Bedload Transport Rates Research Articles

Sediment Transport in River Models with Overbank Flows

Some laboratory sediment-transport experiments are described in which a compound channel with a mobile-bed composed of uniform sand with a d50 of 0.88mm was subjected to overbank flows. The main river channel was monitored to determine the effect of floodplain roughness on conveyance capacity, bed-form geometry, resistance, bed-load transport, and dune migration rate. The floodplain roughness was varied to simulate a wide range of conditions, commensurate with conditions that can occur in a natural river. For a given discharge, the main river channel bed was found to adjust itself to a quasi-equilibrium condition governed by the lateral momentum transfer between the floodplain and main channel flows and the local alluvial resistance relationship appropriate for the proportion of total flow in the main river channel. The sediment transport rate was found to reflect all these influences. The data are summarized in equation form for comparison with other experimental studies and for checking numerical river simulation models.

Role of pressure gradients in sheet flow of coarse sediments under sawtooth waves

Discrete particle model (DPM) simulations have been performed to directly examine the role of horizontal pressure gradients acting on the wave bottom boundary layer (WBBL) during the process of sheet flow transport in the surf zone. The DPM is a two‐phase WBBL model that directly couples a one‐dimensional eddy‐viscosity fluid flow to a three‐dimensional particle model. Newton's Third Law is enforced at every simulation time step through fluid‐particle interaction forces of buoyancy, drag, and added‐mass. Simulations are able to reproduce bedload transport rates from a laboratory data set for coarse sediment distributions. Consequently, simulations of a monochromatic sawtooth wave were performed with three coarse grain size distributions to investigate the relative importance of bed shear stresses, horizontal pressure forces, and fluid‐particle drag forces on bedload transport processes. The simulation results demonstrate that the magnitude of the horizontal pressure force acting directly on sediment embedded in the WBBL is small compared to the magnitude of the particle drag force or particle stress; however, the phasing of the onshore peak in the horizontal pressure force with respect to the particle drag force is important to sediment mobilization and enhances the observed onshore bias in bed load flux. Removing the horizontal pressure force on the sediment particles resulted on average in a 30% reduction in net bed load flux. In the surf zone, free stream fluid accelerations are typically equated to the horizontal pressure gradient. Thus, the simulations may explain why the fluid acceleration can be a successful parameter for aiding in sediment transport predictions in the surf zone.

Effect of sampling time on measured gravel bed load transport rates in a coarse‐bedded stream

This study empirically quantified the effect of sampling time on measured transport rates and fitted rating curves based on results obtained from bed load traps deployed for 2, 10, and 60 min in a coarse‐bedded stream. As expected for a skewed distribution of transport rates, 2 min deployment underpredicted transport rates obtained from 10 and 60 min deployment by factors of 2 and 3 at moderate flows (50–70% Qbkf). At near‐bank‐full flow the underprediction by 2 min versus 10 min sampling increased to a factor of 5, while transport rates collected during 60 min deployments were reduced because of overfilled bags. At flows near incipient gravel motion, 2 min sampling overpredicted transport rates obtained by 10 and 60 min deployment by factors of 2.7 and 3.4. The overprediction is attributed to computational effects arising mainly from the lowest measurable transport rate for each sampling time. Rating curves fitted to transport rates from 2 min sampling were significantly less steep than those for longer deployment times. However, sampling time explains only a small degree of the large difference between rating curves from bed load traps and a Helley‐Smith sampler (2 min sampling).

