Prediction Of Sediment Transport Research Articles

Comparing grain size composition of inter-rill and rill-eroded sediment from cultivated hillslope soils using caesium-134 and cobalt-60 as tracers

Understanding the extent to which textural characteristics of inter-rill and rill-eroded sediment differ across a range of agricultural soils is important in order to more accurately predict sediment transport capacities and thus increase the reliability of erosion models. However, a lack of consensus still exists over how much influence these two mechanisms exert on the physical characteristics of sediment during an erosion event. This communication reports results from a prototype tracing technique using two different artificial radionuclides to compare the grain size composition of inter-rill and rill-eroded sediment from eight different cultivated hillslope soils from southern England. A 1.0 m * 0.2 m * 0.11 m soil box was filled with soil labelled with caesium-134 (134Cs) to a depth of 0.1 m. A second layer of soil labelled with cobalt-60 (60Co) was distributed across the first soil layer, to a depth of 0.01 m, and lightly textured to replicate a fine tilth. The soil box was pitched at a ca. 17% slope (9.5°) and each soil was subjected to ca. 35 mm hr−1 of simulated rainfall until surface runoff was initiated and a rill developed. All runoff-eroded sediment was collected over 90 s intervals, dried and radiometrically assayed. Changes in radiometric signals were used to identify the transition from inter-rill to rill erosion, demarking a shift in sediment-source. Samples recording highest activity of either radionuclide were deemed to contain largest contributions of sediment mobilised by each respective erosion process and were selected for grain size analysis. Grain sizes, measured at 10 percentile intervals, were subjected to a paired-sample statistical analysis to determine whether size-difference was significant. Rill-eroded sediment was significantly coarser than inter-rill eroded sediment for four soils, whereas inter-rill eroded sediment was significantly coarser than rill-eroded sediment for one soil. There was no significant size-difference for two soils, and one soil was omitted from statistical analysis due to excessive contamination by the sub-dominant erosion process.

Sediment Transport and Bed-Form Characteristics for a Range of Step-Down Flows

AbstractEphemeral streams with mobile beds represent a challenging environment for sediment transport prediction. Bed topography from relatively infrequent high flow events may persist after hydrog...

In-situ study of the spatiotemporal variability of sediment erodibility in a microtidal estuary

Seabed erosion is a vital process for sediment transport in estuaries and coasts. In this study, we investigated the seabed erosion process and its spatiotemporal variability in the Pearl River Estuary, China, via in-situ observations and laboratory experiments. It is found that the seabed was more erodible in the channel than at the shoal in most cases due to the difference in sediment size. In a neap-spring-neap tidal cycle, a laminated seabed with different sediment ages was inferred to form due to deposition on flood tides and erosion on ebb tides. The seabed was more erodible during spring tides than neap tides because of the difference in sediment availability at the unconsolidated fresh layer. Furthermore, the erosion curve showed evident flood-ebb asymmetry, which was found to be related to the variations of sediment availability and bottom stress. Besides, erosion experiments with different sediment consolidations revealed that sediment erodibility decreased with the growth of consolidation. This study promotes our understanding on the spatiotemporal variability of sediment erodibility, and has significant implications for simulation and prediction of estuarine sediment transport and morphological evolution.

Nondimensional sediment transport capacity of sand soils and its response to parameter in the Loess Plateau of China

