Bedload Movement Research Articles

Modeling the hydraulic roughness of a movable flatbed in a sand channel while considering the effects of water temperature

The prediction of the flow resistance (usually quantified as the hydraulic roughness) of a movable flatbed is a key issue affecting the calculation accuracy of flood levels in river training projects. Bedload motion on a movable flatbed causes additional energy loss and increases hydraulic roughness. Several theoretical and empirical predictors for characterizing this phenomenon have been proposed, but the accuracy and physical basis of these models should be improved. In this study, the total energy dissipation rate is separated into two components: the energy dissipation rate due to grain drag and the additional energy dissipation rate due to bedload motion. Following the energy dissipation rate balance equation, a new predictor was proposed for movable flatbed flows. The water temperature was empirically coupled with the fluid viscosity and its associated physical variables. A new empirical relation between two dimensionless flow‒sediment combination variables was established to demarcate the various bedform transitions induced by the water temperature. The new predictor was compared with other predictors, and the prediction results were compared to the measured data. The error metric showed that the new predictor provided the highest accuracy, with ∼88.5% of the 826 data points falling within the ±30% error band. The new predictor suggested that the additional drag is nonlinearly proportional to the grain drag, and the scale factor between these two parameters is related to five flow‒sediment variables. In addition, the ability of the new predictor to quantify water temperature effects was examined. The predicted resistance exhibited three change modes with increasing water temperature, and the results suitably agreed with the measurements. The effect of the water temperature on the resistance of a movable flatbed is jointly controlled by the suspension number and roughness Reynolds number. This study provides an effective predictor that can be used by decision makers for modeling the hydraulic roughness of a movable flatbed.

Sounding out the river: Seismic and hydroacoustic monitoring of bedload transport

AbstractSeismological observations provide a non‐invasive and continuous means for indirectly measuring fluvial bedload transport. A significant challenge remains in independently characterising the seismic signature of bedload transport from other sources such as turbulence. We present a unique dataset from an alluvial Scottish river, combining seismic data and hydroacoustic measurements, to analyse bedload transport during three high‐flow events occurring within the same year. By studying three successive events, we assess the consistency of bedload transport thresholds in response to changing flow conditions and explore the presence of hysteresis in seismic data versus water level as an indicator of coarse bedload transport. Through the use of hydroacoustic data to independently characterise bedload transport, our findings reveal that bedload transport occurred during all three events but that the threshold for entrainment varied. These entrainment thresholds were influenced by antecedent events, with a drop of 15%–20% of the threshold flow depth following the largest of the three events. In agreement with recent studies, we also found that hysteresis in the seismic versus water level data is not sufficient for identifying and analysing bedload transport: Distinct hysteresis was only observed during the largest of the three events despite all events experiencing bedload transport as observed through the independent hydroacoustic data. Our work shows the value in combining independent datasets for long‐term monitoring of bedload transport to understand the evolution in the thresholds of bedload motion, providing crucial information for effective river and land‐use management in a changing climate with potentially impacted high‐flow events.

Measuring Bedload Motion Time at Second Resolution Using Benford's Law on Acoustic Data

AbstractBedload transport is a natural process that strongly affects the Earth's surface system. An important component of quantifying bedload transport flux and establishing early warning systems is the identification of the onset of bedload motion. Bedload transport can be monitored with high temporal resolution using passive acoustic methods, for example, hydrophones. Yet, an efficient method for identifying the onset of bedload transport from long‐term continuous acoustic data is still lacking. Benford's Law defines a probability distribution of the first‐digit of data sets and has been used to identify anomalies. Here, we apply Benford's law to continuous acoustic recordings from Baiyang hydrometric station, a tributary of Liwu River, Taroko National Park, Taiwan at the frequency of 32 kHz from stationary hydrophones deployed for 3 years since 2019. We construct a workflow to parse sound combinations of bedload transportation and analyze them in the context of hydrometric sensing constraining the onset, and recession of bedload transport. We identified three separate sound classes in the data related to the noise produced by the motion of pebbles, water flow, and air. We identify two bedload transport events that lasted 17 and 45 hr, respectively, covering about 0.35% of the total recorded time. The workflow could be transferred to other different catchments, events, or data sets. Due to the influence of instrument and background noise on the regularity of the residuals of the first‐digit, we recommend identifying the first‐digit distribution of the background noise and ruling it out before implementing this workflow.