A flume experiment on the development of subaqueous fine‐gravel dunes from a lower‐stage plane bed

Determining the hydraulic conditions whereby gravel dunes first develop in subaqueous environments is fundamental as their presence may influence engineering solutions designed to maintain bed stability. In addition, estimates of the flow conditions associated with preserved gravel bed forms in sedimentary sequences are useful for reconstructing the depositional environments and geometries of, for example, oil‐ and gas‐bearing geological strata. Consequently, a series of experiments considered dune initiation. In these experiments, defects and latterly incipient dunes developed from lower‐stage plane gravel beds during near‐threshold conditions of motion (θ/θcrit = 1.0–1.016) and long periods of marginal bed load transport rates. The three‐dimensional defects were almost imperceptible positive ovoid features with heights of one or two grain diameters and lengths and spans of a few decimeters. After 17 hours of flow, incipient, low‐amplitude, simple two‐dimensional dunes developed from the defects, with heights ranging between 0.029 and 0.055 m, nonequilibrium wavelengths of 1–4 m and spans of 0.6–0.9 m. Continued development over several days, with θ/θcrit ratios of around 1.3, resulted in near‐equilibrium two‐dimensional dunes with wavelengths averaging 2.6–3.5 m and spans equal to the flume width (4 m). The inception of incipient dunes could be predicted using bulk flow models; however, this approach was not suitable for the prediction of defect development. Near‐bed turbulence, in the form of small‐scale sweep events of limited breadth, controls the initiation of defects, but larger‐scale, coherent turbulent structures in the outer flow are related to dune development. Significantly, both defects and incipient dunes can exist at the same time, which indicates that the effects of sweeps on the bed morphology persist at the same time as larger‐scale turbulent structures are beginning to effect sediment transport.

Application of spatial cross correlation to detection of migration of submarine sand dunes

Knowledge of migration rates of bedforms provides an indirect indication of the behavior of tidally averaged bottom currents, enables optimization of hydrographic survey frequency and may enable calculation of bedload transport rate. To measure bedform migration rate, we test the use of spatial correlation as a measurement method, which quantifies and locates a region of maximum similarity between two spatial variables. For the latter, we use consecutive eight‐bit images of spatial gradient, derived from bathymetric digital terrain models, carrying out the correlation over this representation of the shape of the seabed rather than the bathymetric surface. The digital terrain models were compiled from six repeat multibeam surveys of a headland‐associated bank near Saint John, New Brunswick, with a roughly 30‐day interval. Vectors are drawn depicting the movement of a sand dune at time t0 toward a point in the spatial correlation array at a later time, t1. A number of different techniques of picking the end of the migration vector were used. The sinuosity of the dune crest at the scale of the correlation window has an impact on which migration vector is the better pick. Averaging of migration vectors from consecutive epochs diminishes random errors in the correlation picks using any single pair of images and creates a more accurate picture of the migration field. Migration rates and crest‐relative migration directions vary substantially across the sand bank, reflecting the high gradients in bottom shear stress around the headland.

Comment on “A general power equation for predicting bed load transport rates in gravel bed rivers” by Jeffrey J. Barry et al.

Comment on “A general power equation for predicting bed load transport rates in gravel bed rivers” by Jeffrey J. Barry et al.

Reply to comment by Claude Michel on “A general power equation for predicting bed load transport rates in gravel bed rivers”

[1] We thank Michel [2005] for the opportunity to improve our bed load transport equation [Barry et al., 2004, equation (6)] and to resolve the dimensional complexity that he identified. However, we do not believe that the alternative bed load transport equation proposed by Michel [2005] provides either the mechanistic insight or predictive power of our transport equation. [2] Although some bed load transport data exhibit nonlinear trends in log-log plots of transport rate versus discharge, a simple linear function is sufficient to describe our data [Barry et al., 2004, paragraph 43]. The Figure 7 data of Barry et al. [2004] could be fit by a nonlinear function as suggested by Michel [2005], but we believe this to be an unnecessary complication, particularly given how well our simple equation predicts observed transport rates compared to other more complex equations, such as Parker's [1991] three-part bed load transport function [Barry et al., 2004, Figure 11]. Furthermore, an important aspect of our equation, that is not preserved in Michel's alternative, is the between-site variation in the exponent of the transport function that results from supply related channel armoring (i.e., transport capacity in excess of bed load sediment supply) which provides a mechanistic understanding of the bed load transport process [Barry et al., 2004]. Michel [2005] proposes a bed load transport equation that mimics our equation in terms of the range of exponents that we observe (i.e., 1.5–4 [Barry et al., 2004, Figure 8a] but lacks the mechanistic insight and consequent predictive power. Moreover, Michel's equation requires a sufficient number of bed load transport observations across a broad range of discharges to empirically calibrate his a and β values.

