Water-worked Gravel Beds Research Articles

Response of Reynolds stresses and scaling behavior of high-order structure functions to a water-worked gravel-bed surface and its implication on sediment transport

The aim of this experimental study is to investigate the interaction between turbulent flow and a gravel-bed that mimics the actual roughness structures of a natural bed and its implication on sediment transport. In particular, the response of the Reynolds stresses and the role of intermittency to the bed roughness is the primary focus of the current study. To this end, the flow field, measured with an Acoustic–Doppler Velocimeter (ADV), is thoroughly examined, considering the conditional Reynolds shear stresses and the high-order structure functions of velocity. The study results show that the features and the roughness structure of the water-worked gravel-bed (WWGB) have a strong impact on bursting events. The reason for this is attributed to the flow layer above the crest in the WWGBs being primarily affected by sweep events rather than ejection events, although the latter events are prevalent in this flow layer in an immobile, man-made, gravel-bed. These coherent structures, which are primarily responsible for sediment transport, also are the key cause of the presence of an intermittency excess that breaks the Kolmogorov self-similarity hypothesis, leading to multi-fractal behavior of the velocity structure functions.

Structure functions and invariants of the anisotropic Reynolds stress tensor in turbulent flows on water-worked gravel beds

In this paper, a statistical description of turbulence in flow on a water-worked gravel-bed is presented by applying the laws of turbulence in conjunction with the double-averaging methodology. To this end, a laboratory experiment was performed, in which the gravel-bed was worked by flowing water. From the Taylor frozen-in approximation, the energy spectra of streamwise velocity fluctuation reveal the existence of the inertial subrange (lying between the energy containing and the dissipation ranges), where the turbulent kinetic energy (TKE) dissipation rate is constant. It is revealed that, in this range, Kolmogorov’s 4/5-law for the spatial increments of streamwise velocity is valid, thus allowing an accurate estimation of the TKE dissipation rate. Although Monin–Yaglom’s 4/3-law for the third-order mixed structure function provides a behavior similar to that of Kolmogorov’s 4/5-law, the estimation of the TKE dissipation rate by Monin–Yaglom’s 4/3-law is not quite accurate, owing to the departure from the isotropic turbulence at large scales. Therefore, the present study demonstrates the validity of this statistical approach (Kolmogorov’s 4/5-law) to investigate the turbulence at small scales. Besides, the data plots for an anisotropy invariant map suggest that near-bed anisotropic turbulence tends to reduce to the three-dimensional isotropic turbulence with an increase in the vertical distance, indicating a relaxation of the effects of bed roughness on anisotropic turbulence toward the free surface.

Conditional Turbulence Characteristics in Water-Worked and Screeded Gravel-Bed Flows

AbstractThe conditional double-averaged streamwise velocity, spatially averaged (SA) turbulence and dispersive quantities in flows over a water-worked gravel bed (WGB), and a screeded gravel bed (S...

Water-Worked Gravel Bed: State-of-the-Art Review

In a natural gravel-bed stream, the bed that has an organized roughness structure created by the streamflow is called the water-worked gravel bed (WGB). Such a bed is entirely different from that created in a laboratory by depositing and spreading gravels in the experimental flume, called the screeded gravel bed (SGB). In this paper, a review on the state-of-the-art research on WGBs is presented, highlighting the role of water-work in determining the bed topographical structures and the turbulence characteristics in the flow. In doing so, various methods used to analyze the bed topographical structures are described. Besides, the effects of the water-work on the turbulent flow characteristics, such as streamwise velocity, Reynolds and form-induced stresses, conditional turbulent events and secondary currents in WGBs are discussed. Further, the results form WGBs and SGBs are compared critically. The comparative study infers that a WGB exhibits a higher roughness than an SGB. Consequently, the former has a higher magnitude of turbulence parameters than the latter. Finally, as a future scope of research, laboratory experiments should be conducted in WGBs rather than in SGBs to have an appropriate representation of the flow field close to a natural stream.

