Experimental study of depth-limited open-channel flows over a gravel bed

Laboratory experiments of depth-limited open-channel flows over a gravel bed were conducted in the study.Two gravel patches with identical individual element size and different lengths(3.81 m and 7.5 m)were tested.The depth-limited uniform flow regime with relative submergence S_r(= D/k_s) ranging from2.68 to 5.94 was produced by adjusting the tailgate weir.The velocity profiles were measured by using both an ultra-sound velocity profiler(UVP) and an acoustic Doppler velocimeter(ADV).The conventional methods used to determine the zero-plane displacement and estimate the bed shear velocity were then reviewed and compared.The measured double-averaged(DA) velocity profiles were found to fit well with the log law and defect law with a non-universal Karman constant κ./κ-value remains nearly constant and in the range from 0.2 to 0.3 for the long patch(LP) cases and κ-values are scattered within a wider range from 0.3 to 0.5 for the short patch(SP) cases.While the Br-value in log law remains constant and equal to 8.5 for LP cases,the Br-value was found to decrease with the increase of the dimensionless roughness height k_s~+ for SP cases.The streamwise turbulence intensity distributions were found to be independent on the patch length and agree well with the available experimental data in the intermediate region and wall region.The Manning resistance coefficient and Darcy-Weisbach friction factor were analyzed.The κ-value decreases to 0.22 for the fitting of the logarithmic flow resistance law under small relative submergence.The value of the integration constant Ar in the logarithmic law falls within the normal range between 3.25 and 6.25.     

The influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers

Pebble clusters are reported widely as characteristic of gravel river beds and are known to influence the initial entrainment of bedload. A field assessment suggests that their distribution is not ubiquitous, favouring channel bars, but also reveals a tendency towards a preferred stream wise spacing. A series of laboratory flume experiments shows that flow resistance rises to, and falls from, a peak value as the longitudinal spacing of pebble clusters decreases, in a manner similar to that shown by others for strip roughness, isolated blocks, and simulated ripples and dunes. The experiments also reveal a strong inverse relationship between bedload flux rates and the flow resistance induced by the concentration of pebble clusters. It is concluded that pebble cluster spacing tends towards an equilibrium that is regulated by a feedback process involving sediment transport rates and that the spatial concentration of these microforms will adjust to the point where they induce maximum flow resistance.     

Effects of vegetation channel banks and gravel size on flow structure

Vegetation on river banks and bed roughness are important factors affecting flow structure, sediment transport, erosion and geomorphology in rivers. In this experimental study, the impacts of vegetation on flume walls, grain size of bed gravels and aspect ratio on characteristics of shear stress distribution, Coles' wake parameter, the kinematic energy correction factor (α) and the momentum correction factor (β) have been assessed. Reynolds stress distribution illustrates a three-layer pattern when the aspect ratio is smaller than 2. In addition, the aspect ratio and changes of vegetation affect α, β as well as the Coles' wake parameter Π.     

Flow resistance of gravel bed channels

Existing resistance formulas produce a wide range of friction-factor estimates for gravel bed streams. The purpose of this paper is to develop a reliable resistance formula in terms of the Darcy-Weisbach friction factor f Published data were screened and used to establish the formula. The existing formulas have considered thatfis a function of relative roughness D84/R only, where R is the hydraulic radius and 1984 is the particle size referred to the intermediate diameter that equals or exceeds that of 84 percent of bed sediments. In this paper, f is considered as a ftmction of Froude number in addition to the relative roughness, ffor D84/R〉l displays a different trend than that for Dsn/R〈l perhaps due to the invalid assumption of a logarithmic velocity distribution for D84/R〉l. Anfformula for Dsa/R〈l has been established.     