Dimensionless argument: a narrow grain size range near 2 mm plays a special role in river sediment transport and morphodynamics

Abstract. The grain size 2 mm is the conventional border between sand and gravel. This size is used extensively, and generally without much physical justification, to discriminate between such features as sedimentary deposit type (clast-supported versus matrix-supported), river type (gravel bed versus sand bed), and sediment transport relation (gravel versus sand). Here we inquire as to whether this 2 mm boundary is simply a social construct upon which the research community has decided to agree or whether there is some underlying physics. We use dimensionless arguments to show the following for typical conditions on Earth, i.e., natural clasts (e.g., granitic or limestone) in 20 ∘C water. As grain size ranges from 1 to 5 mm (a narrow band including 2 mm), sediment suspension becomes vanishingly small at normal flood conditions in alluvial rivers. We refer to this range as pea gravel. We further show that bedload movement of a clast in the pea gravel range with, for example, a size of 4 mm moving over a bed of 0.4 mm particles has an enhanced relative mobility compared to a clast with a size of 40 mm moving over a bed of the same 4 mm particles. With this in mind, we use 2 mm here as shorthand for the narrow pea gravel range of 1–5 mm over which transport behavior is distinct from both coarser and finer material. The use of viscosity allows the delineation of a generalized dimensionless bed grain size discriminator between “sand-like” and “gravel-like” rivers. The discriminator is applicable to sediment transport on Titan (ice clasts in flowing methane/ethane liquid at reduced gravity) and Mars (mafic clasts in flowing water at reduced gravity), as well as Earth.

3D-CFD analysis of bedload transport in channel bifurcations

Abstract The aim of this research was to numerically reproduce bedload transport processes in channel bifurcations and thereby evaluate the methodology and feasibility of 3D-computational fluid dynamics (CFD) bedload transport simulations. This was carried out by numerically replicating two physical model investigations of channel bifurcations: research conducted by Bulle in 1926 and a large-scale channel bifurcation investigated at the Kassel University hydraulic laboratory. The numerical results were evaluated by comparing bedload distributions in the channel areas, formations of bedload depositions and development of bedload over time. The numerical simulations of Bulle's model were conducted for different boundary conditions, numerical parameters, sediment grain sizes, flow rates, flow distributions on the two channel branches and for longer running simulations. The investigations of the Kassel University bifurcation were carried out for different sediment types, which influences the numerical parameters, as well as for different flow distributions and channel width/depth ratios. Finally, structures for deflection of bedload into the straight channel were investigated. The results showed that bedload movement can be successfully simulated with 3D-CFD models, even in complex hydraulic conditions. Based on the obtained results, important indicators and recommendations for the application of 3D-CFD bedload transport simulations were acquired.

Comparing Field, Probabilistic, and 2D Numerical Approaches to Assess Gravel Mobility in a Gravel‐Bed River