Morphologically based model of bed load transport capacity in a headwater stream

Field measurements of channel and bed material properties are used to develop a network‐based model of bed load transport capacity in Halfmoon Creek, a high‐gradient stream in the Rocky Mountains of Colorado. The formerly glaciated watershed contains self‐formed reaches that are generally disconnected from hillslopes and major sediment perturbations, strung together by a sequence of bedrock and cascade channels. Measurements of channel geometry, slope, and bed sediment texture (surface and substrate) were taken in 27 reaches with gradients ranging from 0.6 to 6% to examine interactions among key variables affecting bed load transport. The analysis shows that the grain size distribution of the substrate is relatively uniform throughout the basin; however, the distinction between surface and substrate texture varies systematically with channel gradient and shear stress. Reaches with high shear stress are characterized by coarse surface layers, with a high threshold for transport, limiting the mobility of the sediment supplied. In reaches with lower gradient and shear stress the difference between surface and substrate texture is less, increasing both the frequency of transport and the mobility of the bed load. While it might appear that the steeper reaches of Halfmoon Creek have a high capacity for carrying fine‐medium gravel, the relative mobility of these sizes is affected by the presence of very coarse sediment on the bed surface. As a result, modeled estimates of bed load transport rate indicate that average annual bed load sediment yields increase downstream nearly linearly with discharge, suggesting a simple scaling of bed load transport capacity with discharge and drainage area.

Coarse sediment transport in mountain streams in Colorado and Wyoming, USA

AbstractSince the early 1990s, US Forest Service researchers have made thousands of bedload measurements in steep, coarse‐grained channels in Colorado and Wyoming, USA. In this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes, including step–pool, plane‐bed, pool–riffle, and near‐braided channels. This effort builds upon previous work where we applied a piecewise regression model to (1) relate flow to rates of bedload transport and (2) define phases of transport in coarse‐grained channels. Earlier, the model was tested using bedload data from eight sites on the Fraser Experimental Forest near Fraser, Colorado. The analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied. The earlier results were, however, derived from data collected at sites that, for the most part, have quite similar geology and runoff regimes. In this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions. The results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs. The transition from primarily low rates of sand transport (phase I) to higher rates of sand and coarse gravel transport (phase II) occurs, on average, at about 80 per cent of the bankfull (1·5‐year return interval) discharge. A comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase I transport. Moreover, the movement and capture of the D16 to D25 grain size of the bed surface seems to correspond with the onset of phase II transport, particularly in systems with largely static channel surfaces. However, while there were many similarities in observed patterns of bedload transport at the 19 studied sites, each had its own ‘bedload signal’ in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system. Published in 2005 by John Wiley & Sons, Ltd.

05/00031 Production of carbon nanotubes from coal: Oiu, J. et al. Fuel Processing Technology, 2004, 85, (5), 1663–1670

Because bedload samplers typically protrude above the bedload transport zone, they inevitably trap some suspended sediment. For the case of bedload transport over dunes, sediment carried over the separation zone in suspension may be trapped by a bedload sampler that is placed in or near the trough. This sediment does not contribute to the downstream movement of the dune, and is consistently finer and better-sorted than at other locations over the dune. It causes distinctive temporal variability in the particle size distribution of bedload and may complicate the estimation of bedload transport rates.