Influence of bank vegetation and gravel bed on velocity and Reynolds stress distributions

This paper presents the results of a laboratory flume experimental study on the interaction of bank vegetation and gravel bed on the flow velocity （primarily on the location of the maximum velocity, Umax） and the Reynolds stress distributions. The results reveal that the dip of the maximum velocity below the water surface is up to 35% of flow depth and the difference between Umax and the velocity at the water surface is considerable in the presence of vegetation on the walls. The zone of the log-law varies from y/h=2 up to 15 percent of flow depth and it does not depend on distance from the wall. Deviation of the velocity profile in the outer layer over a gravel bed with vegetation cover on the walls is much larger than the case of flow over a gravel bed without vegetation cover on the walls. The presence of vegetation on the walls changes uniform flow to non-uniform flow. This fact can be explained by considering the nonlinear Reynolds stress distribution and location of maximum velocity in each profile at different distances across the flume. The Reynolds stress distributions at the distance 0.02 m from the wall have negative values and away from the wall, they change the sign taking positive values with specific convex form with apex in higher location. Average of von Karman constant κ for this study is equal to 0.16. Based on to=0.16, the methods of Clauser and the Reynolds stress are compatible for determination of shear velocity.     

Effects of accelerating and decelerating flows in a channel with vegetated banks and gravel bed

Experiments were conducted in an 8 m long,40 cm wide,and 60 cm deep re-circulating fiume with vegetated banks and gravel bed to study the effects of accelerating and decelerating flows on the flow stru...     

Visualization of the turbulent flow structure in a gravel-bed river

A novel technique for visualizing turbulent flow data from a gravel-bed river is presented. The time development of flow velocity and shear stress at three heights is displayed using a computer program. This can be used to observe how the fluctuations of velocity and shear stress interact both spatially and temporally. We highlight examples of flow events which are important for the understanding of flow dynamics. The visualization suggests that the turbulent flow-field is characterized not only by coherence over time at a point, but also by spatial interdependence between points. We suggest that this new visualization approach will assist further interpretation of statistical analyses of turbulent signals, as well as focusing future measurement strategies by providing a clearer spatio-temporal picture of the flow structure.     

Sheet flow hydrodynamics over a non-uniform sand bed channel

The current study experimentally investigates the flow characteristics and temporal variations in the sheet flow profile of a non-uniform sand bed channel. Experiments were done to explore turbulent structures in the presence of a sheet flow layer with and without seepage. The turbulent events, such as stream wise velocity, Reynolds shear stresses, and turbulence intensities were found to be increasing and vertical velocity was found decreasing with a sheet layer. The presence of a sheet layer also effects the turbulent energy production and energy dissipation. All the turbulence parameters with and without a sheet layer have also been influenced by the presence of downward seepage. The rate of sheet flow movement is increased with seepage, owing to increased turbulence with seepage. The current study used wavelet analysis on temporally lagged spatial bed elevation profiles obtained from a set of laboratory experiments and synchronized the wavelet coefficients with bed elevation fluctuation at different spatial scales. A spatial cross correlation analysis at multiple scales, based on the wavelet coefficients, has been done on these bed elevation datasets to observe the effect of downward seepage on the dynamic behavior of sheet flow at different length scales. It is found that seepage increases average bed celerity and also increases the celerity of sheet flow of similar length scales. This increase in the celerity has been hypothesized as the increase of sheet flow movement as well as the increase in turbulent parameters with seepage, which destabilizes the bed particles resulting in a disruption in the continuous propagation pattern of the sheet flow. The increase of sheet flow celerity with seepage is confirmed from the saturation level of the wavelet power spectra of the bed elevation series. The presence of seepage also affects the non-uniformity of collective sheet material.     

Turbulent structure in uniform inclined open channel flow over different rough porous beds

In this study the effects of the different rough porous beds in an open inclined channel are studied experimentally for impermeable and permeable porous bed.For the simulation of porous bed two different types of permeable bed with the same thickness(s' = 3 cm) and the same porosity ε=0.70 are used:(a) porous filters and(b) gravel bed.Laboratory experiments were used for the calculation of turbulent velocity profiles.Measurements of velocity were taken for inclined channel for three different slopes(S=-0.002,S=-0.004 and S=-0.006) and for five different flow depths(h=5 cm,7 cm,9 cm,11 cm and 13 cm).The total discharge Q varies from 0.78 to 1.31 1/s.The measurements were obtained using a two-dimensional(2D) Particle Image Velocimetry(PIV).The total discharge was estimated using a calibrated venture apparatus.Results showed that the presence of rough porous bed in inclined open channels influence significantly the turbulent characteristics of the flow in comparison with impermeable open channels with the same slopes.     

