional flow structures as a result of the existing natural features (e.g. different boundaries) affecting the flow. This paper experimentally examined the structure of secondary flow in open channel with a patchy gravel and vegetated beds using two different experimental conditions. A high resolution three-dimensional instantaneous velocity measurement over the gravel and vegetated bed were made using the acoustic Doppler velocimeter (ADV) and the secondary flow quantities determined. The results reveal that the magnitude of secondary flow over the gravel bed in experiment one is relatively large with occurrence of down-flow, and up-flow over the flexible vegetated bed. The lateral interaction and momentum transfer between the two-bed roughness (gravel and vegetated beds) is achieved by the secondary flow, at the lower region of the flow, the transverse motion is directed from the gravel bed towards the grass bed, and at the upper region, and the flow is transported laterally in the opposite direction.

Growth Characteristics of <i>Fuirena umbellata</i> in a Surface Flow Constructed Wetland and Its Influence in Nutrients and Faecal Bacteria Removal from Domestic Wastewater in Cameroon

Various configurations of vegetated bed systems with a variety of macrophytes have been tested experimentally in Cameroon, for the treatment of domestic wastewater. The aim of this work was to assess the growth and biomass production of Fuirena umbellata (Cyperaceae) and its potentials in the removal of faecal bacteria and nutrients from primarily treated domestic effluent. A wetland vegetated with this macrophyte and a non-vegetated wetland (control) were continuously fed with primarily treated domestic wastewater at an estimated loading rate of 205 Litres/day in dry and rainy seasons for two consecutive years. Physicochemical and microbiological parameters of the effluent were monitored at the inflow and outflows of the wetlands alongside with the growth and productivity attributes of the young plants during each season. The density of plants ranged in the wetland from 17 - 185 plants/m2 and from 11 - 146 plants/m2 respectively during the first and the second years. More biomasses were instead produced in the dry seasons than in the rainy seasons but with no significant differences observed. As for nutrients removal, higher efficiencies were observed in the vegetated wetland (45% - 73%) compared to the non-vegetated control (17% - 66%). Similar trends were observed for the faecal bacteria but with no significant differences between the seasons. However, the vegetated beds were significantly more efficient than the non-vegetated control in the reduction of many physicochemical parameters and faecal bacteria. This varied with the seasons.

Stability analysis of ecomorphodynamic equations

AbstractIn order to shed light on the influence of riverbed vegetation on river morphodynamics, we perform a linear stability analysis on a minimal model of vegetation dynamics coupled with classical one‐ and two‐dimensional Saint‐Venant‐Exner equations of morphodynamics. Vegetation is modeled as a density field of rigid, nonsubmerged cylinders and affects flow via a roughness change. Furthermore, vegetation is assumed to develop following a logistic dependence and may be uprooted by flow. First, we perform the stability analysis of the reduced one‐dimensional framework. As a result of the competitive interaction between vegetation growth and removal through uprooting, we find a domain in the parameter space where originally straight rivers are unstable toward periodic longitudinal patterns. For realistic values of the sediment transport parameter, the dominant longitudinal wavelength is determined by the parameters of the vegetation model. Bed topography is found to adjust to the spatial pattern fixed by vegetation. Subsequently, the stability analysis is repeated for the two‐dimensional framework, where the system may evolve toward alternate or multiple bars. On a fixed bed, we find instability toward alternate bars due to flow‐vegetation interaction, but no multiple bars. Both alternate and multiple bars are present on a movable, vegetated bed. Finally, we find that the addition of vegetation to a previously unvegetated riverbed favors instability toward alternate bars and thus the development of a single course rather than braiding.

Numerical simulation of dam-break flow and bed change considering the vegetation effects

