Bedload Sediment Transport Rates Research Articles

Study on the motion characteristics of bedload sediment particles under different sediment transport intensities

AbstractBedload studies at the particle scale may help grasp the essence of the problem. Existing studies suffer from short filming durations, limited data volume, and a narrow range of sediment transport intensity variations. This paper employs the high‐speed photography technology and conducts experimental studies on bedload particle motion under 8 different sediment transport intensities. Using the latest image processing technology, over 6 million sediment particle coordinate points and nearly 400,000 particle motion trajectory curves were automatically obtained and used to compare the motion characteristics of bedload particles under different sediment transport intensities. The results show that under low sediment transport intensity, both the number of moving particles and particle motion velocity contribute to the bedload sediment transport rate, while under high‐intensity conditions, the transport rate mainly depends on the number of moving particles. The probability density distribution of sediment transport rate is concentrated and varies within a small range under low‐intensity conditions, exhibiting a tailing phenomenon. In contrast, under high‐intensity conditions, the range of sediment transport rate values increases, and the probability density curve tends to be symmetric, more closely approximating a normal distribution. Additionally, the paper compares the longitudinal and transverse motion velocities of particles and the coefficient of variation of the bedload sediment transport rate.

Developing a numerical model for sediment transport in channel network by considering multigrade bed load sediment

ABSTRACT Predicting transport rates is difficult for the morphological features of marine, river, and coastal ecosystems. To design a model bed load sediment transport rate along with the surface layer, a mesh-based numerical method is used, and for the sediment transport the governing equations are employed and depth-averaged is used to create open channel flow and in three-dimensional horizontal space the evaluation is performed. By utilizing the governing equations, the open channel flow discretization and the numerical hydrodynamic model of sediment transport rate are calculated, and in the flow channel by using the solution method the settling velocity is determined. The complex issues involved nonlinear equations for the numerical modelling with solution methodology. The chicken swarm optimization algorithm is revealed by multi-objective metaheuristics optimization, and a better outcome is acquired by optimization. Finally, if polygonal mesh modelling is used in the diversion flow channel and curved bend channel, several experimental models were employed in the model experiments. A numerical measured and improved model is used to estimate the results in comparison, and the optimally developed model forecast rates with accuracy. In some open-channel flows such as reservoirs and rivers, the struggling flow and bed load sediment transport were dealt with in this model. Based on the results obtained, the proposed technique demonstrates significant improvements in both accuracy and prediction performance compared to the existing techniques. The proposed technique achieves an accuracy of 96.87%, outperforming the BCNN technique with an accuracy of 89.77% and the H-ANN-FA technique with an accuracy of 84.65%. Moreover, the proposed technique exhibits a lower RMSE value of 0.12, indicating a substantial reduction in the average deviation between predicted and observed values. Based on the NSE and R2 results, the proposed technique demonstrates remarkable improvements in prediction accuracy compared to the other techniques. The proposed technique achieves an NSE value of 0.998 and R2 value of 0.99, compared to the existing technique with an improvement of 46.47% and 11.24%.

Development of the Gravel Pressure and Voice Synchronous Observation System and Application in Bedload Transport Measurement

Bedload sediment transport is critical in the natural river environmental and ecological system. It is quite challenging to measure the bedload sediment transport rate in rivers with any degree of accuracy. In this study, the authors developed the Gravel Pressure and Voice Synchronous observation system (GPVS) to estimate the bedload sediment transport rate in rivers. The core of the GPVS includes an underwater high-fidelity audio recorder and pressure sensor. The audio recorder is intended to monitor the low bedload sediment transport rate, whereas the pressure sensor is utilized to detect relatively substantial bedload sediment transport. The GPVS is tested by running flume experiments with natural gravel to evaluate the system’s reliability and feasibility. Results reveal that the GPVS has a very high sensitivity for detecting bedload transport. When the bedload sediment transport rate is relatively low, the audio recorder can measure it quantitatively, showing that the system is not impacted by ambient noise.

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.