Effect of combining biogeotextile and vegetation cover on the protection of steep slope of highway in northern China: A runoff plot experiment

Biogeotextiles can be used to facilitate the formation of vegetation cover and to reduce soil erosion.Studies have demonstrated that only biogeotextile or vegetation cover can greatly reduce soil erosion.However, information about the effects of the combination of biogeotextile and vegetation cover on soil erosion is still limited, despite that the combination is the commonly practical form for bare road slope protection. Experimental plots, consisting of a relatively loose surface layer and a c...     

Analytical solutions for water flow passing over a vegetal area

This study is aimed at investigating the vertical velocity profile of flow passing over a vegetal area by an analytical approach. The soil ground is considered as pervious and thus non-zero velocity at the ground surface can be estimated. The soil and vegetation layers are regarded as homogeneous and isotropic porous media. Therefore the solution of the flow can be obtained by applying the theory of turbulent flow and Biot’s theory of poroelasticity after dividing the flow field into three layers: homogenous water, vegetation and pervious soil. The velocity distribution is compared with the experimental data of [Rowiński PM, Kubrak J. A mixing-length model for predicting vertical velocity distribution on flows through emergent vegetation. J Hydrol Sci 2002;47(6):893–904] to show its validity. In addition, five dimensionless parameters denoting the variation of slope, permeability of soil, Reynolds stress, density of vegetation, and relative height of vegetation are proposed to reveal their effects on the surface water flow. The analytical solutions of flow velocity can also be simplified into simpler expressions to describe the flow passing over a non-vegetated area.     

Experimental investigation on local shear stress and turbulence intensities over a rough non-uniform bed with and without sediment using 2D particle image velocimetry

The current work focuses on locally resolving velocities,turbulence,and shear stresses over a rough bed with locally non-uniform character.A nonporous subsurface layer and fixed interfacial sublayer of gravel and sand were water-worked to a nature-like bed form and additionally sealed in a hydraulic flume.Two-dimensional Particle Image Velocimetry(2 D-PIV) was applied in the vertical plane of the experimental flume axis.Runs with clear water and weak sediment transport were done under slightly supercritical flow to ensure sediment transport conditions without formation of considerable sediment deposits or dunes.The study design included analyzing the double-averaged flow parameters of the entire measurement domain and investigating the flow development at 14 consecutive vertical subsections.Local geometrical variabilities as well the presence of sediment were mainly reflected in the vertical velocity component.Whereas the vertical velocity decreased over the entire depth in presence of sediment transport,the streamwise velocity profile was reduced only within the interfacial sublayer.In the region with decelerating flow conditions,however,the streamwise velocity profile systematically increased along the entire depth extent.The increase in the main velocity(reduction of flow resistance)correlated with a decrease of the turbulent shear and main normal stresses.Therefore,effects of rough bed smoothening and drag force reduction were experimentally documented within the interfacial sublayer due to mobile sediment.Moreover,the current study leads to the conclusion that in nonuniform flows the maximum Reynolds stress values are a better predictor for the bed shear stress than the linearly extrapolated Reynolds stress profile.This is an important finding because,in natural flows,uniform conditions are rare.     

Flow-induced vegetation uprooting in a meandering bend:Experimental investigation

Vegetation uprooting is a complex process which depends on many interrelated factors. In this paper,attention is focused on the flow-induced uprooting in river bends, where the flow characteristics vary as effected by the channel’s curvature and its continuous changing in the downstream direction. Results are presented by an experimental work done in a high-amplitude meandering flume with mature herbaceous vegetation on the bed. In the first part of the paper, the variation along the bend of the...     

The influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers: Comment on a paper by marwan A. Hassan and ian reid

Data from Turkey Brook are used to demonstrate that the interaction between gravel bedforms, flow resistance and bedload transport is a dynamic one, both between and within hydrographs. and that creation of a significant component of form drag through construction of microforms (pebble clusters) may precede the eventual break-up of the bed in a transport event. This process of drag augmentation', which can be seen as a feedback mechanism delaying transport and can be likened to dilation of a soil tested in a direct shear apparatus, itself appears to be dependent on the characteristics of turbulence, and therefore involves feedback at a finer resolution than envisaged by Hassan and Reid (1990).     

3D Numerical modelling of flow divisions at open channel junctions with or without vegetation

The flow division at an open channel junction is affected by the inflow discharge and the downstream water depths of the junction. The growth of vegetation in a channel system is environmental friendly, but its effect on the flow in an open channel junction can be significant. In this work a 3D RANS (Reynolds Averaged Navier–Stokes equation) model has been implemented to investigate the flow phenomena in channel junctions with or without vegetation. The model is first validated by two cases: flow in an open channel T-junction without vegetation, and flow in a single open channel with vegetation. The model is then applied to simulate flow in an open channel T-junction with varying width ratio and vegetation density of the branch channel. The results quantitatively predict the trend of increasing flow in the branch channel with the increase in branch channel width and/or the decrease in vegetation density. The overall energy loss coefficient of the system, however, decreases with the amount of flow in the branch channel.     

Use of Large-Eddy Simulation for the bed shear stress estimation over a dune

Environmental flows are generally characterized by complex bed morphology and high current speeds. Such configurations favor the formation of vortex structures that strongly affect hydrody-namics and sediment transport. Large-Eddy Simulation (LES) enables investigation of the dynam-ics of the largest turbulence scales and, thanks to enhanced calculation resources, has now become applicable for simulating environmental flows. In this paper, a LES approach is developed in a CFD code (TELEMAC-3D), which was originally developed to simulate free surface flows using RANS methods. The present developments involve implementing subgrid models, boundary con-ditions and numerical schemes suitable for LES. The LES version of TELEMAC-3D was validated by comparing results on the model with experimental data for flow past a cylinder. Then, the model was applied to a test case representing flow over dunes. After validating the hydrodynamics, the model was used to assess the bottom shear stress, using both a RANS and a LES approach. Com-parison highlighted the potential contribution of LES to investigating the hydrodynamic forces acting on the bottom.     

Modeling depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation

This paper presents an approach to modeling the depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation. The depth-averaged equation of vegetated compound channel flow is given by considering the drag force and the blockage effect of vegetation, based on the Shiono and Knight method (1991) [40]. The analytical solution to the transverse variation of depth-averaged velocity is presented, including the effects of bed friction, lateral momentum transfer, secondary flows and drag force due to vegetation. The model is then applied to compound channels with completely vegetated floodplains and with one-line vegetation along the floodplain edge. The modeled results agree well with the available experimental data, indicating that the proposed model is capable of accurately predicting the lateral distributions of depth-averaged velocity and bed shear stress in vegetated compound channels with secondary flows. The secondary flow parameter and dimensionless eddy viscosity are also discussed and analyzed. The study shows that the sign of the secondary flow parameter is determined by the rotational direction of secondary current cells and its value is dependent on the flow depth. In the application of the model, ignoring the secondary flow leads to a large computational error, especially in the non-vegetated main channel.     

Effects of fire and vegetation cover on hydrological characteristics of a Mediterranean shrubland soil