AbstractSoil erosion is a major contributor to land degradation in the Loess Plateau in China. To clarify the sediment transport capacity of overland flow influenced by hydraulic parameters, such as shear stress, sand shear stress (hydraulic gradient partition method and hydraulic radius partition method), mean flow velocity, Froude number, stream power, and unit stream power, indoor experiments with eight‐unit‐width flow discharges from 0.0667 × 10−3 to 0.3333 × 10−3 m2·s−1, six slope gradients from 3.49 to 20.79%, and two kinds of sand soils (d50 = 0.17 and 0.53 mm) were systematically investigated. A nondimensional method was adopted in data processing. Results showed that there was a partition phenomenon of relation curves because of the different median grain diameters. The correlation between the nondimensional stream power and nondimensional sediment transport capacity was the highest, followed by the correlation between the nondimensional unit stream power and nondimensional sediment transport capacity. However, there was a poor correlation between the flow intensity indices of velocity category and nondimensional sediment transport capacity. Nondimensional stream power, nondimensional unit stream power, and nondimensional shear stress could predict sediment transport capacity well. Ignoring the partition phenomenon of the relation curves, stream power could be used to predict sediment transport capacity, with a coefficient of determination of .85. Furthermore, a general flow intensity index was obtained to predict sediment transport capacity of overland flow. Finally, an empirical formula for predicting sediment transport capacity with a coefficient of determination of .90 was established by multiple regression analyses based on the general flow intensity index. During the analysis between measured sediment transport capacities in present study and predicted values based on Zhang model, Mahmoodabadi model, and Wu model, it was found that these three models could not accurately predict sediment transport capacities of this study because different models are estimated on the basis of different experimental conditions.

Statistical Analysis of Bed Load Transport over an Armored Bed Layer with Cluster Microforms

River engineers have long been challenged by the need to predict sediment transport, especially over armored riverbeds. This study investigates the statistical properties of bed load transport over an armored bed layer with cluster microforms in laboratory experiments. Particle clusters on the sediment bed were formed by widely graded particles under constant flow. A series of key kinematic parameters computed from particle trajectories recorded by a digital camcorder, including mean squared particle displacement (MSD), particle number activity, particle velocities, step length, and rest period, were analyzed. The scaling growth of the MSD with time showed that the particle diffusion regime was superdiffusive at small time scales, but became subdiffusive at larger time scales. The particle number activity follows a negative binomial distribution, and the probability distributions of streamwise and transverse particle velocities displayed heavy asymptotic tails, which indicates the particle clusters might exert a dual impact on bed load transport: some particles are accelerated in the preferential paths between particle clusters, while others were obstructed by the particle clusters. In addition, the bed load diffusion regime varied with observation time scales. The findings of this study can gain insight into the bed load transport processes over armored riverbeds.

The Feasibility of Integrative Radial Basis M5Tree Predictive Model for River Suspended Sediment Load Simulation

Accurate suspended sediment transport prediction is highly significant for multiple river engineering sustainability. Conceptually evidenced, sediment load transport is highly stochastic, spatial distributed and redundant pattern due to the incorporation of various hydrological and morphological variables such as river flow discharge and sediment physical properties. The motivation of this study is to explore the feasibility of newly intelligent model called Radial basis M5 model tree (RM5Tree) for suspended sediment load (St) prediction for daily scale information at Trenton hydrological station, Delaware River. Numerous input combination attributes are formulated based on the preceding information of sediment and river flow discharge. The prediction accuracy “based statistical and graphical visualizations” of the proposed model validated against numerous well-established predictive models including response surface method (RSM), artificial neural network (ANN) and classical M5Tree based models. The investigated input combinations behaved differently from one case to another. The optimum input combination attributes are included two months lead times of sediment and discharge information to predict one step ahead St. The attained results of the proposed RM5Tree model exhibited a remarkable prediction accuracy with minimal values of root mean square error (RMSE≈2091 ton/day) and coefficient of determination (R2≈0.86). This presenting a percentage of enhancement in the prediction accuracies by (51.6, 53.1 and 26.3) over (RSM, ANN and M5Tree) optimal models over the testing phase.

Assessing and solving multicollinearity in sediment transport prediction models using principal component analysis

ABSTRACT Predicting sediment transport in a river system is necessary when attempting to monitor river changes due to incoming particle deposition from upstream reaches. Previous studies have used various techniques such as probabilistic, regression, and equal mobility approaches to make predictions. However, issues associated with multicollinearity occur when using the regression approach. Significant correlations between the explanatory variables in a regression equation can increase the coefficient variance estimates, which make these estimates very sensitive to minor changes. Thus, we propose the use of principal component analysis as a solution to these issues. We observed that the variance inflation factor is less than 10, which indicates the nonexistence of issues associated with multicollinearity. Fitting components to the regression equation create a new prediction model for sediment transport, which improves upon previous equations using the same technique.

