Bedload Transport Rates Research Articles

Experimental Study of Bedform Development Characteristics and Their Effects on Sediment Transport

Bedforms are complex and varied features on riverbeds created by the interaction between flow and sediment particles. Bedform evolution is crucial for understanding sediment transport, bed resistance, and the ecological environment of rivers. In this study, a series of movable-bed dune experiments were conducted utilizing advanced 3D topographic laser scanning technology to precisely measure the 2D and 3D dune bedforms under various conditions. This enabled the detailed analysis of the bedform parameters, including the dune length and height, the crest height, and the trough depth. Furthermore, this study examined the relationship between bed resistance and dune morphology and investigated how the intensity of bedload transport affects bed resistance. The findings indicate that bedload mass transport on the riverbed significantly contributes to increased bed resistance. Given the limitations of the traditional bedload transport rate formulas for dune bedforms, this research introduces a new formula derived from the experimental data. The accuracy of this formula was confirmed using the nonlinear least squares method. Our study enhances the accuracy of sediment transport predictions and provides scientific support for river management and engineering practices.

Evaluating the Effect of Morphologic Units on Fractional Sediment Mobility and Bedload Transport in a Small Pool‐Riffle Reach

AbstractThis study examines the spatial pattern of fractional sediment mobility and assesses the influence of morphologic units on bedload transport in a small pool‐riffle reach with limited supply. Using a 2D hydraulic model and a subsurface‐based sediment transport model, shear stresses, fractional sediment mobility, and bedload transport were examined for flow events ranging in magnitude between 0.2Qbf and 1.5Qbf. Results reveal that while spatial variations in shear stress decrease as discharge increases, only a small proportion of the bed experiences high transport rates. At the reach scale, riffles are the primary morphological unit contributing to fully mobile sediment for all size fractions. However, at a subunit scale, there is evidence of sediment transport reversal for grains >32 mm at flows near or exceeding bankfull discharge in association with shear stress reversal. These transport reversals are important for maintaining pools despite their infrequent occurrence in the study reach. Sediment transport maps indicate that bed morphology considerably influences sediment transport at low to moderate flows. During these events, the shear stress is sensitive to local bed topography and partial mobility is the dominant transport process. In contrast, variations in bedload transport rates decrease during high flows when the flow is less sensitive to variations in bed topography and the bed becomes fully mobile.

Measurement and Calculation of Sediment Transport on an Ephemeral Stream

Sediment transport remains a significant challenge for researchers due to the intricate nature of the physical processes involved and the diverse characteristics of watercourses worldwide. A type of watercourse that is of particular interest for study is the ephemeral streams, found primarily in semiarid and arid regions. Due to their unique nature, a new measurement algorithm was created and a modified bed load sampler was built. Measurement of the bed load transport rate and calculation of the water discharge were conducted in an ephemeral stream in Northeastern Greece, where the mean calculated streamflow rate ranged from 0.019 to 0.314 m3/s, and the measured sediment load transport rates per unit width varied from 0.00001 to 0.00213 kg/m/s. The sediment concentration was determined through various methods, including nonlinear regression equations and formulas developed by Yang, with the coefficients of these formulas calibrated accordingly. The results demonstrated that the equations derived from Yang’s multiple regression analysis offered a superior fit compared to the original equations. As a result, two modified versions of Yang’s stream sediment transport formulas were developed and are presented to the readership. To assess the accuracy of the modified formulas, a comparison was conducted between the calculated total sediment concentrations and the measured total sediment concentrations based on various statistical criteria. The analysis shows that none of Yang’s original formulas fit the available data well, but after optimization, both modified formulas can be applied to the specific ephemeral stream. The results indicate also that the formulas derived from the nonlinear regression can be successfully used for the determination of the total sediment concentration in the ephemeral stream and have a better fit compared to Yang’s formulas. The correlation from the nonlinear regression equations suggests that total sediment transport is primarily influenced by water discharge and rainfall intensity, with the latter showing a high correlation coefficient of 0.998.

Plant Morphology Impacts Bedload Sediment Transport

AbstractBedload sediment transport plays an important role in the evolution of rivers, marshes and deltas. In these aquatic environments, vegetation is widespread, and plant species have unique morphology. However, the impact of real plant morphology on flow and sediment transport has not been quantified. This study used model plants with real plant morphology, based on the aquatic species Phragmites australis, Acorus calamus and Typha latifolia. The frontal area of these species increases away from the bed, which leads to higher near‐bed velocity than would be predicted from depth‐average frontal area. A plant morphology coefficient was defined to quantify the impact of vertically‐varied plant frontal area. Laboratory experiments confirmed that the plant morphology coefficient improved the prediction of near‐bed velocity, near‐bed turbulent kinetic energy and bedload transport rate in canopies with realistic morphology. Plant morphology can alter transport rates by up to an order of magnitude, relative to the assumption of uniform morphology.