Existing numerical investigations of dam-break flows rarely consider the effects of vegetation. This paper presents a depth-averaged two-dimensional model for dam-break flows over mobile and vegetated beds. In the model, both the consequences of reducing space for storing mass and momentum by the existence of vegetation and dragging the flow are considered: the former is considered by introducing a factor (1−c) to the flow depth, where c is the vegetation density; the later is considered by including an additional sink term in the momentum equations. The new governing equations are discretized by the finite volume method; and an existing second-order central-upwind scheme embedded with the hydrostatic reconstruction method for water depth, is used to estimate the fluxes; the source terms are estimated by either explicit or semi-explicit methods fulfilling the stability requirement. Laboratory experiments of dam-break flows or quasi-steady flows with/without vegetation effects/sediment transport are simulated. The good agreements between the measurements and the numerical simulations demonstrate a satisfactory performance of the model in reproducing the flow depth, velocity and bed deformation depth. Numerical case studies of six scenarios of dam-break flows over a mobile and vegetated bed are conducted. It is shown that when the area of the vegetation zone, the vegetation density, and the pattern of the vegetation distribution are varied, the resulted bed morphological change differs greatly, suggesting a great influence of vegetation on the dam-break flow evolution. Specifically, the vegetation may divert the direction of the main flow, hindering the flow and thus result in increased deposition upstream of the vegetation.

Entropic approach to estimate the mean flow velocity: experimental investigation in laboratory flumes

The paper deals with the linear entropic relationship between the maximum velocity, u max , and the mean flow velocity, u m , through a dimensionless parameter Φ(M), in open-channel flow. The analysis is conducted with the aid of experimental data collected in straight laboratory flumes under different bed and side-walls roughness conditions. In particular, rough/vegetated beds and smooth/rough side-walls conditions have been investigated. The results show that, in the investigated conditions (with exception of low-submergence vegetated bed—h/k v < 2), Φ(M) can be assumed equal to a value that is very close to that found in natural channels. This demonstrates that Φ(M) is able to implicitly reflect the different hydraulic behavior which is determined in rough and submerged vegetated beds. Thus, the entropy-based Manning’s roughness formula has been validated and the sensitivity analysis of Manning’s coefficient with the values of y o (location of the zero-velocity plane) has been also performed. It is found that this formula is quite robust to represent the observed flow resistance also in the presence of vegetation.

Effects of short flexible seagrass Zostera noltei on flow, erosion and deposition processes determined using flume experiments

AbstractInnovative flume experiments were conducted in a recirculating straight flume. Zostera noltei meadows were sampled in their natural bed sediments in the field at contrasting stages of their seasonal growth. The aims of this study were: (i) to quantify the combined effects of leaf flexibility and development characteristics of Zostera noltei canopies on their interaction with hydrodynamics; and (ii) to quantify the role of Zostera noltei meadows in suspended sediment trapping and bed sediment resuspension related with changes in hydrodynamic forcing caused by the seasonal development of seagrasses. Velocity within the canopy was significantly damped. The attenuation in velocity ranged from 34 to 87% compared with bare sediments and was associated with a density threshold resulting from the flow‐induced canopy reconfiguration. The reduction in flow was higher in dense canopies at higher velocities than in less dense canopies, in which the reduction in flow was greater at low velocities. These contrasted results can be explained by competition between a rough‐wall boundary layer caused by the bed and a shear layer caused by the canopy. The velocity attenuation was associated with a two to three‐fold increase in bottom shear stress compared with unvegetated sediment. Despite the increase in near‐bed turbulence, protection of the sediment against erosion increased under a fully developed meadow, while sediment properties were found to be the main factor controlling erosion in a less developed meadow. Deposition fluxes were higher on the vegetated bed than on bare sediments, and these fluxes increased with leaf density. Fewer freshly deposited sediments were resuspended in vegetated beds, resulting in an increase in net sediment deposition with meadow growth. However, in the case of a very high leaf area index, sediment was mostly deposited on leaves, which facilitated subsequent resuspension and resulted in less efficient sediment trapping than in the less developed meadow.

Performances of a Yard-Scale Surface Flow Wetland Vegetated with Echinochloa Crus-Pavonis in the Removal of Nutrients and Faecal Bacteria from Domestic Wastewater