Effect of stem cover on hydraulic parameters of overland flow

Vegetation can effectively prevent soil loss and play an important role in soil and water conservation. Accurate estimation of hydraulic parameters is critical for soil erosion models. Hydraulic data for different levels of vegetation stem cover, slope gradient and flow discharge are quite limited. The objectives of this study were to compare different measured methods of flow velocity, evaluate the influence of vegetation stem cover, slope gradient and discharge on hydraulic parameters and predict the shear stress (τ), stream power (Ω) and unit stream power (ω) and emphasize the significance of hydraulic radius in the calculation of the shear stress (τ), stream power (Ω), and unit stream power (ω). A non-erodible flume bed was used in this study. The discharge ranged from 0.5 × 10−3 to 2.0 × 10−3 m3 s−1, the slope gradient ranged from 8.8% to 25.9%, and an artificial stem cover of approximately 0, 1.25, 2.5, 5, 10, 15, 20, 25 and 30% was used to represent the natural vegetation. The stems were 2 mm in diameter and randomly arranged. The electrolyte pulse method was preferred for measurement of the overland flow velocity. The flow velocity decreased as the stem cover increased, and flow discharge and slope gradient decreased. The reduction in flow velocity was as high as 90% for 30% stem cover based on the experiments. The flow depth increased as the stem cover and flow discharge increased as the slope gradient decreased. The Reynolds and Froude number values decreased with increasing stem cover. The shear stress, stream power and unit stream power were all significantly affected by the stem cover and could all broadly be described using an exponential delay function of the stem cover. The ω was no significantly impacted by hydraulic radius. Thus, the unit stream power is an improved hydraulic parameter for predicting sediment transport capacity.

Application of the two-fluid model to prediction of sediment transport in turbulent open channel flow

The two-fluid model (TFM) formulation was used to predict the liquid and sediment transport in an open channel flow for dilute concentrations. A low-Reynolds-number k–ε model was used to represent the turbulence in the liquid phase; it included the effect of the sediment particles on the turbulence field. The particle-wall interactions were implemented using the boundary condition of Johnson and Jackson (1987), which resulted in a finite value for the sediment velocity at the bottom wall. The values of the specularity coefficient and coefficient of restitution for the particle-wall collisions were specified. Overall, the TFM prediction for the liquid velocity, sediment concentration, and turbulent statistics agreed well with two different experimental test cases by Lyn (1987). A key parameter for determining the sediment phase transport was the granular temperature, the level of which was set by the value of the specularity coefficient at the wall. In the context of the TFM, the presence of the sediment particles was found to enhance the wall shear stress of the liquid phase.

Predicting macroturbulence energy and timescales for flow over a gravel bed: Experimental results and scaling laws

The fluvial science community lacks predictive equations for macroturbulence energy and timescales, which may serve useful for predicting sediment transport in rivers. Therefore, we investigated the systematic variation of macroturbulence scales by varying hydraulic parameters in an experimental facility, collecting velocimetry data and measuring macroturbulence scales with spectral analyses, and testing scaling laws for macroturbulence prediction that consider inner variables, outer variables, and turbulence kinetic energy. Experiments spanned across 11 energy gradients and aspect ratio combinations including 116 testing conditions each with 30 minute instantaneous turbulence time-series collected. Results showed velocimetry data and time-average turbulence parameters supported the presence of an equilibrium outer region and coherent double-layer assumed in the experimental design. Macroturbulence and bursting wavenumbers were constant in the outer region when scaled with the flow depth, and were independent of distance above the boundary, aspect ratio, energy gradient, Froude number, or Shields parameter. Data results agreed with the derivation herein and showed macroturbulence energy scales with the streamwise turbulence kinetic energy. Semi-theoretical equations for turbulence kinetic energy scaled macroturbulence energy reasonably well. A novel equation is presented for the macroturbulence period, and the equation's transferability is supported by consistency with past macroturbulence studies, bursting formula, and flow visualization studies. Novel equations are presented for macroturbulence energy, and transferability is supported on theoretical and empirical grounds; however open questions remain regarding a universal function for streamwise turbulence kinetic energy and the net contribution of macroturbulence to total turbulence energy. Results showed the macroturbulence wavenumber required a relatively long duration of data (i.e., 17 min) to stabilize. Results were used to formulate a time scale for minimum measurement length of turbulence data when information about macroturbulence is sought after. We discuss macroturbulence linkages with sediment transport, including, prediction of the sediment carrying capacity via macroturbulence formula as one application to help close the gap between turbulence and fluvial sediment transport.