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

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

Distinctive hydrodynamic properties and ecological responses of multi-thread rivers under different degrees of multiplicity in the Upper Yellow River

Alluvial rivers that exhibit multi-thread patterns are common in nature and can be the dominant channel morphology in large rivers. However, their ecological properties in response to diverse and dynamic channel morphology has gained limited attention and remained poorly understood. In this study, we adopted an eco-hydraulic model by integrating a hydrodynamic, a sediment-transport, and a habitat-suitability model to assess habitat quality for fish species (Schizopygopsis pylzovi and Platypharodon extremus) in three anabranching reaches with each exhibiting a distinct anabranching morphology in the Upper Yellow River, eastern Qinghai-Tibet Plateau. Based on the hydrologic data and actual channel morphology, we modeled the hydrodynamic and sediment-transport conditions for a period spanning ten years, and simulated habitat conditions under a potentially changing environment with different flow magnitudes and frequencies. The results indicated that the average flow velocity in the low and mid-order anabranching reaches is higher than that in the high-order, complex anabranching reaches. Meanwhile, the bedload transport rate was higher in the high and mid-order anabranching reaches than that in the low-order anabranching reach, demonstrating a greater transport efficiency of multi-thread systems with a greater multiplicity. Consequently, the habitat suitability shows a deteriorating trend over the ten-year modeling period and Schizopygopsis pylzovi shows better habitat status than Platypharodon extremus. The flow magnitudes and frequency also have a significant impact on the distribution of high habitat suitability index among the different river patterns in Upper Yellow River. This study can provide valuable information to optimize ecological outcomes and provide valuable insights for future dam operation strategies and consideration efforts aimed at preserving and restoring riverine ecosystems.

Spring erosional processes and small sapping valleys in southwestern lower Michigan, USA

AbstractTributary valleys along the Pigeon River in southwestern Michigan USA possess many previously defined criteria for sapping valleys. Direct observations of seasonal variations in groundwater levels, hydraulic gradients and water temperatures confirm that perennial springs and their streams are present in these small valleys. Estimated spring‐fed stream discharges are low and variable, but continuous. Direct observations, grain‐size distributions and bedload transport estimates indicate that significant transport of fine to medium sand occurs in all seasons.We present a process‐form model for small‐scale sapping processes. Stream terraces and paleo‐meander scarps are being modified by sapping, and elongated valleys are likely being lengthened headward by groundwater sapping. Sapping processes have been occurring for at least the past ~4,500 years after Pigeon River adjusted to a fall in base level from the Nipissing high stand to the modern level of Lake Michigan. Sapping processes are currently producing a variety of small‐scale landforms dependent on four main factors: 1) hydraulic gradients and spring discharges, 2) available grain size of material being eroded, 3) transport rate of sand bedload by spring‐fed streams and finally 4) time available for groundwater sapping process to occur.

Bedload transport fluctuations, flow conditions, and disequilibrium ratio at the Swiss Erlenbach stream: results from 27 years of high-resolution temporal measurements

Abstract. Based on measurements with the Swiss plate geophone system with a 1 min temporal resolution, bedload transport fluctuations were analysed as a function of the flow and transport conditions in the Swiss Erlenbach stream. The study confirms a finding from an earlier event-based analysis of the same bedload transport data, which showed that the disequilibrium ratio of measured to calculated transport rate (disequilibrium condition) influences the sediment transport behaviour. To analyse the transport conditions, the following elements were examined to characterise bedload transport fluctuations: (i) the autocorrelation coefficient of bedload transport rates as a function of lag time (memory effect), (ii) the critical discharge at the start and end of a transport event, (iii) the variability in the bedload transport rates, and (iv) a hysteresis index as a measure of the strength of bedload transport during the rising and falling limb of the hydrograph. This study underlines that above-average disequilibrium conditions, which are associated with a larger sediment availability on the streambed, generally have a stronger effect on subsequent transport conditions than below-average disequilibrium conditions, which are associated with comparatively less sediment availability on the streambed. The findings highlight the important roles of the sediment availability on the streambed, the disequilibrium ratio, and the hydraulic forcing in view of a better understanding of the bedload transport fluctuations in a steep mountain stream.

