Graded Sediment Transport Research Articles

Numerical modeling of graded sediment transport with limited bed material supply – A case study in the Middle Yangtze River

Suspended sediment concentration in the Middle Yangtze River (MYR) has reduced sharply below the Three Gorges Dam since 2003, and led to significant channel degradation and riverbed coarsening. However, the complex interaction between riverbed coarsening and sediment transport is essential for channel evolution processes but is poorly understood. This paper presents a one-dimensional model for simulating graded sediment transport when the bed material supply is limited. The model takes account of the effect of bed material composition (BMC) and threshold probability on graded sediment transport, and thus improve the accuracy of recovery coefficient. The modeling results of graded sediment concentrations was calibrated and then satisfactorily verified using field observations in the JingJiang Reach from 2015 to 2016. The results showed bed material composition affect the simulated fine, medium, and coarse sediment concentrations, and the improvements of normalized errors are 17.6%, 45.9%, and 330.7%, respectively. With the BMC effect considered, the recovery factor of fine and medium were far less than coarse sediment in the MYR. In addition, the recovery ratios of medium and coarse sediment concentrations along the MYR were improved by 88.6%-265.4% and 75.1%-105.9% respectively, owing to the bed material supply after the TGD operation; however, there was no significant effect on fine sediment, with a low recovery ratio of 8.3%-8.6%. Therefore, the effect of bed material composition on graded sediment transport and channel evolution needs to be considered for higher simulation accuracy in the MYR.

Investigation of uniform and graded sediment wash-off in an urban drainage system: Numerical model validation from a rainfall simulator in an experimental facility

Understanding sediment wash-off in urban environments plays an essential role in sediment transport management; and is critical for accurate pluvial flood control to assist in adaptation and mitigation strategies. Sediment transport models have been researched previously, though challenges still arise due to the complicated nature of graded sediment transport. This study tested the accuracy of the van Rijn model using a sparse distribution of particle sizes using the geometric mean. As such, this study used high-resolution datasets collected in a water laboratory to investigate sediment wash-off and transport on an urban street. This included the interaction of two gully pots receiving sediment loads that were washed off from a hypothetical urban surface by three rainfall intensities. The results showed that the model was able to simulate uniform sediments entering the gully pots accurately when the sediment size was assigned to a median diameter. Using the grain diameter to represent the geometric mean can improve the model performance for simulating a graded sediment.

Analytical Model for Graded Sediment Transport in Velocity‐ and Acceleration‐Skewed Oscillatory Sheet Flow

AbstractAn analytical model is proposed for graded sediment transport in velocity‐ and acceleration‐skewed oscillatory sheet flow. The model consists of periodic velocity and sediment concentration formulae over a mobile sediment bed. An active layer is set below the mobile sediment bed level to handle erosion differences among size fractions. The model accounts for velocity phase lead, sediment phase lag, velocity and acceleration asymmetries, and gradation effects. The model is validated using datasets of velocity, concentration, sediment flux, and transport rate. Better performance is obtained by the model when compared to previous formulae. Discussion is conducted for net boundary layer flow, total, and fractional sediment fluxes under both velocity‐ and acceleration‐skewed oscillatory flows, and the result shows the existence of inhibited or promoted transport for different size fractions. The model is applied to describe the sediment size grading curve at different levels. The composition of transported sediment is different from the initial bed composition. The fining of sediment above the initial bed and the coarse below the initial bed are reproduced by the model, which reveals the vertical sorting processes at different locations. Meanwhile, interactions between fractions are substantial in the sheet flow layer but are insignificant in the suspension layer. The model is suitable for non‐cohesive sediment mixtures consisting of fine, medium, and coarse proportions and has implications for predicting nearshore‐graded sediment transport and sorting processes.

An Experimental Study on Progressive and Reverse Fluxes of Sediments with Fine Fractions in the Wave Motion over Sloped Bed

The purpose of the study was to collect experimental data on the vertical structure of sediment fluxes during the wave crest and trough phase over sloped bed. The first stage of the experimental work included measurements of these fluxes using the particle image method, while the second stage, measurements of sediment transport rates and granulometric distributions of sediments collected in the traps on both sides of the sloped initial area. The experimental data were compared both with the results collected previously over flat bed as well as with a theoretical analysis based on a three-layer model of graded sediment transport. This model does not take into account the effects related to the presence of fine and very fine fractions and neglects the effects related to the bed slope, i.e., to gravitational forces and to additional pressure gradients. Hence, a modification of this model is proposed that is based on four coefficients that corrected for sediment fluxes over sloped bed. The consistency of the sediment transport calculations according to the modified model with measurements was achieved within plus/minus a factor of 2 of the measurements.