AbstractSediment transport is a key process that affects the morphology and ecological habitat diversity of rivers. As part of a gravel augmentation program to mitigate sediment deficit below a dam, gravel mobility in the Ain River in Eastern France was investigated by tracking of a large amount (n = 1,063) of PIT‐tagged gravels in the field, conducting a probabilistic approach based on published tracer studies, and performing two‐dimensional (2D) numerical modeling of flow and bedload transport. This comparative study highlights the strengths, weaknesses, and complementary aspects of the three approaches to the understanding of river gravel mobility. Thanks to recent technological improvements, PIT‐tagged gravels provide an increasingly reliable and accurate representation of bedload movement in the field, although it remains limited in spatio‐temporal resolution. Based on an exponential distribution, the probabilistic approach correctly reproduces the average trend in travel distances by the different classes of particles over hydrological periods, including one or several significant floods. Furthermore, the 2D numerical modeling accounts for the variability of local hydrodynamic conditions and can simulate realistic displacement distributions for the different classes of particles with high spatio‐temporal resolution. Numerical modeling is a very encouraging approach, which makes our study original because it is the first time that the estimation of mean travel distances, the application of an exponential distribution, and the comparison with a hydrodynamic model are combined. A more effective modeling strategy involves incorporating a probabilistic transport model in the 2D numerical model to reproduce the observed scatter of the individual particle trajectories.

Transient Bedload Transport During Flashflood Bores in a Desert Gravel‐Bed Channel

AbstractEvidence of the effect of unsteady flow on bedload sediment transport is meager, particularly in desert rivers, despite its scientific, environmental and engineering importance. Our study of the phenomenon was conducted in Nahal Eshtemoa, Israel, an ephemeral river located in a semi‐arid area and subjected to flash floods. During 4 years, we monitored bedload transport and contemporaneous hydraulics, focusing on two intervals during which flow changed most rapidly: (a) the arrival of flood bores over dry river beds and (b) the propagation of subsequent bores over flowing water. Monitoring involved deployment of plate microphones, video cameras, surface velocity radar and a 3D velocimeter, enabling synchronized and unique data acquisition at high temporal resolution. We hypothesized that turbulence would be considerably elevated during the arrival of a flood bore and that this would induce transient entrainment of bedload. Our results demonstrate that bedload is indeed transported by the bore front and ceases soon after the arrival of most bores, even though water depth continues to rise. Bores arriving on a dry river bed usually generate higher bedload fluxes than bores advancing over flowing water. Elevated values of shear stress, turbulent kinetic energy and water‐surface slope occur as a bore impinges but then decrease. This is likely to be the explanation for high transient rates of bedload transport. Once the threshold depth previously determined as inducing continuous bedload motion is attained (≈40 cm), transport rates rise considerably relative to those associated with the bores.

Study on Characteristics of Pipeline Hydraulic Transportation of Coarse Particles Based on LBM-DEM Method

Pipeline hydraulic transportation is an important method for transporting solid materials. An LBM–DEM-based simulation method is established in the Visual Studio platform using the Fortran language, which can model the hydraulic transportation process of coarse particles in a pipeline. To verify the feasibility of this numerical simulation method, we also conduct pipeline hydraulic transportation experiments and find that the simulation results are consistent with the experimental results. This method is used to investigate the motion characteristics of coarse particles in horizontal pipelines under different conditions because of its advantages of clear physical meaning, ability to deal with complex boundary conditions, and suitability for parallel computing. The results show that the bedload motion is the main motion mode of coarse particles in horizontal pipelines. Changes in the pipeline flow velocity and particle diameter can cause some particles to saltate, and the critical pipeline flow velocity of saltation particles has a linear relationship with the particle diameter. During the process of coarse particles changing from the static state to stable state, the velocity curve of coarse particles gradually changes from “C” type to “S” type with the increase of particle diameter. Moreover, there is a linear relationship between the pipeline flow velocity and the stable velocity of coarse particles, while the particle diameter has no significant influence on the stable velocity. This study provides guidance for the development of pipeline hydraulic transportation technology of coarse particles.

Detection of the Bedload Movement with an Acoustic Sensor in the Yangtze River, China

The acoustic method, which enables continuous monitoring with great temporal resolution, is an alternative technique for detecting bedload movement. In order to record the sound signals produced by the impacts between gravel particles and detect the bedload motion, in this study, a hydrophone is placed close to the riverbed at the upper Yangtze River. Three categories of raw audio signals—moving gravel particles, ship engines, and flow turbulence—are collected and investigated. Signal preprocessing is performed using spectral subtraction to reduce the noise of the background sound, and the sound signal characteristic parameters are then calculated. In this paper, we propose a novel method for detecting and extracting bedload motion parameters, including peak frequency, pitch frequency, and energy eigenvector. When a segment of a speech signal meets the indicators for all three feature parameters simultaneously, the segment signal is classified as a bedload motion sound signal. Further work will be conducted to investigate bedload transport using the extracted audio signal.