An improved formula for bed-load rate in open channel flows with emergent vegetation

There is an urgent need to predict the bed-load transport rate in vegetated river ecosystems to support restoration efforts. In response, we have developed a novel model for estimating the effective shear stress acting on the riverbed. This model is based on the energy equation and considers the intrinsic relationship between energy loss in the mean flow and turbulence generated by vegetation in open channel flows with emergent vegetation. Using this bed shear stress model, we assessed the performance of the Meyer-Peter–Müller (MPM) formula in predicting the bed-load transport rate in vegetated flows by comparing it with collected literature experimental data. The results revealed that the MPM formula does not provide accurate predictions. It tends to overestimate the bed-load transport rate when the dimensionless effective shear stress is approximately less than one and underestimate them when the dimensionless effective shear stress is approximately greater than one. This suggests that vegetation enhances and decreases the sediment transport rate when the dimensionless effective shear stress is approximately larger or lower than one, respectively. Consequently, we modified the coefficients of the MPM formula using extensive experimental data, leading to the development of a novel predictive formula for the bed-load transport rate in vegetated flows. This new formula outperforms existing literature equations and is effective for predicting the bedload transport rate, even for umbrella-like vegetation.

Re-examination of the effect of Reynolds number on local scour around a pipeline under steady current

Herein, a two-dimensional numerical model was established to examine the effect of Reynolds number on local scour around a pipeline under steady current by solving the Naiver-Stokes equation with shear stress turbulence (SST) k-ω closure. Suspended load and bedload transport rates were considered in the model. The numerical results revealed that the Reynolds number can considerably affect the local scour around the pipeline. However, earlier works often ignored the effect of Reynolds number on local scour. An empirical equation for the variations in the maximum scour depth with Reynolds number was presented based on present study.

Influence of Sediment Supply Timing on Bedload Transport and Bed Surface Texture During a Single Experimental Hydrograph in Gravel Bed Rivers

AbstractChannel stability and sediment transport in gravel bed streams depend on temporally and spatially variable fluid forces, bed surface structures, armoring, and sediment supply/storage. Of particular interest here is the influence of sediment supply timing on bedload transport rate and grain size distribution, bed surface composition and channel morphology. We conducted flume experiments in a sediment feed flume with poorly sorted sediment. A symmetrical, identical stepped hydrograph was used with five different sediment feeding schemes: no feed, constant feed, rising‐limb only feed, falling‐limb only feed, and variable feed. The same sediment mass of 800 kg was fed during each experiment. Sediment transport rates ranged over five orders of magnitude regardless of feeding scheme. Clockwise hysteresis was observed for bedload transport rate and bedload grain size, that is, the transport rate was larger and coarser during the rising limb. Counterclockwise hysteresis was observed for the grain size distribution of the bed surface, that is, the bed surface was finest during the rising limb. In all experiments sediment yield during the rising was higher than during the falling limb, indicating that the rising limb is more capable to transport the supplied sediment. Our study provides insight on how timing of sediment supply influences sediment transport and bed surface during a single hydrograph, essential information for artificial sediment supply projects to restore and habilitate gravel bed streams.

Sediment Transport Beneath a Simulated Partial Ice Cover: Effects of Asymmetric Border Ice

With the onset of winter in cold regions, border ice begins to form, impacting sediment transport rate and distribution. Understanding the effect of ice cover is crucial in regions with prolonged sub-freezing temperatures, as water bodies remain frozen for a considerable part of the year. While the existing literature includes many studies on sediment transport in open channel flow and several studies on completely ice-covered flow, there is limited research on sediment transport in partially ice-covered channels. Addressing this gap, the current study conducted laboratory experiments in a rectangular flume at the Hydraulics Research and Testing Facility, University of Manitoba, Canada. The investigation focused on examining the influence of asymmetric border ice, varying coverage ratios in asymmetric partially ice-covered flow, and changing flow strengths on bedload transport rate and distribution. Additionally, a comparison was made with symmetric partially ice-covered flow conditions. The findings indicate that the presence of asymmetric border ice indeed affected the bedload transport distribution within the channel, causing non-uniform bedload transport distribution across the channel width, with peak values concentrated in the center of the open flow section. Increased asymmetry in border ice leads to greater asymmetry in bedload transport rate distribution. Despite these pronounced differences in bedload transport rate across the channel width, the cross-section-averaged bedload transport rate could still be estimated utilizing the conventional equations, used for open channel flow, fully ice-covered flow, and symmetric partially ice-covered flow, with the effect of ice cover accounted for by incorporating an additional boundary in the calculation of the wetted perimeter, leading to adjustments in the flow strength.