Viscous effects on bedload sediment transport rates

Viscous effects on bedload sediment transport rates have been seldom examined in previous studies. They may become significant in the low transport regime, as evidenced by experimental data. However, the problem cannot be resolved using classical bedload models that involve the shear excess, for example, Meyer-Peter–Müller formula. By applying the concept of the critical pickup probability for incipient sediment motion, this study derives a new bedload formula that takes the fluid viscosity into consideration. The results show that the relationship between the dimensionless bedload rate and the Shields number varies significantly with the viscosity-dependent parameter in the low transport regime, which partially accounts for the scattering of bedload data. In addition, the present study proposes a simple approach to the evaluation of the critical bedload transport rate that is associated with the Shields curve for incipient sediment motion.

Hysteretic Implications for Graded Bed Load Sediment Transport in Symmetrical Hydrograph Flows

Differential parametric values associated with bed load sediment transport, that result at the same discharge levels on the rising and falling limbs of a flood hydrograph, are usually defined as bed load hysteresis. This hysteresis in bed load sediment transport rates is of considerable interest in the field of fluvial hydraulics. Within this study, a series of well-defined, symmetrical hydrograph flows are generated over a graded, mobile sediment bed to fully examine the hysteresis of the resulting bed load sediment transport in terms of the threshold of motion, and differential bed load transport rates and bed load yields during the hydrographs. The experiments are conducted in a titling flume without sediment supply specified at the upstream inlet, thereby representing typical river reach conditions immediately downstream of a dam that are exclusively subject to net in-channel bed degradation from sediment transport initiated during flood events. Our results show that the fractional bed load transport of defined fine, medium and coarse size classes within the graded sediment bed generally display clockwise, no/mixed and counter-clockwise hysteresis patterns, respectively, with clockwise hysteresis most commonly found for the coarse size class mobilised by hydrographs with long durations. By contrast, counter-clockwise hysteresis is usually observed for fine size class transported by hydrographs with short durations. Accordingly, the corresponding reference stresses for each size class vary between different hydrographs and are primarily controlled by the hydrograph flashiness (i.e. unsteadiness) and magnitude (i.e. total water work). Moreover, it is shown that the hysteresis effect, particularly for those size classes and hydrograph combinations that result in clockwise and counter-clockwise behaviour, should be fully accounted for when reproducing bed load transport rates using separate-limb based method. Finally, we investigate the relative fractions of the overall bed load yields generated during the rising and falling limbs of all symmetrical hydrographs (i.e. the bed load yield ratio), which are found to be primarily dependent on bed load transport hysteresis. Finally, the relationship between the bed load yield ratio and the ratio of reference stresses for the fractional sediment motion of each size class on both limbs is found to follow a power law.

Validation of an Experimental Procedure to Determine Bedload Transport Rates in Steep Channels with Coarse Sediment

The current study presents an experimental procedure used to determine bedload sediment transport rates in channels with high gradients and coarse sediment. With the aim to validate the procedure for further investigations, laboratory experiments were performed to calculate bedload transport rates. The experiments were performed in a laboratory tilting flume with slopes ranging from 3% to 5%. The sediment particles were uniform in shape (spheres). The experiments were divided into four cases based on sediment size. Three cases of uniform sizes of 10 mm, 15 mm and 25 mm and a case with a grain size distribution formed with the uniform particle sizes were considered. From the experimental results a mathematical bedload transport model was obtained through multiple linear regression. The experimental model was compared with equations presented in the literature obtained for gravel bed rivers. The experimental results agree with some of the models presented in the literature. The closest agreement was seen with models developed for steep slopes especially for the highest slopes considered in the present study. Therefore, it can be concluded that the methodology used can be replicated for the study of bedload transport rates of channels with high gradients and coarse sediment particles to study more general cases of this process such as sediments with non-uniform shapes and sizes. However, a simplified model is proposed to estimate bedload transport rates for slopes up to 5%.