Erosion depth and concentration profile formulae for graded sediment transport in asymmetric oscillatory sheet flow

The transport of graded sediments is much more complex and universal compared with that of uniform sediments. This paper proposes formulae of erosion depth and concentration profile for graded sediments in asymmetric oscillatory sheet flow. The erosion depth is obtained by taking the summation of each size fraction whilst considering the phase residual, phase shift and acceleration modification. The erosion depth includes a hiding-exposure factor of exposure height and a phase shift correction of gradation, which represent the selective transport (promotion or inhibition) of graded sediments. The sediment concentration for each size fraction is proposed using the idealised exponential or power function, which is based on erosion depth considering mass conservation. The upper sheet flow boundary and sheet flow layer thickness are further identified from the erosion depth and concentration profile. Experimental data for validation are recalculated by an improved method that considers mass conservation and actual stationary bed. The proposed formulae demonstrate a better performance in predicting the erosion depth and concentration profile of graded sediments compared with previous uniform and graded sediment formulae. The proposed formulae are able to identify the selective transport and grade recombination on mobile sediment bed. The hiding-exposure effect and phase lag discrepancy for different size fractions are also discussed.

An Experimental Study on Progressive and Reverse Fluxes of Sediments with Fine Fractions in Wave Motion

The purpose of the study was to collect experimental data on the vertical structure of sediment fluxes during the wave crest and trough phase. The first stage of the experimental work included measurements of these fluxes using the particle image method, while in the second stage, measurements of sediment transport rates and granulometric distributions of sediments were collected in the traps on both sides of the initial area. The experimental data were compared with the results of a theoretical analysis based on a three-layer model of graded sediment transport. The comparison of the calculations with the measurements was conducted separately for fluxes of fine and very fine fractions in the diameter range di < 0.20 mm, coarse, and total fractions all outgoing in the crest and trough phase from the initial area and deposited in adjacent control areas. As this model did not take into account both the effects of vertical mixing and the phase-lag effects related to the presence of fine and very fine fractions, a modification of this model was proposed that was based on four coefficients that corrected for fluxes. The consistency of the sediment transport calculations according to the modified model with measurements was achieved within plus/minus a factor of 2 of the measurements.

An integrated approach for investigating the correlation between floods and river morphology: A case study of the Saalach River, Germany

Man-made structures in the Saalach River have changed the hydromorphological characteristics of the river regime. In some river reaches, the Saalach has lost the high morphological versatility and high variation in sediment transport characteristic of a mountain river. Among the negative effects, an extreme flow discharge in combination with riverbed variation could be one of the possible causes of flood disasters along the river. For example, the heavy and long lasting rainfall in June 2013 led to a peak discharge of 1100 m3/s, which was slightly above the 100-year flood return period, inundating a nearby city. However, the influence of the man-made structures on this flood event in this reach is unclear. In this study an integrative hydromorphological model is applied to evaluate this impact by a comparison with a standard clear water model with fixed bed. Moreover, a comparative analysis of a three-and two-dimensional flow model is performed to assess the models suitability representing the flow in this river stretch. The integrative model concept is based on the software TELEMAC-MASCARET, in an enhanced version for better representing graded sediment transport in rivers. In contrast to our integrative model, the standard clear water model with fixed bed overestimates the water elevations as it cannot take the significant changes in morphology into account. Results demonstrate that our proposed model more accurately represents the inundation in the floodplain and could thus be used to provide more reliable predictions to decision-makers for improved flood protection strategy.