Numerical Study of the Flow and Blockage Ratio of Cylindrical Pier Local Scour

A three-dimensional large eddy simulation model is used to simulate the turbulent flow dynamics around a circular pier in live-bed and clear-water scour conditions. The Navier–Stokes equations are transformed into a σ-coordinate system and solved using a second-order unstructured triangular finite-volume method. We simulate the bed evolution by solving the Exner-Polya equation assisted by a sand-slide model as a correction method. The bedload transport rate is based on the model of Engelund and Fredsœ. The model was validated for live-bed conditions in a wide channel and clear-water conditions in a narrow channel against the experimental data found in the literature. The in-house model NSMP3D can successfully produce both the live-bed and clear-water scouring throughout a stable long-term simulation. The flow model was used to study the effects of the blockage ratio in the flow near the pier in clear-water conditions, particularly the contraction effect at the zone where the scour hole starts to form. The scour depth in the clear water simulations is generally deeper than the live-bed simulations. In clear-water, the results show that the present model is able to qualitatively and quantitatively capture the hydrodynamic and morphodynamic processes near the bed. In comparison to the wide channel situation, the simulations indicate that the scour rate is faster in the narrow channel case.

Improved XBeach model and its application in coastal beach evolution under wave action

ABSTRACT Beach evolution is one of the key issues in coastal engineering. High computational efficiency model of beach evolution is limited by strong wave nonlinearity, flow acceleration, and sediment phase lag. The XBeach model is improved to cover the three limitations in the study. First, the near-bed velocity of nonlinear wave is reconstructed by using nonlinear parameters. Second, flow acceleration and sediment phase lag are taken into account for the near-bed sediment conditions to calculate the equilibrium concentration of suspended sediment and the transport rate of bed load. The response of sediment is not necessarily instantaneous to near-bed velocity in practical problems, but separated into a phase (time) shift between sediment concentration and velocity and a phase residual denoting sediment amount in moving at flow reversal. The improved XBeach model performs better results than the original XBeach model. Numerical experiments are carried out using the improved XBeach model to investigate the effects of wave height and sediment size. The numerical experiments show that the volume and range of erosion/deposition increase with the increase of wave height and decrease of sediment size, and deposition center moves to offshore. The perspective of wave energy and phase lag can explain the erosion/deposition variation.

Non-Equilibrium Bedload Transport Model Applied to Erosive Overtopping Dambreach

Bedload sediment transport is an ubiquitous process in natural surface water flows (rivers, dams, coast, etc), but it also plays a key role in catastrophic events such as dyke erosion or dam breach collapse. The bedload transport mechanism can be under equilibrium state, where solid rate and flow carry capacity are balanced, or under non-equilibrium (non-capacity) conditions. Extremely transient surface flows, such as dam/dyke erosive collapses, are systems which always change in space and time, hence absolute equilibrium states in the coupled fluid/solid transport rarely exist. Intuitively, assuming non-equilibrium conditions in transient flows should allow to estimate correctly the bedload transport rates and the bed level evolution. To get insight into this topic, a 2D Finite Volume model for bedload transport based on the non-capacity approach is proposed in this work. This non-equilibrium model considers that the actual bedload sediment discharge can be delayed, spatial and temporally, from the instantaneous solid carry capacity of the flow. Furthermore, the actual solid rate and the adaptation length/time is governed by the temporal evolution of the bedload transport layer and the vertical exchange solid flux. The model is tested for the simulation of overtopping dyke erosion and dambreach opening cases. Numerical results seems to support that considering non-equilibrium conditions for the bedload transport improves the general agreement between the computed results and measured data in both benchmarking cases.

Influence of bed surface structure on bedload transport in mountain rivers

AbstractMountain rivers are characterized by wide grain size distributions and complex bed surface structures, which significantly affect bedload transport. Owing to the lack of a clear understanding of the quantitative influence of the bed surface structure on the bedload transport rate, existing methods for estimating the bedload transport rate in mountain rivers produce large errors. Based on theoretical analysis and flume experiments, this study reveals the influence of bed surface structure on nonuniform bedload transport and proposes a method for estimating bedload transport rate considering the quantitative influence of bed surface structure. The findings of the present study provide theoretical methodological support for predicting the sediment transport and bed evolution in mountain rivers.