Bedload Sediment Rate Prediction for the Sand Transport Along Coastal Waters in Ocean Management Strategy

Interactions among different landforms and varied complicated physical processes cause sediment transport in coastal regions being the interest of ocean management planning studies. In coastal zones, the derivation of the bedload sediment transport rate and the flow velocity distribution is done by entropy theory which assumes the modified spatiotemporal disorder power index (MSTDPI) and the time-averaged flow velocity (as a random variable). Studying the deposition trend of bedload sediment transport rate for the sand particle (BLSTRS) and estimating the coastal erosion rate as a case study, the Makran coast is selected. To analyze the spatiotemporal patterns, the disorder power index (entropy-power) method is applied in this study where the monthly data of six Makran coastal sections from January 1970 to December 2015 are used. The studied data are mainly focused on the correlation of the flow rate of the sediment from the Makran River, and the spatiotemporal patterns of BLSTRS. Despite their meaningful spatiotemporal variability, it is not very easy to explain how the abovementioned variables perform together; the entropy-power index allows a better understanding of the combined performance of such parameters as the flow velocity and sediment transport by showing clearer signals for the assessment of coastal engineering issues at very large (coastal) scales.

Observations of wave attenuation, scour, and subsurface pore pressures acrossbthree marsh restoration sill structures on a sandy bed

With rising sea levels and more frequent exposure to extreme storms, coastlines worldwide are vulnerable to increased erosion and loss of natural marsh lands. In an effort to lessen these impacts, there is a growing practice of adapting hard or “gray” coastline protection techniques to more nature-based features that promote habitat and ecosystem health. Living shoreline marsh restorations utilize natural and naturebased materials to protect marsh shores from erosion while also allowing intertidal flushing to promote the health and diversity of the marsh. Our study investigates three types of living shoreline sill designs exposed to average and storm-energy wave conditions at varying water levels. The sills were designed to mimic constructed sills in practice (rock, oyster shell, tree root wads), but more generally vary in structure porosity and material dissipation potential. Large-scale laboratory experiments were conducted in the large wave flume at the O.H. Hinsdale Wave Research Laboratory. Wave transmission and reflection are used to demonstrate wave attenuation capability of each sill structure. Scour of the sill, bedload sediment transport rates on the seaward and shoreward sides of the sill, and sediment pore-water vertical hydraulic gradients were used to demonstrate the potential for sediment transport and liquefaction. Results will contribute to understanding the effect of sill material porosity and mass on structure stability, and the effectiveness of using green living shoreline sill structures in the continued effort to establish design criteria for living shoreline implementation.

Simple Formulation of Bedload Sediment Transport Rate Based on Novel Definition of Pickup Probability

AbstractThe pickup probability of bedload sediment can be defined as the channel width fraction that is only associated with entrained particles. With this definition, the dimensionless bedload tra...

Observation of Sediment Dynamics in the Nearshore Region using Fluorescent Sand Tracers

The field observations of cross-shore sediment transport from the swash zone to the offshore side of the outer bar using fluorescent sand tracers were conducted in 2014 and 2015 at the Hasaki coast, Japan facing the Pacific Ocean during storm wave conditions. The topography difference between the two years data set is that outer bar shape existed in 2014 and not existed in 2015. During the observations, wave height and wave period were observed, and sand core samplings were conducted. By using the observed wave data and the analyzed results of collected fluorescent sand tracers from the cores, the sediment movements from the offshore side of the outer bar area to the swash zone during storms were investigated. The results indicate that the sediment movements during the storms were affected by the outer bar shape. If the outer bar existed, the sediment of the onshore side of the outer bar tend to move landward, and it also moves to seaward due to undertow until the outer bar location. The sediment at the offshore side of the outer bar tends to move to onshore ward due to bedload sediment transport. However, the transport rate decrease in the trough region. The beach topography without the bar shape, the bed load sediment transport rate at the trough region was higher than that with the bar shape. Thus, the sediments at the offshore side of the bar also could move until the swash zone.

History‐Dependent Threshold for Motion Revealed by Continuous Bedload Transport Measurements in a Steep Mountain Stream

AbstractTo explore the causes of history‐dependent sediment transport in rivers, we use a 19‐year record of coarse sediment transport from a steep channel in Switzerland. We observe a strong dependence of the threshold for sediment motion (τc) on the magnitude of previous flows for prior shear stresses ranging from 104 to 340 Pa, resulting in seasonally increasing τc for 10 of 19 years. This stabilization occurs with and without measureable bedload transport, suggesting that small‐scale riverbed rearrangement increases τc. Following large transport events (>340 Pa), this history dependence is disrupted. Bedload tracers suggest that significant reorganization of the bed erases memory of previous flows. We suggest that the magnitude of past flows controls the organization of the bed, which then modifies τc, paralleling the evolution of granular media under shear. Our results support the use of a state function to better predict variability in bedload sediment transport rates.