Resistance to overland flow due to bed‐load transport on plane mobile beds

AbstractDuring bed‐load transport by overland flow, momentum is transferred from the flow to the bed via grain collisions, resulting in a decrease in flow velocity and an increase in flow resistance, herein termed bed‐load transport resistance. In overland flow on mobile plane beds, total flow resistance f consists of grain resistance fg and bed‐load transport resistance fbt. In order to identify and evaluate the relative importance of the factors controlling fbt, 38 flume experiments were performed on slopes of 2·7 and 5·5° using sediment with median diameters of 0·74 and 1·16 mm. All flows were supercritical and turbulent.This study is an extension of a recent study by Gao and Abrahams (Earth Surface Processes and Landforms 2004, vol. 29, pp. 423–435). These authors found that fbt is controlled by three factors: sediment concentration C, dimensionless sediment diameter D*, and relative submergence h/D, where h is flow depth, D is median sediment diameter. However, a new dimensional analysis identifies two additional factors: Froude number F and slope S. Multiple regression analyses reveal (1) that these five factors together explain 97 per cent of the variance of fbt, and (2) that S controls fbt entirely through C. The variable C is therefore redundant, and a new functional equation relating fbt to D*, h/D, S and F is developed. This equation may be used to predict fbt. An advantage of this equation is that it may be used to predict fbt without measuring bed‐load transport rate. Copyright © 2004 John Wiley & Sons, Ltd.

A general power equation for predicting bed load transport rates in gravel bed rivers

A variety of formulae has been developed to predict bed load transport in gravel bed rivers, ranging from simple regressions to complex multiparameter formulations. The ability to test these formulae across numerous field sites has, until recently, been hampered by a paucity of bed load transport data for gravel bed rivers. We use 2104 bed load transport observations in 24 gravel bed rivers in Idaho to assess the performance of eight different formulations of four bed load transport equations. Results show substantial differences in performance but no consistent relationship between formula performance and degree of calibration or complexity. However, formulae containing a transport threshold typically exhibit poor performance. Furthermore, we find that the transport data are best described by a simple power function of discharge. From this we propose a new bed load transport equation and identify channel and watershed characteristics that control the exponent and coefficient of the proposed power function. We find that the exponent is principally a factor of supply‐related channel armoring (transport capacity in excess of sediment supply), whereas the coefficient is related to drainage area (a surrogate for absolute sediment supply). We evaluate the accuracy of the proposed power function at 17 independent test sites.

Numerical Modeling of Bed Deformation in Laboratory Channels

A depth-average model using a finite-volume method with boundary-fitted grids has been developed to calculate bed deformation in alluvial channels. The model system consists of an unsteady hydrodynamic module, a sediment transport module and a bed-deformation module. The hydrodynamic module is based on the two-dimensional shallow water equations. The sediment transport module is comprised of semiempirical models of suspended load and nonequilibrium bedload. The bed-deformation module is based on the mass balance for sediment. The secondary flow transport effects are taken into account by adjusting the dimensionless diffusivity coefficient in the depth-average version of the k-e turbulence model. A quasi-three-dimensional flow approach is used to simulate the effect of secondary flows due to channel curvature on bed-load transport. The effects of bed slope on the rate and direction of bed-load transport are also taken into account. The developed model has been validated by computing the scour hole and the ...

Site specificity of bed load measurement using an acoustic Doppler current profiler

Concurrent measurements of bed load transport velocity (v) from the bottom tracking feature of an acoustic Doppler current profiler (aDcp) and bed load transport rate (gb) from conventional pressure difference samplers are presented. Data sets were collected from both gravel bed and sand bed reaches of the Fraser River, covering a bed material range of 0.25–25 mm. Strong relations, in which v explained >70% of the variability of measured gb, were observed in a gravel bed and a sand bed reach. Differences in correlation between v and gb among the contrasting environments are attributed to variations in both the bed load particle size and aDcp operating parameters. Similar values of v were associated with lower mass transport rates in a sand bed than in a gravel bed reach. A nondimensional data collapse, accounting for differences in bed load particle size, explained 42% of the observed variance of the combined data set. Longer averaging times were required in the gravel bed reach, likely due to the stochastic nature of bed load entrainment in gravels and the resulting heterogeneous bed load velocity field. Bed load transport was modeled using both shear stress models and a kinematic model that utilizes the estimated bed load velocity. The standard shear stress models provided poor matches to the measured bed load transport rates: the sand bed data were overpredicted, and the gravel bed predictions correlated poorly with the measured predictions. Use of the kinematic model yields an estimate for the product of bed load concentration and bed load layer depth. This work highlights the potential of acoustic techniques for estimating bed load.