An experimental study based on the effects of fire on soil hydrology was developed at the Experimental Station of ‘La Concordia’ (Valencia, Spain). It is located on a calcareous hillside facing SSE and composed of nine erosion plots (4 × 20 m). In summer 2003, after eight years of soil and vegetation recovery from previous fires in 1995 (with three fire treatments: T1 high‐intensity fire, T2 moderate intensity, and T3 not burnt), experimental fires of low intensity were again conducted on the plots already burnt, to study the effects of repeated fires on the soil water infiltration, soil water content and runoff. Infiltration rates and capacities were measured by the mini‐disk infiltrometer method (MDI), assessing the effects of vegetation cover by comparing the under‐canopy microenvironment (UC) and its absence on bare soil (BS), immediately before and after the fire experiments. Soil properties like water retention capacity (SWRC) and water content (SWC) were also determined for the different fire treatments (T1, T2 and T3) and microsites (UC and BS). Hydrological parameters, such as runoff and infiltration rate, were monitored at plot scale from July 2002 to July 2004. In the post‐fire period, data displayed a 20% runoff increase and a decrease in infiltration (18%). Differences in the steady‐state infiltration rate (SSI) and infiltration capacity (IC) were tested with the MDI on the different treatments (T1, T2 and T3), and between the UC and BS microsites of each treatment. After fire, the SSI of the UC soil declined from 16 mm h⁻¹ to 12 mm h⁻¹ on T1, and from 24 mm h⁻¹ to 19 mm h⁻¹ on T2. The IC was reduced by 2/3 in the T1 UC soil, and by half on T2 UC soil. On the BS of T1 and T2, the fire effect was minimal, and higher infiltration rates and capacities were reached. Therefore, the presence/absence of vegetation when burnt influenced the post‐burnt infiltration patterns at soil microscale. On the T3, different rates and capacities were obtained depending on the microsites (UC and BS), with higher SSI (25 mm h⁻¹) and IC (226 mm h⁻¹) on BS than on UC (SSI of 18 mm h⁻¹ and IC of 136 mm h⁻¹). The SWRC and SWC were recovered from 1995 to 2003 (prior to the fires). The 2003 fire promoted high variability on the SWC at pF 0·1, 2 and 2·5, and the SWRC on burnt soils were reduced. To summarize, the IC and SSI post‐fire decreases were related to the lower infiltration rate at plot scale, the significant differences in the SWRC between burnt and control treatments, and the increase in the runoff yield (20%). According to the results, the MDI was a useful tool to characterize the soil infiltration on the vegetation patches of the Mediterranean maquia, and contrary to other studies, on the UC soil, the infiltration rate and IC, when soil was dry, were lower than that obtained on BS. Once the soil gets wet, similar values were found on both microenvironments. Copyright © 2010 John Wiley & Sons, Ltd.     

Flow resistance of one-line emergent vegetation along the floodplain edge of a compound open channel

Experiments have been conducted in straight compound channels with and without one-line emergent vegetation along the floodplain edge, in which stream-wise velocities and boundary shear stresses have been measured. The experimental results show that the velocity distribution in the vegetation case is considerably different from that in the no vegetation case and the boundary shear stress is also significantly reduced by the additional flow resistance caused by the vegetation at a similar relative water depth. The apparent shear stress distribution which has been calculated with the boundary shear stress and weight component in the vegetation case is totally different from that in the no-vegetation case. New formulae for friction factors for the with and without vegetation cases are developed using vegetation density and flow parameters. The drag force caused by the vegetation is obtained for two different vegetation density cases and the magnitude of its effect on total flow resistance is then investigated. The force balance method is used to predict discharge and this is compared with the discharge predicted by the new formula. A further analysis of the selection of vegetation spacing is carried out, determining its effect on stage-discharge.     

Turbulence, suspension and downstream fining over a sand-gravel mixture bed

Flume experiments were carried out to study the turbulence and its impact on suspension and segregation of grain-sizes under unidirectional flow conditions over the sand-gravel mixture bed. The components of fluid velocity with fluctuations were measured vertically using 3-D Micro-acoustic Doppler velocimeter (ADV). The theoretical models for velocity and sediment suspension have been developed based on the concept of mixing length that includes the damping effect of turbulence due to sediment suspension in the flow over the sand-gravel mixture bed. Statistical analysis of segregation of grain-sizes along downstream of the bed has been performed using the principle of unsupervised learning or clustering problem. Exploratory data analysis suggests that there is a progressive downstream fining of sediment sizes with selective depositions of gravels, sand-gravels and sand materials along the stream, which may be segmented into three regions such as, the upstream, the transitional and the downstream respectively. This contribution is relevant to understand the direction of ancient rivers, the bed material character in the river form, sorting process and its role in controlling the sediment flux through landscape.