Development of a machine learning model for river bed load

Abstract. Prediction of bed load sediment transport rates in rivers is a notoriously difficult problem due to inherent variability in river hydraulics and channel morphology. Machine learning (ML) offers a compelling approach to leverage the growing wealth of bed load transport observations towards the development of a data-driven predictive model. We present an artificial neural network (ANN) model for predicting bed load transport rates informed by 8117 measurements from 134 rivers. Inputs to the model were river discharge, flow width, bed slope, and four bed surface sediment sizes. A sensitivity analysis showed that all inputs to the ANN model contributed to a reasonable estimate of bed load flux. At individual sites, the ANN model was able to reproduce observed sediment rating curves with a variety of shapes without site-specific calibration. This ANN model has the potential to be broadly applied to predict bed load fluxes based on discharge and reach properties alone.

Characteristics and selective mobility of bedload sheets

Bedform-driven flux is a fundamental component of bedload transport along riverbeds, resulting in unsteady transport rates. For bedload sheets, these fluctuations emerge through the preferential sorting of coarser grains in the leading front of the bedform. This study establishes key characteristics of bedload sheets formed in a sand-dominated unimodal sediment and, using fractional transport rates, determines the state of grain mobility to evaluate a proposed bedform regime (Venditti et al., 2017). A series of 10 flume experiments were executed using a water-worked sediment bed, with Shields numbers ranging from 0.037 to 0.15. Sediment supply was set by operating the flume in sediment recirculation mode. For each run bedform dimensions and migration rate were measured as well as hydraulic conditions. Sampling and analysis of bedload and bed sediment were used to derive fractional transport rates and grain size distributions to determine scaled transport rates and transport-supply ratios. Bedload sampling partitioned transport into bedload sheets and the load between sheets. Bedload sheets initially formed at a Shields number equal to 0.056 and persisted to a value of 0.13. Over this range, sheets grew in size and frequency, migrated faster, and produced elevated bedload transport rates that were consistently coarser than the flux between sheets. Scaled transport rates indicate selective mobility when sheets existed but this state of grain mobility does not uniquely define bedload sheets. Nonetheless, the mobility of larger grains relative to those in the surface sediment and the sediment supply conditions during runs fall within the bounds that define bedload sheets in the bedform regime diagram evaluated. Incorporating these conditions for bedload sheets into fractional bedload transport rates could improve predictions of bedload fluxes.

Statistical Roughness Properties of the Bed Surface in Braided Rivers

Braided rivers are widespread in nature, and their bed morphology is complex and variable. This paper aims to investigate and quantitatively analyze the bed surface roughness of braided rivers utilizing statistical theory. In this paper, a physical model of braided rivers is developed, and four constant discharge experiments are carried out. Based on Structure-from-Motion photogrammetry and direct measurement of bedload transport using a load cell, data on bedload transport rate, bed morphology, and bed elevation are obtained, facilitating the in-depth investigation of the correlations between these parameters. The results show that the morphological active width increases with increasing discharge. There was a significant positive correlation between the morphological active width and the bedload transport rate, although there is considerable scatter due to the inherent variability in braided river morphodynamics. The elevation probability distribution of bed surfaces shows negative skewness and leptokurtic distribution. There is a relatively significant correlation between skewness and the dimensionless bedload transport rate. The two-dimensional variogram values of bed elevation are variable, and the bed is anisotropic. Additionally, both the longitudinal sill and correlation length values exhibit an increase with the rise in stream power. Remarkably, the correlation between the dimensionless sill and correlation length, as well as the dimensionless bedload transport rate, proves to be highly significant. Consequently, this correlation can serve as a reliable general factor for predicting bedload transport rate in the reach.

Experimental investigation of sediment transport in partially ice-covered channels

It is important to understand the effects of ice cover on sediment transport in cold climates, where sub-freezing temperatures affect water bodies for a significant part of the year. The literature contains many studies on sediment transport in open channel flow, and several studies on sediment transport in completely ice-covered flow. There has been little or no research on sediment transport in partially ice-covered channels. In the current study, laboratory experiments were done in a rectangular flume to quantify the impact of border ice presence on the sediment transport rate. The effects of ice cover extent and changing flow strengths on sediment transport distribution also were investigated, and the results were compared to those for fully ice-covered and open channel flow. The ice coverage ratios considered were 0 (representing the open water condition), 0.25, 0.50, 0.67, and 1 (representing fully ice-covered flow). The partial ice cover was found to impact the sediment transport distribution within the channel. The effect of ice coverage extent on sediment transport distribution was more significant at lower flow strengths and became negligible at higher flow strengths. The conventional equations for sediment transport in open channel flow and fully ice-covered flow that relate the dimensionless bedload transport rate to the flow strength were found to be applicable to estimate the total cross-section-averaged bedload transport for partially ice-covered flow when modified appropriately. Empirical coefficients for these equations were determined using the experimental data.