Magnitude‐frequency analysis of bed load data in an Alpine boulder bed stream

The purpose of this study is to investigate the connection between bed load, channel processes, and sediment sources in mountain basins using data from the Rio Cordon basin (northeastern Italian Alps). The main channel is a steep, boulder bed, step pool stream, and bed load volumes are measured at a special facility where particles >20 mm are trapped. Results from a combined frequency analysis of peak water discharges and total bed load volumes based on 17 years of field data are presented, focusing on discrepancies between recurrence intervals of peak discharge and bed load volume for each event. A cause of major disturbance is a high‐magnitude, low‐recurrence event that occurred in 1994. Dimensional and nondimensional bed load intensity‐duration curves are also reported, which emphasize differences between short‐ and long‐duration events. The relationship flow–bed load rates (investigated using a total of 180 data) shows no breakpoints, and two different curves are evident, representing pre‐1994 and post‐1994 floods, respectively. The analysis demonstrates both the control exerted by sediment availability on bed load transport rates and the persisting long‐term impact of major floods on mountain streams.

Experimental investigation of bedload transport processes under unsteady flow conditions

AbstractHydraulic engineering is usually based on theoretical analysis and/or numerical modelling simulation. As the dynamic behaviour of sediment movement under unsteady flow is still unclear, and field measurement is comparatively difficult during a large flood, prior investigations through flume experiments are required. A series of flume experiments, conducted using different inflow hydrographs without sediment supply from upstream, was carried out to investigate the sediment transport process under unsteady flow conditions.A series of triangular hydrographs were performed in the experiments. The results indicate that a temporal lag was found between the flow hydrograph peak and the sediment hydrograph peak because large size sand dunes lasted for a short period in the falling limb of the flow hydrograph. The temporal lag was found to be about equal to 6–15% of the flow hydrograph duration. Owing to the temporal lag, the total bedload yield in the rising period was less than that in the falling period. Furthermore, the measured total bedload yield in the unsteady flow experiments was larger than the predicted value, which was estimated by using the results obtained from the equivalent steady flow experiment. The peak bedload transport rate for unsteady flow conditions was also larger than the predicted value. The ratios of the measured to the predicted quantities mentioned above were found to be constant values for different shapes of hydrographs. It is, therefore, expected that the analytical results of sediment transport from equivalent steady flow can be a good reference for sediment transport under unsteady flow conditions. Copyright © 2004 John Wiley & Sons, Ltd.

Sand Transport in Nile River, Egypt

Measurements of bed-load and suspended-load transport rates were carried out successfully at four cross sections of the Nile River, in Egypt, along the entire length from Aswan to Cairo using a mechanical sampler called the Delft Nile Sampler. The measured transport rates were compared to similar data sets from two other large scale rivers: the Rhine-Waal River in the Netherlands and the Mississippi River in the USA. The bed-load transport rates in the Nile River and in the Rhine-Waal River are in very good agreement. Comparison of suspended transport rates in the Nile River and in the Mississippi River shows that both data sets are complementary, revealing a very consistent trend of suspended transport against current velocity; suspended transport is roughly proportional to (Vav)3to4. Three formulas for the prediction of bed-load transport were tested using the Nile data: Meyer-Peter–Muller, Bagnold, and Van Rijn. The prediction formula of Van Rijn produced significantly better results than the other two formulas; the average relative error was about 60%. The formula of Van Rijn was modified to extend it to conditions with slightly nonuniform sediment mixtures by introducing a correction factor for the bed shear parameter. Based on a limited number of flume experiments, the correction factor was found to be dependent on the characteristics of the sediment mixture (d10,d50,d90, and σg). Comparison of bed-load transport measured in the Nile River with computed transport rates of the modified formula showed improved results; the average relative error decreased to about 30%. The formulas of Bagnold and Van Rijn were also used to compute the suspended transport rates in the Nile River. The computed transport rates were found to be within a factor of 2 of measured values; the formula of Bagnold performed slightly better. The total load transport formula of Engelund–Hansen was also successfully used (computed values within a factor of about 2 of measured values).