Simulation of the fate of faecal bacteria in estuarine and coastal waters based on a fractionated sediment transport model

A two-dimensional depth-integrated numerical model is refined in this paper to simulate the hydrodynamics, graded sediment transport process and the fate of faecal bacteria in estuarine and coastal waters. The sediment mixture is divided into several fractions according to the grain size. A bed evolution model is adopted to simulate the processes of the bed elevation change and sediment grain size sorting. The faecal bacteria transport equation includes enhanced source and sink terms to represent bacterial kinetic transformation and disappearance or reappearance due to sediment deposition or re-suspension. A novel partition ratio and dynamic decay rates of faecal bacteria are adopted in the numerical model. The model has been applied to the turbid water environment in the Bristol Channel and Severn estuary, UK. The predictions by the present model are compared with field data and those by non-fractionated model.

Experimental investigations of graded sediment transport under unsteady flow hydrographs

Natural fluvial channels can experience significant variations in sediment transport rates under unsteady flow conditions, especially during flood hydrograph events. At present, however, there is a distinct lack of understanding of the interaction between unsteady hydrograph flow properties and temporal variability in graded sediment transport rates. In the current study, a series of parametric experiments were conducted to investigate the response of two-graded sediment beds to a range of different unsteady hydrograph flow conditions. Investigations of the total and fractional bed-load sediment transport rates revealed strong temporal variations in transport over the hydrographs, with size-dependent temporal lag effects observed between peak flow conditions and peak bed-load transport rates. Specifically, coarse gravels had increased mobility during the rising limb of the hydrographs, attaining their peak bed-load transport rate either prior to, or near, peak flow conditions. By contrast, the finer grades tended to have enhanced mobility during the receding limb of the hydrographs, with peak transport rates measured after peak flow conditions had passed. Grain size distributions measured from the collected bed-load samples also indicated material coarsening over the rising limb and fining during the receding limb, while corresponding image analysis measurements of bed surface composition showed only marginal variation over the hydrographs. Computation of total and fractional sediment yields revealed that the bimodal sediment mixture tested was transported at significantly higher rates than the uni-modal mixture over all hydrograph conditions tested. This finding indicated that the uni-modal sediment bed was inherently more stable than the bimodal bed due to the increased abundance of medium-sized gravels present in the uni-modal sediment grade. The parametric dependences established in the study have clear implications for improved understanding of fractional inter-granular effects at the bed surface and their influence on graded sediment transport processes within natural fluvial channels under flood flow hydrographs.

Numerical modelling of riverbed grain size stratigraphic evolution

For several decades, quantification of riverbed grain size stratigraphic evolution has been based upon the active layer formulation (ALF), which unfortunately involves considerable uncertainty. While it is the sediment exchange across the bed surface that directly affects the riverbed stratigraphy, it has been assumed in the ALF that the sediment fraction at the lower interface of the active layer is a linear function of the sediment fraction in the flow. Here it is proposed that the sediment fraction of the sediment exchange flux is used directly in estimating the sediment fraction at the lower surface of the active layer. Together with the size-specific mass conservation for riverbed sediment, the modified approach is referred to as the surface-based formulation (SBF). When incorporated into a coupled non-capacity modelling framework for fluvial processes, the SBF leads to results that agree as well or better than those using ALF with laboratory and field observations. This is illustrated for typical cases featuring bed aggradation and degradation due to graded bed-load sediment transport. Systematic experiments on graded sediment transport by unsteady flows are warranted for further testing the modified formulation.

Modelling Graded Sediment Transport and Bed Evolution in a Tidal Harbour

ABSTRACT Yang, C.; Jiang, C., and Lin, B. 2013. Modelling graded sediment transport and bed evolution in a tidal harbour. This paper presents the development of a sediment transport model to predict the bed evolution processes in estuarine and coastal waters. The model is based on an existing hydrodynamic and sediment transport model, with significant refinements being made to enhance its capability for simulating the transport of graded sediments under nonequilibrium conditions. A multifraction sediment transport model has been developed to replace the existing single-fraction model. The sediment mixture is divided into several fractions according to the grain size. For each fraction, the particle size is considered to be uniform, and its transport form, either as suspended load or bed load, is determined by the magnitude of a suspension index. A bed evolution model is developed to simulate the processes of bed level change and sediment grain size sorting. The model is applied to a laboratory model harbo...