Near-bed turbulence structures in water-worked and screeded gravel-bed flows

Coherent structures and their impact on the near-bed time-averaged flow structure in a water-worked gravel-bed (WGB) and a screeded gravel-bed (SGB) are analyzed. Instantaneous velocities were measured using a particle image velocimetry system in the WGB and SGB flows in a flume with rectangular cross section. To ascertain the response of the WGB with respect to the SGB to the coherent structures, the time- and double-averaged flow, and the spatially averaged (SA) turbulence parameters, the experimental flow conditions for both the beds were kept identical. The surface gravels in the WGB were spatially organized owing to the water action. By contrast, the surface gravels in the SGB were randomly poised. These result in a higher roughness height in the WGB than in the SGB. Time series analysis for the instantaneous velocity and vorticity on a central vertical plane along the streamwise direction proves that the coherent structures in the near-bed flow zone are constituted by rapidly and slowly moving fluid streaks. Besides, the time-averaged streamwise velocity, vorticity, turbulence level, third-order correlations, and turbulent kinetic energy (TKE) budget are analyzed in the WGB and SGB. Their contours are plotted on the central vertical plane to study their spatial distributions. In addition, the SA higher-order correlations and TKE budget in the WGB and SGB are examined. A comparative study infers that the higher roughness in the WGB than in the SGB causes both the time-averaged and SA turbulence parameters in the former to be greater than those in the latter.

Spatially averaged dissipation rate in flows over water-worked and screeded gravel beds

The characteristics of the spatially averaged (SA) turbulent kinetic energy (TKE) dissipation rate are studied experimentally in flows over water-worked gravel beds (WGBs) and a screeded gravel bed (SGB) measuring the instantaneous velocity field by the Particle Image Velocimetry system. To study the response of water work roughness to the TKE related parameters, the flow Froude numbers in both the WGB and SGB were maintained identical in the experiments. Owing to the action of water work, the surface gravels in the WGB are more spatially organized than those of the SGB, where they are randomly oriented, resulting in a higher roughness in the WGB than in the SGB. Examination of the second- and the third-order velocity structure functions reveals the evidence of an inertial subrange in both the WGB and SGB. Kolmogorov’s two-thirds and four-fifths laws are preserved within the inertial subrange, providing an accurate estimation of the TKE dissipation rate in both the beds. However, owing to the higher roughness in the WGB than in the SGB, the TKE and the scaling law parameters are greater in the former than in the latter case. Furthermore, to examine the effects of the shear Reynolds number on the TKE dissipation rate and the scaling law parameters, two additional experiments with different shear Reynolds numbers were conducted with the WGBs. The comparative results suggest that the SA TKE dissipation rates estimated from Kolmogorov’s two-thirds and four-fifths laws increase with an increase in the shear Reynolds number in the WGBs.The characteristics of the spatially averaged (SA) turbulent kinetic energy (TKE) dissipation rate are studied experimentally in flows over water-worked gravel beds (WGBs) and a screeded gravel bed (SGB) measuring the instantaneous velocity field by the Particle Image Velocimetry system. To study the response of water work roughness to the TKE related parameters, the flow Froude numbers in both the WGB and SGB were maintained identical in the experiments. Owing to the action of water work, the surface gravels in the WGB are more spatially organized than those of the SGB, where they are randomly oriented, resulting in a higher roughness in the WGB than in the SGB. Examination of the second- and the third-order velocity structure functions reveals the evidence of an inertial subrange in both the WGB and SGB. Kolmogorov’s two-thirds and four-fifths laws are preserved within the inertial subrange, providing an accurate estimation of the TKE dissipation rate in both the beds. However, owing to the higher rough...

Hydrodynamics of water-worked and screeded gravel beds: A comparative study

The turbulence characteristics within flows over water-worked gravel beds (WGBs) and screeded gravel beds (SGBs) were examined by measuring the instantaneous flow velocity field using a two-dimensional particle image velocimetry system. To compare the responses of a WGB and an SGB to velocity and various turbulence characteristics, the flow Froude number was kept identical for both the beds that remained immobile. The roughness structures of both the beds were measured using a laser scanner. The results showed that the bed surface roughness was higher in the WGB than in the SGB. However, the longest axis of the gravels of WGBs was oriented streamwise owing to the action of water work, but the gravels of SGBs were randomly poised. The distribution of bed roughness fluctuations was negatively skewed in the WGB and positively skewed in the SGB. Double averaging methodology was applied to analyze the flow parameters. In this paper, the vertical profiles of the double-averaged streamwise velocity and the turbulence parameters, specifically the spatially averaged (SA) Reynolds shear and normal stresses, form-induced shear and normal stresses, turbulent kinetic energy (TKE) and form-induced TKE fluxes, quadrant analysis of SA Reynolds shear stress, etc., are presented and analyzed critically by focusing on comparisons between a WGB and an SGB. A comparative study reveals that in the near-bed flow zone, the SGB underestimates the turbulence parameters compared to the WGB. Therefore, in order to represent the prototypical flow in laboratory, the experiments should be performed in a WGB.