Characterisation and Assessment of a Flash Flood in the Himalaya: Understanding the Significance of High Magnitude Events in Sediment Mobilisation

Abstract An increase in the frequency of high magnitude events (climatological, geomorphic and meteorological) has been observed in the recent times. The observation has been closely linked to anthropogenic activities and climate change. The events have been studied on the basis of their trends, frequency, geographical distribution and impacts. A flash flood from 2018 in a small catchment in Uttarakhand, North West Himalayas is reported here. The event has been characterized based on the possible causes and observed effects. The runoff generation tendency of major sub-catchments within the study area was assessed and the area was mapped based on sediment connectivity. The hydrological characteristics of the event was calculated using Manning's equations. The event generated a peak discharge of 9953 m3/s flowing at 15 m/s with a stream power of 6.4 MW. Based on the hydrological assessment, the thresholds of bedload movement, sediment transport and redistribution was analysed. The peak flow had a capacity of mobilising boulders of 6m diameter. Putting together the field and modelled data, the difference in the nature of the of the topography was analysed that caused only a particular sub-catchment to respond to the same climatic forcing significantly more than the others and also highlighted the significance of such high magnitude stochastic events in controlling the rates of long term geomorphic processes that shape the landscape.

Experimental analysis of the effect of bed-load movement on flow hydraulic characteristics

Abstract In this study, the effect of bed-load movement on mean flow characteristics was evaluated in two rigid rectangular flumes. The experiments consisted of creating flow conditions carrying sediments with mean diameters of D50 = 0.5, 0.6, and 2.84 mm over both smooth and rough beds. Various sediment concentrations were injected at the upstream end of the flume at non-deposit injection rates to study the effect of various concentrations on flow resistance. The effect of sediment movement on flow resistance was examined by comparing the results with those of clear-water flows (without sediment injection on both smooth and rough beds). The results showed that the sediment transport at maximum injection rate may increase the friction factor up to 50% and 58% in the smooth bed, and up to about 75% and 80% in the rough bed with mean diameter of 0.5 and 0.6 mm. In addition, for D50 = 2.84 mm, the friction factor decreased in the smooth bed and increased up to 50% in the rough bed. In general, it can be concluded that bed-load transport can be increased by the flow friction factor. The results also showed that bed-loads may decrease the average velocity and increase shear velocity with extraction of momentum from the flow, and both the mentioned factors may increase the flow friction factor. Raising the bed-load concentration in the flow may increase the elevation of the friction factor, approaching a constant value after reaching the aggregation threshold and generation of bed-forms.

Incipient Motion of Gravel Bedload Considering the Secondary Flow in Bend

The transport of gravel bedload in river bend is one of the basic problems of river dynamics. However, both the complicated bend flow structure and the stochastic bedload transport make explaining the problem theoretically difficult. Flow is the power of the bedload transport, so obtaining the precise theoretical flow structure is fundamental. Through considering the velocity-dip phenomenon and giving the matched boundary conditions, we have obtained the completed three-dimensional bend flow structure in the previous study. Combining the flow structure and the influences of the two-way exposure, we did further research on the incipient motion of the gravel bedload in bend in this paper. It turns out that, because of the existence of the secondary flow, the particle in bend forced by both the longitude and the transverse velocities in the plane. The two-way velocities form an intersection angle which influences the incipient velocity and direction of the particle movement. Moreover, the non-uniform distribution of the bend flow structure along the cross-section decides the differences of the intersection angle, incipient velocity, transport speed of the bedload in the bed surface. Then, the above differences result in the change of the sediment gradation and partition behaviour of the bedload transport.