Microforms in Gravel Bed Rivers: Formation, Disintegration, and Effects on Bedload Transport

This research aims to advance current knowledge on cluster formation and evolution by tackling some of the aspects associated with cluster microtopography and the effects of clusters on bedload transport. The specific objectives of the study are (1) to identify the bed shear stress range in which clusters form and disintegrate, (2) to quantitatively describe the spacing characteristics and orientation of clusters with respect to flow characteristics, (3) to quantify the effects clusters have on the mean bedload rate, and (4) to assess the effects of clusters on the pulsating nature of bedload. In order to meet the objectives of this study, two main experimental scenarios, namely, Test Series A and B (20 experiments overall) are considered in a laboratory flume under well-controlled conditions. Series A tests are performed to address objectives (1) and (2) while Series B is designed to meet objectives (3) and (4). Results show that cluster microforms develop in uniform sediment at 1.25 to 2 times the Shields parameter of an individual particle and start disintegrating at about 2.25 times the Shields parameter. It is found that during an unsteady flow event, effects of clusters on bedload transport rate can be classified in three different phases: a sink phase where clusters absorb incoming sediment, a neutral phase where clusters do not affect bedload, and a source phase where clusters release particles. Clusters also increase the magnitude of the fluctuations in bedload transport rate, showing that clusters amplify the unsteady nature of bedload transport. A fourth-order autoregressive, autoregressive integrated moving average model is employed to describe the time series of bedload and provide a predictive formula for predicting bedload at different periods. Finally, a change-point analysis enhanced with a binary segmentation procedure is performed to identify the abrupt changes in the bedload statistic characteristics due to the effects of clusters and detect the different phases in bedload time series using probability theory. The analysis verifies the experimental findings that three phases are detected in the bedload rate time series structure, namely, sink, neutral, and source.

Bed load transport under irregular waves plus current from Monte Carlo simulations of parameterized models with application to ripple migration rates observed in the field

The bed load transport rate under random waves plus current has been predicted for a large range of wave–current conditions. A parameterized model valid for regular waves plus current has been used in Monte Carlo simulations, assuming the wave amplitudes to be Rayleigh distributed. The mean value, standard deviation and numerical estimates of the probability density function of the bed load transport rate are presented for a wide range of wave–current conditions. It appears that overall the effect of the current is dominating the bed load transport rate. Moreover, a significant scatter of the bed load transport rate under random waves plus current is found. Such a scatter is also found in field measurements by Amos et al. [J. Coast. Res. 15 (1999) 1]. Predicted ripple migration rates in the bed load regime have been compared with those obtained from field measurements by Amos et al. [J. Coast. Res. 15 (1999) 1], taking the bed load transport rate to be proportional to the ripple migration rate times the ripple height. Overall, the predictions capture the qualitative as well as the quantitative behaviour of the ripple migration rates in a wide range of wave–current conditions; the ripple migration rates, and thereby the bed load, are predicted within the correct order of magnitude for a wide range of wave–current conditions.

Supply limited sediment transport in a high‐discharge event of the tropical Burdekin River, North Queensland, Australia

AbstractInteractions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m3 s−1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m3 s−1). About 3·7 × 106 tonnes of suspended sediment and 3 × 105 tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river.