Bedload Research Articles

Numerical Analysis of Sediment Transport Rates from Rip Currents at an Open Inlet between Low Crested Breakwaters (LCB): The Role of Infra-Gravity Waves

Lee, J.L. and Cho, Y.-J., 2021. Numerical analysis of sediment transport rates from rip currents at an open inlet between Low Crested Breakwaters (LCB): The role of infra-gravity waves. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 489–493. Coconut Creek (Florida), ISSN 0749-0208. Low Crested Breakwaters (LCB), the most preferred structural type in the coastal zone management project by the Korean Ministry of Ocean and Fisheries, has been designed to mitigate beach erosion by high waves occurring only a few times a year. Now that moderate sea conditions are dramatically different from those in rough seas, these poor design practices could interrupt the grand circulation process of natural beaches over the course of a year. As a result, the beach stabilizing effect of LCB often falls short of our expectations. In this study, in order to test this hypothesis, 3-D numerical simulation was carried out to analyze the rip currents and its associated sediment transport at the open inlet between LCB when LCB is subject to infra-gravity waves, which play an indispensable role in the beach restoring process in a mild sea with an annual prevalence rate of over 80%. Numerical results show that in the case of LCB with lower crest freeboard, rip currents gets increased by 2.6 times when compared to the one before the deployment of LCB since the water mass influx toward the down-wave side of LCB by overflowing LCB by the preceding waves is redirected toward the open inlet. The sediment transport rate was estimated using Bailard's model, the most referred cross-shore sediment model in the literature. It was shown that the primary sediment transport mode at the open inlet between LCB was bed load, and sand of 5.62 ×10–5 m3 / m was leaving the inner zone of LCB per unit wave period.

Numerical Study of Local Scour around a Submarine Pipeline with a Spoiler Using a Symmetry Boundary Condition

A two-dimensional numerical model for solving the Navier–Stokes equations was developed to investigate the local scour around a submarine pipeline with a spoiler. Both the suspended load and the bed load were considered in the present numerical model. The focus of the present study is to investigate the effects of the spoiler length on the hydrodynamic forces on the pipeline and the spoiler as well as the local scour around the submarine pipeline. The corresponding numerical results show that the mean drag coefficients of the pipeline and the spoiler increase with the increase of the spoiler length. As for the mean lift coefficient, a general decreasing trend with the increasing spoiler length is observed for the pipeline. However, the mean lift coefficient of the spoiler first increases and then decreases with the increasing spoiler length. In addition, it is found that a larger spoiler length leads to a deeper scour depth, and an empirical equation was proposed for predicting the non-dimensional scour depth of submarine pipelines with non-dimensional spoiler length based on the numerical results.

Effect of Increasing Antecedent Flows on Equilibrium Bed-Load Transport Rates in a Laboratory Channel with a Sand and Gravel Bed Channel

Experiments were conducted in a laboratory flume in which gravel and total bed-load rates were measured continuously using independent methods that allowed evaluation of the effect of four antecedent flow conditions on the transport of bed load under standard flow. It was found that both mean total bed-load rate and gravel transport rates are related to the magnitude of antecedent flow. The changes in mean bed-load rates were attributed to changes in the stability of the coarsest fractions of the bed material, caused by the previous high-flow event. This work indicates that long-term mean bed-load transport rates for a sand and gravel bed are not just a function of grain size and flow rate but also vary with the magnitude of antecedent flow.

Analisa Angkutan Sedimen Pada Hulu Bendung Aepodu Kabupaten Konawe Selatan

Research on Aepodu Weir Sediment Transport Analysis in South Konawe District, based on observations in the field, Aepodu Weir hasa sediment buildup that has now exceeded the height of the weirlight house. The purpose of the study was to analyze the magnitudeof Aepodu river flow and to analyze the amount of sedimenttransport that occurred in the Aepodu dam. The method used todetermine the amount of bed load transport uses stchoklitscht, whilefor transporting suspended load using forcheimer.The results of the analysis of the average flow of the Aepodu riverwere 3,604 m3/ second. Sediment transport that occurs in Aepoduweir is Bedload transport (Qb) of 291625.771 tons / year, andsuspended load transport (Qs) of 16972,423 tons / year, so that thetotal sediment transport (QT) is 308598,194 tons / year.

Gravel excavation and geomorphic evolution of the mining affected river in the upstream reach of the Yangtze River, China

As economic development upstream in the Yangtze River basin has progressed in recent decades, the demand for sediment has rapidly increased and contributed to an expansion in sediment excavation that may affect the river's stability and navigation safety. In the current study, the distribution of gravel mining in the upstream reach of the Yangtze River was investigated using field measurements obtained from 2008 to 2017. An experimental investigation was then done to analyze the bed load behavior in a typical mined channel using a physical river model. Finally, a two-dimensional hydrodynamic-sediment transport numerical model was developed to predict the evolution of the mined channel. The results indicate that gravel mining was mainly distributed among the following nine river sections: Tongluoxia–Yibin, Erlong–Xianglutan, Naxi–Jinkou, Binpangqi–Hongyanqi, Chayuqi–Shenbeizui, Yangshipan–Shangbaisha, Dongxikou, Baisha–Wenzhongba, and Jiangjin–Shenzhongba. The gravel shoals near the riverbank have been destroyed, and the bed topography has been altered by up to 1.65 × 109 m3 over the past 10 years. A bed load velocity formula was proposed to describe bed load transport in the mined channel based on dimensional analysis. The recovery process of the excavation pits was slow in both the experiments and simulations, with only 0.12% and 1.39%, respectively, of the mined amounts of gravel being restored. Finally, the recovery rate of the mining pits gradually slowed as the sediment supply from upstream decreased, suggesting that the destroyed gravel shoals rarely returned to their original forms due to the combined operation of hydro-projects in upstream areas and limited sediment storage in the channel.