Upscaling Sediment‐Flux‐Dependent Fluvial Bedrock Incision to Long Timescales

AbstractFluvial bedrock incision is driven by the impact of moving bedload particles. Mechanistic, sediment‐flux‐dependent incision models have been proposed, but the stream power incision model (SPIM) is frequently used to model landscape evolution over large spatial and temporal scales. This disconnect between the mechanistic understanding of fluvial bedrock incision on the process scale, and the way it is modeled on long time scales presents one of the current challenges in quantitative geomorphology. Here, a mechanistic model of fluvial bedrock incision that is rooted in current process understanding is explicitly upscaled to long time scales by integrating over the distribution of discharge. The model predicts a channel long profile form equivalent to the one yielded by the SPIM, but explicitly resolves the effects of channel width, cross‐sectional shape, bedrock erodibility, and discharge variability. The channel long profile chiefly depends on the mechanics of bedload transport, rather than bedrock incision. In addition to the imposed boundary conditions specifying the upstream supply of water and sediment, and the incision rate, the model includes four free parameters, describing the at‐a‐station hydraulic geometry of channel width, the dependence of bedload transport capacity on channel width, the threshold discharge of bedload motion, and reach‐scale cover dynamics. For certain parameter combinations, no solutions exist. However, by adjusting the free parameters, one or several solutions can usually be found. The controls on and the feedbacks between the free parameters have so far been little studied, but may exert important controls on bedrock channel morphology and dynamics.

Large eddy simulation of sediment transport in high flow intensity by discrete particle method

The processes of sediment transport in high flow intensity are studied through numerical simulation using a coupled CFD (computational fluid dynamics)-DPM (discrete particle method) model with a focus on discussing the co-existence problem of bed load and suspended load and the corresponding sediment concentration distribution. It is shown that for the sediment-laden flows, the components of turbulence intensity decrease slightly as sediment concentration increases. The bed load and suspended load both coexist in a transition layer. Particle collision plays an important effect on the velocity of bed load movement. The total sediment transport rate obviously increases with the increase of flow intensity, but the bed load transport rate does not clearly increase. In the present study, the maximum value of suspended sediment concentration curve is about 7%, and increases with the increase of the flow intensity. The simulation results are generally consistent with experimental observation.

Hydrodynamic modeling and radiotracer investigations in Kolkata Port Trust, Kolkata, West Bengal, India

In this study, hydrodynamic modeling and radiotracer investigations were conducted in Kolkata Port Trust, Kolkata, West Bengal, India to investigate the dispersion of sediments and to evaluate the suitability of three different dumping sites (Upper Auckland, Lower Auckland and Eden) for dredged sediments along a navigation channel. Scandium-46 (148-240 GBq) comprising scandium glass powder with a particle size distribution ranging from 75-100 μm was used as a radiotracer. The radiotracer was injected onto the seabed at the selected site using a remotely operated injection system and its movement on the seabed was tracked over a period of three months using waterproof NaI(Tl) scintillation detector. The tracer concentration curves were plotted on a site map of the area for different trackings and isocount contours were plotted. Qualitative information was obtained from the isocount contours such as the general direction of movement and spread of the radiotracer. The isocount contours showed that the sediment predominantly moved away from the navigation channel. From the isocount, a transport diagram was plotted based on isocount counter and sediment transport parameters such as the general direction of movement by sediments on the seabed, transport velocity, transport thickness and bed load movement rate were determined for all the sites. The dispersion patterns obtained for the sediments by modeling were compared with the experimental results and they were found to be similar. Thus, the model results were validated and the proposed sites were found to be suitable for dumping the dredged material because the sediments did not return to the navigation channel.