Comparison of Morphodynamic Models for the Lower Yellow River1

Xia, Junqiang, Zhengbing Wang, Yanping Wang, and Xin Yu, 2012. Comparison of Morphodynamic Models for the Lower Yellow River. Journal of the American Water Resources Association (JAWRA) 1‐18. DOI: 10.1111/jawr.12002Abstract: Significant channel adjustments often occur during flood seasons in the Lower Yellow River (LYR), and it is a challenging work to accurately simulate the morphodynamic processes in the LYR using numerical models. A comparison of two morphodynamic models (Delft3D and 2DLLCDM) for the LYR is presented herein to identify critical improvements for these models. The concepts of these models are first compared with each other. The models were then used to simulate the processes of flood routing, sediment transport, and morphological changes occurring in a braided reach of the LYR. The differences were investigated between the simulated results from these models and corresponding field measurements, and the results indicate that: (1) the hydrodynamic processes calculated by both models agree closely with the measurements if an appropriate Manning’s roughness coefficient is used; (2) the concentrations of suspended load at the downstream boundary calculated by the models agree reasonably with the observed data; and (3) the predicted cross‐sectional profiles obtained from these models do not correspond well with the measurements. Based on these findings, the weak aspects of the models are clarified, and three critical improvements are recommended, including: (1) the development of roughness predictor; (2) the refinement of graded sediment transport capacity formulation; and (3) the consideration of bank erosion module. These improvements need to be implemented in the future.

Process‐based, morphodynamic hindcast of decadal deposition patterns in San Pablo Bay, California, 1856–1887

This study investigates the possibility of hindcasting‐observed decadal‐scale morphologic change in San Pablo Bay, a subembayment of the San Francisco Estuary, California, USA, by means of a 3‐D numerical model (Delft3D). The hindcast period, 1856–1887, is characterized by upstream hydraulic mining that resulted in a high sediment input to the estuary. The model includes wind waves, salt water and fresh water interactions, and graded sediment transport, among others. Simplified initial conditions and hydrodynamic forcing were necessary because detailed historic descriptions were lacking. Model results show significant skill. The river discharge and sediment concentration have a strong positive influence on deposition volumes. Waves decrease deposition rates and have, together with tidal movement, the greatest effect on sediment distribution within San Pablo Bay. The applied process‐based (or reductionist) modeling approach is valuable once reasonable values for model parameters and hydrodynamic forcing are obtained. Sensitivity analysis reveals the dominant forcing of the system and suggests that the model planform plays a dominant role in the morphodynamic development. A detailed physical explanation of the model outcomes is difficult because of the high nonlinearity of the processes. Process formulation refinement, a more detailed description of the forcing, or further model parameter variations may lead to an enhanced model performance, albeit to a limited extent. The approach potentially provides a sound basis for prediction of future developments. Parallel use of highly schematized box models and a process‐based approach as described in the present work is probably the most valuable method to assess decadal morphodynamic development.

MEDIUM-TERM MORPHODYNAMIC MODELING OF MIXED MUD AND SAND IN THE

A morphodynamic model for the Jadebusen basin based on the current DELFT3D model is established. With
 morphological information such as qualitative measurements of SPM-concentration (Suspended Particular Matter) and
 bottom change estimates as well as detailed surveys of a sand pit refilling process the model parameters for cohesive
 transport (Partheniades 1965) are calibrated. Within a period of 8 month, which is condensed to a representative
 period of 28 days for the modeling, almost 45% of the initial pit volume was refilled with mud. Despite the lack of
 specific field measurements a setup of mud transport parameters can be defined, which allows further investigations of
 sand pit locations in the Jadebusen basin. On major outcome is that critical erosion shear stresses for mud should be
 spatially varied in order to account for the different consolidation states of the mud fraction. Furthermore the paper
 addresses the problem of the initial grain size distribution for graded sediment transport. Wadden flat types are
 characterized based on aerial observations which build the underlying information for a relocation model run.