Development of an Ice Jam Flood Forecasting System for the Lower Oder River—Requirements for Real-Time Predictions of Water, Ice and Sediment Transport

Despite ubiquitous warming, the lower Oder River typically freezes over almost every year. Ice jams may occur during freeze-up and ice cover breakup phases, particularly in the middle and lower reaches of the river, with weirs and piers. The slush ice and ice blocks may accumulate to form ice jams, leading to backwater effects and substantial water level rise. The small bottom slope of the lower Oder and the tidal backflow from the Baltic Sea enhance the formation of ice jams during cold weather conditions, jeopardizing the dikes. Therefore, development of an ice jam flood forecasting system for the Oder River is much needed. This commentary presents selected results from an international workshop that took place in Wrocław (Poland) on 26–27 November 2018 that brought together an international team of experts to explore the requirements and research opportunities in the field of ice jam flood forecasting and risk assessment for the Oder River section along the German–Polish border. The workshop launched a platform for collaboration amongst Canadian, German and Polish scientists, government officials and water managers to pave a way forward for joint research focused on achieving the long-term goal of forecasting, assessing and mitigating ice jam impacts along the lower Oder. German and Polish government agencies are in need of new tools to forecast ice jams and assess their subsequent consequences and risks to communities and ship navigation along a river. Addressing these issues will also help research and ice flood management in a Canadian context. A research program would aim to develop a modelling system by addressing fundamental issues that impede the prediction of ice jam events and their consequences in cold regions.

Developing a Stochastic Model for Studying and Simulating Sediment Transport in Ports and Harbors

A particle model to describe and predict sediment transport in shallow water is developed with the use of random walk models. The model is developed by showing consistency between the Fokker-Plank equation and the Advection diffusion equations. Erosion and deposition process in the model are developed probabilistically where the erosion term is considered to be a constant and deposition term is taken as a function by relating sediment settling velocity and diffusion coefficient. Eventually, we simulated the particle model by considering three environment tests. In each environment test the simulations show the distribution of particle and the position of each particle at any given time t. The simulations also show the particles that will finally remain in suspension state and the particles that will be deposited during the transport process following the deployment of 10,000 particles. It was also established that there is a uniform distribution of particles in the test environment I and III and a linear dependence between the number of particles in a different grid cell and the water depth in test environment II.

SEDIMENT ADVECTION AND DIFFUSION BY OBLIQUELY DESCENDING EDDIES

Three-dimensional vortex structures involving obliquely descending eddies (ODE), produced by depth-induced breaking-waves, has been proved to be associated with local sediment suspension in the surf zone (Zhou et al., 2017); vertical velocity fluctuations around the ODEs induces sediment suspension near the bed. Otsuka et al. (2017) explained the mechanical contributions of the ODEs to enhance local sediment suspension under the breaking waves and modeled the vortex-induced suspension to predict the profile of the equilibrium sediment concentration in the surf zone. In order to predict local behaviors of sediment, however, sediment-turbulence interactions in the transitional turbulence under breaking waves need to be understood. The interaction may be described in terms of Schmidt number (Sc). Sc has been empirically determined for trivial steady flows such as open channel or pipe flows. In the surf zone where organized flows evolve into a turbulent bore, the interaction may vary with the transitional feature of turbulence during a wave-breaking process, and thus Sc may be variable in time and space. No appropriate Sc model has been proposed for the surf zone flow. A parametric study on the sediment motion with respect to the variation of Sc is required for better prediction of sediment transport in the surf zone. In this study, contributions of the sediment advection and diffusion in the vortex structure to the concentration are computationally investigated. Effects of Sc to the sediment suspension and diffusion process will be also discussed in this work.