Turbulent kinetic energy flux and budget in a water-worked gravel bed

Turbulent flow over a water-worked gravel bed (WGB) was investigated using the double-averaging methodology (DAM). The flow measurements were carried out by the particle image velocimetry (PIV) technique. The double-averaged (DA) turbulent characteristics (DA Turbulent kinetic energy (TKE) components, form-induced TKE components, DA TKE fluxes, form-induced TKE fluxes, DA TKE budget) were analyzed for the WGB. To understand the effect of changed bed topography on the turbulent characteristics, the flow measurements were carried out over a screeded gravel bed (SGB), keeping the flow Froude number same as in case of WGB. Owing to water work, the bed topography of WGB was dissimilar to that of SGB, resulting in higher roughness size for the former than that for the latter. Comparative study of the DA turbulent characteristics of both the beds infers that especially in the near-bed flow zone, the flow parameters of the WGB are attaining higher values than those of the SGB. However, they are almost alike for both the beds in the flow outer layer.

Laboratory studies on bedload transport under unsteady flow conditions

Abstract Two sets of triangular hydrographs were generated in a 12-m-long laboratory flume for two sets of initial bed conditions: intact and water-worked gravel bed. Flowrate ranging from 0.0013 m3 s-1 to 0.0456 m3 s-1, water level ranging from 0.02 m to 0.11 m, and cumulative mass of transported sediment ranging from 4.5 kg to 14.2 kg were measured. Then, bedload transport rate, water surface slope, bed shear stress, and stream power were evaluated. The results indicated the impact of initial bed conditions and flow unsteadiness on bedload transport rate and total sediment yield. Difference in ratio between the amount of supplied sediment and total sediment yield for tests with different initial conditions was observed. Bedload rate, bed shear stress, and stream power demonstrated clock-wise hysteretic relation with flowrate. The study revealed practical aspects of experimental design, performance, and data analysis. Water surface slope evaluation based on spatial water depth data was discussed. It was shown that for certain conditions stream power was more adequate for the analysis of sediment transport dynamics than the bed shear stress. The relations between bedload transport dynamics, and flow and sediment parameters obtained by dimensional and multiple regression analysis were presented.

Measurement of Gravel-Bed Topography: Evaluation Study Applying Statistical Roughness Analysis

In this two-part study, experiments are conducted to evaluate available topography measurement techniques for gravel beds in a laboratory flume and to study their suitability for statistical roughness analysis. The available instruments for this study include (1) an acoustic bed profiler; (2) a hand-held laser scanner; and (3) two digital consumer cameras forming a stereophotogrammetric system, and are employed to obtain digital elevation models (DEMs) of water-worked gravel beds. In the first part of the study, the three measurement techniques are reviewed and their feasibilities for future grain-scale roughness work assessed, based on the obtained elevation datasets. Water-worked gravel-bed topographies are measured with all three available measurement techniques. The analysis of the DEMs concentrates on using probability distribution functions (PDFs) and second-order structure functions of bed elevations. Roughness coefficients are determined and used as a benchmark for comparison of the three measurement techniques. Although, visually, differences in the DEMs obtained with different measurement techniques are observed, the results of the chosen statistical analysis do not disclose the visual differences to the same extent. It is shown that the used stereophotogrammetric system, although theoretically allowing a fast and high-resolution recording process, lacks behind in accuracy. Thus, the second part of the study identifies and presents steps to improve the quality of the obtained stereophotogrammetric DEMs. A checklist is provided, highlighting the improvements made in the follow-up study, in order to obtain a high-quality stereophotogrammetric DEM. The overview will be useful for other researchers to make use of available low-cost and high-quality consumer camera equipment, to setup their own, non-proprietary stereophotogrammetric system.