Numerical modelling of non-equilibrium graded sediment transport in a curved open channel

The computer code FAST3D has been developed to calculate flow and sediment transport in open channels. In the code, the flow field is calculated by solving the full Reynolds-averaged Navier–Stokes equations with k–ε turbulence model; the bed-load transport is simulated with a non-equilibrium model containing an important parameter, the so-called non-equilibrium adaptation length, which characterizes the distance for sediment to adjust from a non-equilibrium state to an equilibrium state; the bed deformation is obtained from an overall mass-balance equation for sediment transport. The governing equations are solved numerically with a finite volume method on an adaptive, non-staggered grid. The former model assumed uniform bed material. In order to take into account the influence of grain-size distribution of the bed-surface on the evolution of the bed topography and consequently also on the flow field, a sediment transport module has been presently developed by the authors at the Institute of Hydraulic and Water Resources Engineering, Technische Universität München, Germany, for fractional sediment transport using a multiple layer model. This paper presents the numerical results for sediment sorting and the bed deformation in a curved alluvial channel under unsteady-flow conditions according to Yen and Lee (1995). The calculations were compared with data from laboratory measurements. Further, the sensitivity of the simulated results to the non-equilibrium adaptation length is investigated.

A graded sediment transport and bed evolution model in estuarine basins and its application to the Yellow River Delta

Details are given in present paper of the development of a sediment transport and morphological model to predict the bed evolution processes in tidal flows. The model is based on an existing hydrodynamic and sediment transport model (DIVASTSED), but with significant refinements being made. A single size sediment transport module is refined to include capability for simulating the transport of graded sediments under non-equilibrium conditions. Both cohesive sediment and non-cohesive sediment are taken into consideration in present study. The fall velocity of suspended sediment is modified in present model due to the high sediment concentration. A 3-layer approach is adopted to simulate the variations of sediment gradations of bed materials. The model is used to simulate the bed evolution in the Yellow River Delta from 1992 to 1995, the numerical results show how the morphology developed in the Yellow River Delta and agree well with the field data. The present model could provide an effective tool for the management of wetland in the Yellow River Delta.

Modelling of the Silting up of Navigation Channels

A mathematical model describing transport of non-uniformly graded sediments has been applied to analyzing the silting up of approach routes (navigation channels) leading to ports. This model distinguishes three layers in the movement of sediments, assuming that the vertical sorting occurs only in the process of picking up grains in the contact layer. It is also assumed that along the widnward edge of the route sediments are transported in the bedload and contact layers during the wave crest phase and – as suspended sediments – in the outer region under the influence of the resultant current. On the leeward side sediments are transported only during the wave trough phase in the bedload and contact layer. The computations have demonstrated that the above model can be a useful tool for predicting both the rate and volume of sediments silting up navigation channels as well as grain-size distribution of sediments which fill up a water route.

Modelling of the Silting up Processes In Water Routes of Łeba and Tolkmicko Ports

A theoretical three-layer model of non-uniformly graded sediment transport has been used to analyse the silting up of water routes in the port of Łeba and Tolkmicko. Comparison of the results derived from model calculations with the measurements and dredging work data has demonstrated that the model is useful for prediction of the silting up extent as well as the distribution of grain-size fractions in sediments captured in water routes of ports, which are different in bathymetric as well as hydrodynamic parameters, and where sea bottom deposits are different in nature.

Numerical simulation of graded sediment transport

River engineers commonly need to predict sediment transport rates in the absence of adequate field data, and so make frequent use of numerical sediment transport models. This paper compares the performance of two such computer models for the prediction of graded sediment transport. The intention is to inform model users of the level of accuracy that can be expected for typical applications. One model uses the Parker bedload transport equation and the other uses the Meyer-Peter and Müller sediment transport formula modified for use with graded sediments. The computer model predictions are compared with detailed experimental data collected for the purpose of computer model validation. The Meyer-Peter and Müller-based model produces better predictions of sediment transport rates, whereas the Parker-based model performs better in terms of the grain sizes of both the bedload and the bed material. These results illustrate the limitations of present models and indicate areas requiring further investigation.

Numerical simulation of graded sediment transport

River engineers commonly need to predict sediment transport rates in the absence of adequate field data, and so make frequent use of numerical sediment transport models. This paper compares the performance of two such computer models for the prediction of graded sediment transport. The intention is to inform model users of the level of accuracy that can be expected for typical applications. One model uses the Parker bedload transport equation and the other uses the Meyer-Peter and Müller sediment transport formula modified for use with graded sediments. The computer model predictions are compared with detailed experimental data collected for the purpose of computer model validation. The Meyer-Peter and Müller-based model produces better predictions of sediment transport rates, whereas the Parker-based model performs better in terms of the grain sizes of both the bedload and the bed material. These results illustrate the limitations of present models and indicate areas requiring further investigation.