Quantification of bedform dynamics and bedload sediment flux in sandy braided rivers from airborne and satellite imagery

AbstractImages from specially‐commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanned aerial vehicle (UAV) surveys, and Planet CubeSat satellites are used to quantify dune and bar dynamics in the sandy braided South Saskatchewan River, Canada. Structure‐from‐Motion (SfM) techniques and application of a depth‐brightness model are used to produce a series of Digital Surface Models (DSMs) at low and near‐bankfull flows. A number of technical and image processing challenges are described that arise from the application of SfM in dry and submerged environments. A model for best practice is presented and analysis suggests a depth‐brightness model approach can represent the different scales of bedforms present in sandy braided rivers with low‐turbidity and shallow (< 2 m deep) water.The aerial imagery is used to quantify the spatial distribution of unit bar and dune migration rate in an 18 km reach and three ~1 km long reaches respectively. Dune and unit bar migration rates are highly variable in response to local variations in planform morphology. Sediment transport rates for dunes and unit bars, obtained by integrating migration rates (from UAV) with the volume of sediment moved (from DSMs using MAV imagery) show near‐equivalence in sediment flux. Hence, reach‐based sediment transport rate estimates can be derived from unit bar data alone. Moreover, it is shown that reasonable estimates of sediment transport rate can be made using just unit bar migration rates as measured from 2D imagery, including from satellite images, so long as informed assumptions are made regarding average bar shape and height. With recent availability of frequent, repeat satellite imagery, and the ease of undertaking repeat MAV and UAV surveys, for the first time, it may be possible to provide global estimates of bedload sediment flux for large or inaccessible low‐turbidity rivers that currently have sparse information on bedload sediment transport rates. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Bed-load transport rate based on the entrainment probabilities of sediment grains by rolling and lifting

A function for the bed-load sediment transport rate is derived. This function is obtained by using the entrainment probabilities of the rolling and lifted sediment grains, and by introducing two travel lengths, respectively. The predictions from the new bed-load function agree well with experimental results over the entire experimental range and show significant improvement over the commonly used formula for the bed-load transport rate. The new function shows that, in terms of contributing to the bed-load transport rate, the total entrainment probability of the sediment grains is a weighted summation of those for the lifted and rolling grains, rather than a simple addition of the two. The function is also used to predict the total entrainment probability, saltation length, and the bed layer thickness at a high bed-load transport rate. These predictions all agree well with the experimental results. It is found that, on average, the travel length for the rolling sand grains is about an order of magnitude less than that of the lifted grains.

An entropy theory for the spatiotemporal patterns of the environmental matrix in the nearshore parameters

Understanding the temporal and spatial variability of sediment transport at regional scales is of great value for various purposes; however, relevant studies are still limited and have yielded inconsistent findings about the primary controls on regional temporal and spatial variability of bed load sediment transport rate (BLSTR). In this paper, sediment transport was investigated by a combination of spatiotemporal patterns and entropy theory modelling. Effort has been made in this paper to develop an entropy theory by assuming the time-averaged wave height as a random variable and applying the disorder power index to derive wave height distribution and rate of bed load sediment transport in coastal zones for ocean management planning. To analyze the spatio-temporal patterns, this study uses: A method that needs a disorder power index (entropy-power), uses the long time (1970–2015 monthly) data were retrieved of six large-scale monitoring networks located in the Makran coastal area (located in southeastern Iran), and shows the index suitability for hydrological/marine ecosystem components analyses using a coastal area case study, and studies flow velocity speed, wave and spatio-temporal patterns of BLSTR each of which is dependent on time and has an important role in terrestrial coastal/ocean studies. The combined performance of the above mentioned variables is difficult to describe (although their spatio-temporal variability is mostly meaningful) and, regarding this context, entropy-power index shows signals useful to assess coastal engineering issues and makes the understanding of wave height and sediment transport parameters interactions possible at nearshore zone scales. The advantage of using the entropy theory is that it permits quantification of uncertainty associated with concentration and determination of parameters in terms of specified information, such as mean concentration.