MODELING COARSE SAND TRANSPORT UNDER SKEWED OSCILLATORY FLOW USING A CFD-DEM APPROACH

Onshore/offshore sediment transport in the nearshore is an important mechanism driving the evolution of coastal morphology. The so-called sheet flow is a transport regime, in which the flow forces are intense such that a large amount of transport occurs in a concentrated layer near the bed. Onshore transport is often associated with flow skewness/asymmetry. In the nearshore zone, due to the bottom slope and wave shoaling, the wave velocity tends be onshore skewed before breaking in the surf zone. For breaking waves, the velocity asymmetry (or acceleration skewness) may also play a key role in determining net sediment transport. Understanding the net sediment transport rate in response to wave skewness/asymmetry is fundamental to a better prediction of sediment transport in regional scale morphodynamic models. In this study, we used an Euler-Lagrange two-phase model to study sheet flow transport of coarse sand under oscillatory flows subject to velocity/acceleration skewness.

Effective Nikuradse Roughness on the Mobile Plan Bed

Nikuradse roughness (ks) is very important in the sediment transport prediction because it is related to the evaluations of the velocity distribution, shear stress and erosion depth. Dimensionless Nikuradse roughness (ks/D, where D is the sediment diameter) is usually given 1–2.5 on the immobile plan bed or at low shear stress. But it behaves differently on the mobile plan bed at high shear stress with much sediment picked up to movement when the Shields parameter (Θ) is larger than 0.8–1.0. The effective Nikuradse roughness on the mobile plan bed was derived indirectly from the erosion depth correlated to the mobile plan bed thickness considering the mass conservation in the present study. The proposed erosion depth confirmed the relation to the Shields parameters with an extra factor consisting of suspended sediment and its damping to turbulence. The decrement of the erosion depth caused by the increment of the sediment diameter at large shear stress was obtained, which was usually absent in classical empirical formulas based on the bedload theory. Good agreement with experiments was achieved by the present prediction of the Nikuradse roughness, erosion depth and sediment transport rate. Discussion was mainly focused on the prediction improvement caused by considering the impact of suspended sediment and its damping to turbulence.

Effects of ridge height and spacing on the near-surface airflow field and on wind erosion of a sandy soil: Results of a wind tunnel study

In wind tunnel experiments, we measured the airflow field near the ground surface and the resulting sand transport rate over a sand bed that contained widely but uniformly spaced non-erodible ridges. We found that when airflow passed over the bed, the wind velocity changed drastically around the first ridge, but the magnitude of the change decreased gradually moving downwind until a relatively stable airflow developed, characterized by regular fluctuation over the uniformly spaced ridges. The distance from the first ridge to the zone of stable airflow increased with increasing ridge height or spacing between ridges. This airflow field led to a non-uniform distribution of wind erosion that affected total sediment transport from the sand bed. Using our wind tunnel simulation results as inputs for existing sediment transport equations, we developed a model to predict sediment transport (q) over a sand bed that contained widely spaced ridges: q=C'dDρgu*2u*-u*texp⁡(-aHu*3n). The model accounts for the ridge height (H), number of ridges per unit length (n), and the wind’s free-stream friction velocity (u*) and threshold friction velocity (u*t). We discussed the different mechanisms by which ridges influence the near-surface airflow at large spacing (> 5H) and small spacing (≤ 5H). The ridges appear to affect the airflow as roughness elements when the spacing is small, but act as barriers to the wind when the spacing is large.

Evaluation of estuary shoreline shift in response to climate change: A study from the central west coast of India

AbstractThe prediction of coastal sediment transport and rate of change of shorelines into the future are traditionally done by analysis of historical satellite imageries and field observations or by empirical/numerical modeling. The modeling is traditionally done on the basis of historical data. Instead, here, we suggest the use of future conditions impacted by the climate change for this purpose and a procedure based on regional climate models. Further, considering the difficulty at some places to acquire a large amount of data of various parameters to run a numerical model, we propose the use of simple neural network as an alternative. The location studied belongs to the shoreline adjoining the estuary of River Gangavali, along the central west coast of India. Waves were simulated using a numerical wave model for past and future time periods of 36 years each, and a numerical coastal evolution model was run with this input. It was found that in future, the wave activity at this site would intensify along with certain shift in the direction of wave attack. This will push the net and gross sediment transports up by 131.7% and 114.3%, respectively, and also enhance the shoreline change rate. It was noticed that the future shifts in the wave direction could be as influential as those in the wave height and can cause more accretion of the shoreline. The study emphasizes the importance of considering the projected climate over the past one in planning a regional coastal ecosystem.