Does flow variance affect bedload flux when the bed is dominated by grain roughness?

Previous studies have shown that spatial variance in fluid and critical shear stress, caused by form roughness, can increase bedload flux. Others have revealed that variance in flow velocity increases with relative submergence and that bed mobility is reduced at lower submergences. The paper explores the link between these observations and addresses the following questions: is grain roughness sufficient to cause variance in fluid shear stress and an increase in bedload flux; if this variance changes with submergence, does this mean that the increase is dependent on submergence; and does this explain the change in mobility with submergence? A simple, statistical bedload model, based on spatial distributions of fluid and critical shear stress, has been used to explore these effects over a water-worked gravel deposit. Estimates of spatially distributed fluid shear stress were gained from laboratory flume measurements of near-bed flow velocity, and a distribution of critical shear stress was simulated using a discrete particle model of the sediment distribution used in the flume. The velocity data were used to describe the change in the spatial distribution of near-bed velocity with relative submergence, which allowed the effects of submergence on flux to also be considered. The main conclusions were: (i) spatial variance in fluid shear stress from grain roughness was not sufficient to have an appreciable effect on bedload flux over water-worked gravel beds with a spatial distribution of critical shear stress; (ii) spatial variance in critical shear stress, caused by grain roughness, had a much larger influence and increased bedload flux. This was a similar level of increase observed in studies for conditions where form roughness was high; (iii) spatially averaged estimates of fluid and critical shear stress should not be used to estimate bedload flux even if form roughness is low; and (iv) a rise in relative submergence increased bedload flux. This was due to changes in the spatial distribution of near-bed velocity and not due to a lowering in the local flow velocity as has been suggested by previous studies.

Water‐worked gravel beds in laboratory flumes – a natural analogue?

AbstractAn investigation has been conducted to identify the key parameters that are likely to scale laboratory sediment deposits to the field scale. Two types of bed formation were examined: one where sediment is manually placed and screeded and the second where sediment is fed into a running flume. This later technique created deposits through sequential cycles of sediment transport and deposition. Detailed bed surface topography measurements have been made over a screeded bed and three fed beds. In addition, bulk subsurface porosity and hydraulic conductivity have been measured. By comparing the four beds, results revealed that certain physical properties of the screeded bed were clearly different from those of the fed beds. The screeded bed had a random organization of grains on both the surface and within the subsurface. The fed beds exhibited greater surface and subsurface organization and complexity, and had a number of properties that closely resembled those found for water‐worked gravel beds. The surfaces were water‐worked and armoured and there was preferential particle orientation and direction of imbrication in the subsurface. This suggested that fed beds are able to simulate, in a simplified manner, both the surface and subsurface properties of established gravel‐bed river deposits. The near‐bed flow properties were also compared. It revealed that the use of a screeded bed will typically cause an underestimation in the degree of temporal variability in the flow. Furthermore, time‐averaged streamwise velocities were found to be randomly organized over the screeded bed but were organized into long streamwise flow structures over the fed beds. It clearly showed that caution should be taken when comparing velocity measurements over screeded beds with water‐worked beds, and that the formation of fed beds offers an improved way of investigating intragravel flow and sediment–water interface exchange processes in gravel‐bed rivers at a laboratory scale. Copyright © 2008 John Wiley & Sons, Ltd.

The spatial organisation of time-averaged streamwise velocity and its correlation with the surface topography of water-worked gravel beds

An examination was made into the spatial pattern of time-averaged streamwise velocity in the near-bed region over two water-worked gravel beds. Laboratory observations revealed that there is considerable spatial variability in velocity. It was organised into streamwise streaks of high-speed fluid which were overlain by spots of low-speed fluid. This spatial pattern was found to be consistently and heavily dependent on relative submergence. The spatial pattern of velocity was shown to have little linear coherence with bed surface topography at the grain-scale. It suggested that for flows above the two beds studied here, bed surface topography at the grain-scale exerted less of an influence on the spatial organisation of time-averaged streamwise velocities than relative submergence.