Interplay between grain protrusion and sediment entrainment in an experimental flume

AbstractBed load sediment transport is typically formulated as a nonlinear function of the shear stress exerted on the bed in excess of a critical value. However, due to the inherent spatial variability in grain packing and protrusion on water‐worked beds, the critical stress used in transport models represents a spatial averaging of the critical entrainment stresses of many individual particles on the bed. We perform a series of flume experiments in which we quantify, for the first time, the evolution of topography on the particle scale during low‐flow periods. We link this topography to subsequent bed load sediment transport rates and explore implications for particle critical stresses. We exploit the observed dependence of bed load flux on antecedent low‐flow duration to isolate the relationship between grain protrusion and bed load flux over a wide array of transport rates at the same applied stress. A synthesis of high‐resolution bed topography surveys and bed load flux data show that bed load transport rates characteristic of gravel channels are governed by the portion of grains that protrude highest above the bed and that this portion corresponds to ~1–5% of the total bed elevation distribution in our experiments. This result supports the argument that only a small portion of grain entrainment thresholds for a riverbed is exceeded during transport. Further, these results emphasize that subtle changes in bed topography can have dramatic effects on bed load sediment transport. We also find that transport of these highest protruding particles enhances the local erosion of surrounding grains.

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.

Characteristics of Sediment Concentration and Suspended Sediment Transport Due to Horizontal and Vertical Asymmetric Waves

Wave phenomenon in ocean often has the shape in the horizontal and vertical asymmetric waves when propagating in the near-zone with high non-linearity. The orbital velocity due to the wave asymmetriesand nonlinearities could generate the flow patterns that drives cross-shore sediment transport in the near-shore zone. A new calculation method for bottom shear stresses and bed-load sediment transport rate for the horizontal and vertical asymmetries waves proposed by1,2was implemented to model the concentration sediment and suspended sediment transport. The characteristics of the concentration sediment and suspended load induced by the horizontal and vertical asymmetries waves have been examined. The modeling results showed that the smaller α and the higher Ni indicate more wave skew-ness and more non-linearity of waves causing a higher the bottom shear stress and the net bed load transport while the net suspended load transport decrease with the decreasing of the wave skew-ness. The higher skewed waves indicated the higher acceleration causing increase the value of the net onshore transport rate for bed load sediment transport and decrease the value of the net offshore transport rate for suspended sediment transport.

Seepage effects on bedload sediment transport rate by random waves

The mean net bedload sediment transport rate beneath random waves is predicted taking into account the effect of seepage flow. This is achieved by using wave half-cycle bedload sediment transport formulas valid for regular waves together with a modified Shields parameter including the effect of seepage flow. The Madsen and Grant (1976) bedload sediment transport formula is used to demonstrate the method. An example using data typical to field conditions is included to illustrate the approach. The analytical results can be used to make an assessment of seepage effects on the mean net bedload sediment transport based on available wave statistics. Generally, it is recommended that a stochastic approach should be used rather than using the rms values in an otherwise deterministic approach.

Particle velocity and concentration profiles in bedload experiments on a steep slope

ABSTRACTDepth profiles of particle streamwise velocity, concentration and bedload sediment transport rate were measured in a turbulent and supercritical water flow. One‐size 6 mm diameter spherical glass beads were transported at equilibrium in a two‐dimensional 10% steep channel with a mobile bed. Flows were filmed from the side by a high‐speed camera. Particle tracking algorithms made it possible to determine the position, velocity and trajectory of a very large number of particles. Approximately half of the sediment transport rate was composed by rolling grains, and the other half by saltation. This revealed a complex structure, with several concentration and flux peaks due to rolling, and one peak due to saltation. With an increase of the sediment transport rate, the depth structure remained the same at the water/granular interface, with peak value increases but with no shift in elevations. The saltation region expanded towards higher elevations with an increase of the particle velocity commensurate to the water velocity. The proportion of the sediment transport rate in saltation did not vary significantly. The particle streamwise velocity profiles exhibited three segments: an exponential decay in the bed, a linear increase where rolling and saltation co‐existed, and above this, a logarithmic‐like shape due to saltating particles. These results are comparable to profiles measured and modelled in dry granular free surface flows and in more intense bedload such as sheet flows. Copyright © 2014 John Wiley & Sons, Ltd.