Numerical investigation of particles\u2019 transport, deposition and resuspension under unsteady conditions in constructed stormwater ponds

BackgroundTotal suspended solids (TSS) loads carried by stormwater runoff is a major pollutant source on receiving water bodies. Stormwater ponds are widely used for controlling TSS discharge. However, the trap efficiency is not satisfactory because it is affected by many complex factors, which are not fully understood. Therefore, there is a necessity to gain insight into the sediment process in stormwater ponds for optimization design of stormwater ponds. To address this issue, we propose a novel modeling framework based on discrete phase model (DPM), aiming to fully represent the sediment transport, settling, and resuspension at grain scale under time-dependent conditions.ResultsIn the newly proposed method, heterogeneous characteristics of sediments’ loads, varying flows and sediment loads, settling and resuspension effect at grain scale, time-dependent conditions, and turbulent effect are all well considered. The proposed models have been coded with C language and hooked in computational fluid dynamics software Fluent, and the methods were tested with a case of laboratory experimental setup. Different bed boundary conditions are tested and compared with the observation data for optimization parameters’ identification. The simulation results demonstrated that the physically based DPM with the newly developed method can well reproduce the evolution of sediment transport, settling and resuspension behaviors compared with the scale experiment.ConclusionsThe newly proposed method can accurately predict the trap efficiency and temporal–spatial sediment distribution. The decomposition of bed load motion at grain scale is a necessary and valid way to represent the sedimentation process in shallow ponds. The developed model could be a tool to help us gain insight into the sediment transport phenomena at grain scale in shallow tanks since it can provide detailed information which the experiment cannot.

Dispersal and aging of terrigenous organic matter in the Pearl River Estuary and the northern South China Sea Shelf

Large rivers discharge great amounts of terrigenous organic carbon (OCterr) to the ocean, 90% of which are trapped in the coastal areas. The OCterr processing during transport to and within the coastal oceans are not well known. The Pearl River is the second largest river in China in terms of annual runoff. Here, we studied OCterr delivered from the river in the Pearl River Estuary (PRE) and the northern South China Sea (SCS) shelf using stable and radioactive carbon isotope compositions (δ13C and F14C) of OC in surface sediments to (1) constrain sources of OC, (2) explore the role of hydrodynamic processes for OCterr transport, and (3) better understand the burial and degradation processes of OCterr.A three end-member mixing model based on δ13C and 1/(C/N) values of bulk OC was used to calculate relative contributions of OCterr, OC derived from marine (OCmar) and riverine primary production (OCRpp). Results showed that the PRE and its western coastal inshore mud deposit are dominated by OCterr (70 ± 3% and 54 ± 10%, respectively), but other areas receive increasing contributions from OCmar: the slope (49 ± 10%), the eastern coast (58 ± 2%) and the outer shelf (68 ± 3%). OCRpp accounted for substantial proportions in the PRE (14 ± 6%) and rapidly decreased in the offshore area (3–5%), likely due to extensive aerobic respiration of organic matter (OM). Subsequently, average F14C values of OCterr were calculated ranging from 0.271 to 0.639 using a Monte-Carlo simulation strategy and based on the assumption that F14C values of OCRpp and OCmar varied within narrow ranges. Together with OCterr contents and grain sizes of sediments, F14C values of OCterr were used to distinguish two regions of distinctive sedimentological characteristics in the study area. Region I, including the PRE and inner shelf, showed a decrease of OCterr content along the trajectory of westward along-shelf transport, reflecting resuspension-dominated conditions for OCterr transport. Region II, including the outer shelf and one site on the inner shelf (E701), exhibited relatively old OCterr (9180 ± 730 yr BP), which is attributed to preferential accumulation of coarser sediments by bedload movement. Finally, a first-order degradation rate constant was calculated from contents and 14C ages of OCterr, yielding a slow OCterr degradation rate of (2.88 ± 0.61) × 10–4 yr−1, indicating a more refractory nature and likely effective mineral protection of OCterr. The ΣCO2 efflux due to OCterr degradation was estimated to be 3.68 ± 1.39 Gg C yr−1 for the entire surface sediments in the Pearl River derived mud belt, equivalent to only 0.7 ± 0.3% of the total particulate OC flux of the Pearl River. This, hence, suggests that the OCterr is more persistent and the preservation of OCterr in the marine system is better than previously thought, but experiences substantial degradation on centennial to millennial timescales.