Several macrophytes are being tested in experimental wetlands systems in Cameroon, for the treatment of domestic wastewater. The aim of the present research was to assess the performances of a yard scale surface flow wetland vegetated with Echinochloa crus-pavonis, in the removal of nutrients and faecal bacteria from primarily treated effluent. A wetland vegetated with the macrophyte and a non-vegetated wetland (control) were continuously fed with primarily treated domestic wastewater at organic loading rates varying from 20.74 to 27.15 g BOD/m 2 /day in dry season and the rainy season for two consecutive years. Physicochemical and microbiological characteristics of water were monitored at the entries and exits of the wetlands alongside the growth attributes of the young plants during each season. Plant densities in the bed increased with time during each season of the experiment, and not significantly lowered during the following season after harvest. Percentage removal ranges of 89-95 % and 75-93 % were observed for faecal bacteria during the dry and the rainy seasons respectively in the vegetated beds, as compared to ranges of 25-71 % and 24-73 % recorded in the control bed. The same trend was observed for nitrates and orthophosphates with removal rates of up to 62 % and 82 % respectively as compared to 40 % and 47 % recorded in the control. The vegetated bed was significantly more efficient than the control in the reduction of several parameters, but this varied with seasons.

Spatial Variation of Longitudinal Dispersion in LECA-Based Vegetated Beds

The evaluation of the dispersion in vegetated beds may allow indentifying mechanisms that affect the transport and reaction of solutes, namely organic and nitrogen compounds. A set of non-reactive tracer experiments (slag injection) was performed in a vegetated bed (a mesocosm with a LECA-based substratum and colonized withPhragmites australis) used for the removal of organic and nitrogen pollutant loads. Loads of approximately 300 mg COD/L and 30 mg NH4-N/L and a hydraulic loading rate of 3.5 cm/d were used. The results showed a delay in all the residence time distribution (RTD) curves and a variation in the dimensionless residence time (μ(m,θ)) of the E(θ) curves, which means that the mass centre of the impulse was late relatively to the expected one. A strong dispersion and tracer retention (due to the presence of stagnated areas and internal recirculation) was observed, especially in the first 33 cm of the bed, which seems to have been related to the presence of complex clusters of roots, solid material, biofilm and LECA particles. An analytical solution of the Multiple-Tanks-in-Series (MTS) model well represents the RTD curves obtained in the tracer experiments. The detected dispersion and dead volume ratios (7% to 12%) did not affect the performance of the bed, which presented mean removal efficiencies of 85% and 60.4% for COD and NH4-N, respectively.

Investigation of turbulence characteristics in channel with dense vegetation

In this experimental study, the turbulent flow in a channel with vegetation by using sprouts of wheat on channel bed was investigated. Two different aspect ratios of channel were used. An Acoustic Doppler Velocimetry was used to measure parameters of turbulent flow over submerged sprouts of wheat, such as velocity profiles. The log law and the Reynolds shear stress distribution were applied. Results indicate that the position of the maximum turbulence intensity superposes on the inflection point situated over the top of submerged vegetation cover. Quadrant analysis shows that near the vegetation bed, the sweeps and ejections appear to be the most dominant phenomenon, while far from the vegetated bed, the outward is dominant event. Results also show that the aspect ratio plays an important role on the contribution of the different bursting events for Reynolds stress determination.

Drag coefficient of unsubmerged rigid vegetation stems in open channel flows

Results of an experimental study on the drag force measurement involving a single stem kept in a channel flow stem array are presented. The data collected herein and those from literature indicate that the stem drag coefficient logarithmically increases with the areal stem density. The stem Reynolds number is noticed to have only a small effect on the stem drag coefficient which was however found to depend on the stem staggering pattern. The drag coefficient is less influenced by the Froude number in subcritical flows but it decreases with the Froude number in supercritical flows. New relationships are proposed for the stem drag coefficient which appear useful in partitioning the total flow resistance of vegetated bed streams into the stem and the bed particle resistances. The bed particle resistance applies to sediment transport through such vegetated flows for which the average flow velocity is available.

Flow analysis in a channel with flexible vegetation using double-averaging method

The paper addresses the problem of the resistance due to vegetation in an open channel flow, characterized by partially and fully submerged vegetation formed by colonies of bushes. The flow is characterized by significant spatial variations of velocity between vertical profiles that make the traditional approach based on time averaging of turbulent fluctuations inconvenient. A more useful procedure, based on time and spatial averaging (Double-Averaging Method) is applied for the flow field analysis and characterization. The vertical distribution of mean velocity and turbulent stresses at different spatial locations has been measured with a 3D Acoustic Doppler Velocimeter (ADV) for two different vegetation densities where fully submerged real bushes (salix pentandra) have been used. Velocity measurements were completed together with the measurements of drag exerted on the flow by bushes at different flow depths. The analysis of velocity measurements allows depicting the fundamental characteristics of both the mean flow field and turbulence. The experimental data show that the contribution of form-induced stresses to the momentum balance cannot be neglected. The mean velocity profiles and the spatially averaged turbulent intensity profiles allow inferring that the vegetation density is a driving parameter for the development of a mixing layer at the canopy top in the case of submerged vegetation. Moreover, the net upward turbulent momentum flux, evaluated with the methodology proposed by Lu and Willmarth (1973), appears to be damped for increased vegetation density; this finding can rationally explain the reduction of the suspended sediment transport capacity typically observed in free surface flows over a vegetated bed.