Influence of turbulent horseshoe vortex and associated bed shear stress on sediment transport in front of a cylinder

This study concerns the flow and associated sediment transport in front of a cylinder in steady currents. The study comprises (i) flow characteristics induced by the turbulent horseshoe vortex (THV), (ii) bed shear stress within the THV region, and (iii) predicted sediment transport rates. The velocity fields in front of a wall-mounted circular cylinder were measured using time-resolved particle image velocimetry (PIV). The flow characteristics show that two time-averaged THVs are formed, and the dynamics of instantaneous THVs exhibit a quasi-periodic process from generation to death. Both the mean and fluctuations of bed shear stress within the THV region are significantly amplified, and their values are comparable. The probability density function of the instantaneous bed shear stress exhibits a double-peaked distribution and cannot be represented by the normally-used log-normal distribution for uniform channel-open flows. The comparisons of sediment transport rates where turbulent fluctuations in the bed shear stress are, or are not, taken into account show that the sediment transport rates calculated by the mean bed shear stress are under-predicted. Furthermore, a new sediment transport model incorporating the influence of bed shear stress fluctuations is proposed and validated by comparing the initial scour rate in front of the cylinder.

Erosion potential of dynamic ice breakup in Lower Athabasca River. Part I: Field measurements and initial quantification

As part of the Oil Sands Joint Monitoring Program, the present study aims to understand and quantify the erosional and sediment transport capacity of the Lower Athabasca River during the ice breakup period. It is motivated by past findings indicating that ice breakup processes can generate dominant contributions to the annual sediment load, which cannot be estimated by existing analytical methods and numerical models. Of special interest is the relatively steep reach above Fort McMurray, which potentially generates very large erosional forces during breakup and is a potential natural source of polycyclic aromatic hydrocarbons. A two-year field program has furnished unique data on the spatiotemporal water level variation during the breakup event. This information can serve to quantify key hydrodynamic variables and provide the requisite validation data for future model development to predict sediment transport under highly dynamic ice-influenced flow conditions. Bed shear directly indexes the erosional and sediment transport capacity of the flow; initial estimates based on the field measurements suggest that it exceeds all previously known values during breakup in other rivers. The power of the breakup is also manifested in the suddenness and height of various javes, in exceptionally high speeds of accompanying ice runs, as well as in bulk erosion of the river banks and bed in shallow areas.

Transport and deposition of fine sediment in a channel partly covered by flexible vegetation

Riparian plants exert flow resistance and largely influence the flow structure, which affects erosion, deposition and transport processes of fine sediments. Predicting these vegetative effects is important for flood, sediment and nutrient management. However, predictions on the fate of sediments are complicated by uncertainties associated with the suitable parameterization of natural plants and the associated effects on the turbulent flow field and on the variables in the transport equations. The aim of this study is to quantify deposition and transport of fine sandy sediment in a partly vegetated channel under laboratory conditions. Care was taken to reproduce conditions typical of vegetated floodplain flows including dense flexible grassy understory as a starting point. The experiments were conducted in a flume that is specifically designed to recirculate fine sediment. We measured suspended sediment concentrations with optical turbidity sensors and determined patterns of net deposition over the vegetated parts of the cross section. The flow field was determined with acoustic Doppler velocimetry. Our investigations are intended to improve future predictions of fine sediment storage and transport in natural or constructed vegetated channels, and the first results reported herein were useful in designing further, on-going experiments with complex combinations of vegetation and channel geometry. Key words: sediment transport, suspended sediment, deposition, riparian vegetation, flow field.