Field Application and Validation of a Seismic Bedload Transport Model

AbstractBedload transport drives morphological changes in gravel‐bed streams and sediment transfer in catchments. The large impact forces associated with bedload motion and its highly dynamic spatiotemporal nature make it difficult to monitor bedload transport in the field. In this study, we revise a physically‐based model of bedload‐induced seismic ground motion proposed by Tsai et al. (2012) and apply it to invert bedload flux from seismic measurements alongside an Alpine stream. First, we constrain the seismic response of a braided river reach with a simple active experiment using a series of large‐rock impacts. This allows the characterization of surface wave propagation and attenuation with distance from the impact source. Second, we distinguish bedload‐generated ground vibrations from those caused by turbulent flow using frequency‐based scaling relationships between seismic power and discharge. Finally, absolute bedload transport rates are quantified from seismic measurements using inverse modeling based on a simplified formulation of bedload particle motion. The results are verified with a large data set of bedload samples (between 0.01 and 1 kg/m/s), demonstrating that seismic measurements can provide an indirect measure for bedload flux with an uncertainty of less than one order of magnitude. Larger deviations may be due to the contribution of turbulent flow effects and remaining uncertainties in particle impact mechanics (elasticity and velocity). When constraining these uncertainties, seismic monitoring may provide an accurate and continuous means to investigate bedload dynamics in gravel‐bed streams.

Experimental Study on Uniform and Mixed Bed-Load Sediment Transport under Unsteady Flow

The scouring and deposition of sediment caused by unsteady flows (e.g., storm waves and floods) produces many secondary disasters. The resultant bed-load movement exhibits different transport laws compared with that by steady flow. In this study, the flume experiments were performed to study the bed-load movement under unsteady flow with different velocity skewness. The movement of uniform and non-uniform non-cohesive sediment under unsteady flow as well as the influence of the steady and unsteady flow on sediment transport rate are compared. Additionally, the non-uniform sediment transport formula of fine-to-coarse particle diameter ratio was investigated. The results showed that the sediment transport rate between uniform and non-uniform sand under the same median diameter is different. The non-uniform sediment transport rate is 1.27-, 3.19-, and 0.68-times as large as that in uniform sediment under d50 = 0.664, 1.333, and 2.639 mm under unsteady flow, respectively. For non-uniform sand, the transport rate of non-uniform sand with a larger adjacent particle size ratio (δ = 0.29) was 1.31-times greater than that of the non-uniform sand with a smaller adjacent particle size ratio (δ = 0.50). Moreover, theoretical deduction was carried out and the incipient sediment motion was analyzed from the force mechanism. A new unsteadiness parameter based on the acceleration concept was proposed. The relationship between the travel distance and velocity skewness of sediment particles was set up. The experimental results and theoretical analysis showed that sediment under unsteady flow were easier to start and transport than those under steady flow in the same flow effect. The travel distance of sediment particles was longer under unsteady flow than that under steady flow.

Particle image velocimetry validation for quantifying bedload movement

A Particle Image Velocimetry (PIV) technique was validated for predicting movements of bedload particles. Synthetic images, similar to those from testing a physical model, were created with different image particle densities and particle displacements. A set of surface particles were digitally moved by rotating and translating them on top of a layer of stationary particles to replicate the mobile-bed layer in the physical model. The images were processed using PIV software and performance of the software to measure these movements was evaluated. The effect of image particle density, particle displacement, and interrogation window size on the PIV software performance was investigated. Study results showed that all investigated variables had a significant effect on the predicted velocities. Guidelines are provided for application of PIV to bedload transport based on study results. Optimum bedload velocity predictions were found with the combination of highest particle density and highest particle displacements.