Constructed wetlands for wastewater treatment in the Czech Republic

The first constructed wetland (CW) for wastewater treatment was built in the Czech Republic in 1989. This recent survey shows that at the end of 1999 101 systems are in operation and several more are under construction. 95 CWs are designed with sub-surface horizontal flow, 6 systems are hybrid with a combination of vertical and horizontal flow beds. Most systems (56) were designed for the treatment of municipal and domestic sewage while 38 CWs were designed for the treatment of wastewater from combined sewer systems. The most commonly used size of vegetated beds is 1,001-2,500 m2 (31 systems) followed by the area between 51-250 m2 (19%). The area of vegetated bed of the largest system is 4,493 m2. Size distribution is quite evenly spread from very small systems (PE = 3 or 4) up to 1,000 PE. However, most systems (44) were sized to treat wastewater from sources between 101 and 500 PE. The most commonly used macrophyte is Common reed (Phragmites australis) which is used in 34 systems as a monotypic stand and in 44 systems in combination with other macrophytes, most frequently with Reed canarygrass (Phalaris arundinacea) (31 systems) and cattails (Typha spp.) (8 systems).

Nitrogen removal in a combined system: vertical vegetated bed over horizontal flow sand bed

Pig farm wastewater creates various problems in many areas throughout Thailand. Constructed wetland systems are an appropriate, low cost treatment option for tropical countries such as Thailand. In this study, a combined system (a vertical flow bed planted with Cyperus flabelliformis over a horizontal flow sand bed without plants) was used to treat settled pig farm wastewater. This system is suitable for using in farms where land is limited. The average COD and nitrogen loading rate of the vegetated vertical flow bed were 105 g/m2 x d and 11 g/m2 x d respectively. The wastewater was fed intermittently at intervals of 4 hours with a hydraulic loading rate of 3.7 cm/d. The recirculation of the effluent increased total nitrogen (TN) removal efficiency from 71% to 85%. The chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) removal efficiencies were 95% and 98%. Nitrification was significant in vertical flow Cyperus bed, and the concentration of nitrate increased by a factor of 140. The horizontal flow sand bed enhanced COD removal and nitrate reduction was 60%. Plant uptake of nitrogen was 1.1 g N/m2 x d or dry biomass production was 2.8 kg/m2 over 100 days.

NUMERICAL STUDY ON TURBULENT FLOW OVER RIGID VEGETATION-COVERED BED IN OPEN CHANNELS

Open channel flow over rigid vegetation is analyzed numerically by horizontaly averaged Reynolds equations based on k-e turbulence model. Not only uniform flow over vegetated bed but also a transitional flow from area over a smooth bed to area over vegetated bed is studied. The experiments were previously conducted for the former case. For the latter case, the flow experiments are conducted. The experimental data are well described by the numerical model. Furthermore, the effect of the vegetation density on the turbulent structure is investigated with numerical prediction of flow by the present model.

Suspended Sediment Concentration Distribution of Flow over Vegetated Sand Bed

Hydraulics in vegetated streams increasingly becomes important for designing comfortable river environments. In this study, suspended sediment transport is investigated. Firstly, the turbulent structure of open channel flow over a vegetated bed is studied by means of both experiments and numerical analysis based on k-A turbulence model. Secondly, the turbulent flux of suspended sediment is estimated by assuming a similarity between the change of turbulent flux distribution of suspended sediment due to the presence of a vegetated layer and that of turbulent momentum flux (Reynolds-stress distribution). Based on the above, the concentration distribution of suspended sediment of flow in a vegetated-bed channel is predicted against the vegetation density.