Sediment transport mechanisms and selective removal of soil particles under unsteady-state conditions in a sheet erosion system

Selective removal of particles and nutrients by water erosion is a key factor in soil erosion studies. Most agricultural soils are located on gentle slopes where fertility is high; however, until now, the main attention on sediment transport mechanisms was paid to high-slope gradients, where soil erosion is intense, but soils are less productive. Despite the importance of sediment size distribution (SSD) and transport mechanisms under unsteady-state conditions, few studies have been done on this issue. Higher sediment concentrations in the early stages of the runoff indicate the need to deal with unsteady-state conditions. To address this issue, sheet erosion experiments were done using a 0.2 m by 1 m tilting flume under controlled laboratory conditions. Six inflow rates (75, 100, 125, 150, 175, and 200 mL/s) were applied on two contrasting soils, including an adjacent agricultural soil and a not cultivated soil (namely Cropland soil and Control soil, respectively) at two low slope gradients (1.5% and 2%). No rill formation was observed during the experiments. The sediment-laden runoff was sampled during unsteady-state flow to determine the SSDs and sediment transport mechanisms of eroded particles for unsteady-state conditions. The results indicated different trends in the selective removal of sediment particles depending on hydraulic conditions and soil aggregate size. The contribution of suspension-saltation (SS) to the total sediment load for the Cropland and Control soils varied from 22% to 68% and from 35% to 59%, respectively, while up to 78% and 65% of soil particles were transported by bed load (BL), respectively. However, SS and BL indicated a reverse trend with stream power. The Cropland soil showed a single peak in the SSD at low stream powers because of the selective removal of fine particles (0.042 mm), whereas the SSD was shifted to a bimodal distribution at higher stream powers with the selective depletion of both fine (≤ 0.084 mm) and coarse (1.5 mm) particles. The Control soil experienced a unimodal SSD in a range of sizes between 0.109 and 0.175 mm for all stream powers. The findings of the current study highlight the need for further study on the erosion of low-slope agricultural soils, where small stream powers can remove fine and fertile soil particles.

A river flowing through a desert: late Quaternary environments in the Nile basin – current understanding and unresolved questions

Late Quaternary environments in the Nile Basin reflect the influence of the African summer monsoon upon plant cover, sediment yield and flood discharge in the Ethiopian and Ugandan headwaters of the Nile. Intervals of prolonged and very high Nile flow coincide with times of stronger summer monsoon and have been dated using a combination of 14C, OSL and 10Be methods. Periods of high Nile flow into the eastern Mediterranean coincide with the formation of highly organic sedimentary layers termed sapropels. Ages obtained so far for these times of sustained middle to late Pleistocene high flow in the Blue and White Nile are broadly coeval with sapropel beds S8 (ca 217 ka), S7 (ca 195 ka), S6 (ca 172 ka), S5 (ca 124 ka), S4 (ca 102 ka) S3 (ca 81 ka), S2 (ca 55–50 ka) and S1 (10–6.5 ka). Sapropel 5 (ca 124 ka) was synchronous with extreme Blue Nile floods and the formation of the 386 m lake in the lower White Nile Valley, as well as with a prolonged wet phase in the eastern Sahara. Fluctuations in Nile flow and sapropel formation reflect the influence of the precessional cycle upon the East African monsoon. Between 75 ka and 19 ka the climate in the Nile headwaters region became progressively colder and drier. During the Last Glacial Maximum, Lake Tana in Ethiopia and Lake Victoria in Uganda became dry, flow in the White Nile was reduced to a trickle, and the Blue Nile and Atbara became highly seasonal bed–load rivers. The return of the summer monsoon at 14.5 ka ushered in extreme Blue Nile floods, widespread flooding across the Nile Basin and the formation of the 382 m lake in the lower White Nile Valley. There was a brief return to aridity during the Younger Dryas (12.8–11.5 ka), after which the climate again became wetter and widespread flooding in the Nile Valley resumed. The early Holocene floods were later followed by incision and creation of the modern relatively narrow flood plain.

Bedrock erosion in subglacial channels.

The Labyrinth in the McMurdo Dry Valleys of Antarctica is characterized by large bedrock channels emerging from beneath the margin of Wright Upper Glacier. To study the morphodynamics of large subglacial channels cut into bedrock, we develop herein a numerical model based on the classical theory of subglacial channels and recent results on bedrock abrasion by saltating bed load. Model results show that bedrock abrasion in subglacial channels with pressurized flow reaches a maximum at an intermediate distance up-ice from the glacier snout for a wide range of sediment grain sizes and sediment loads. Close to the snout, the velocity is too low and the sediment particles cannot be mobilized. Far from the snout, the flow accelerates and sediment is transported in suspension, thus limiting particle impacts at the channel bottom and reducing abrasion. This non-monotonic relationship between subglacial flow and bedrock abrasion produces concave up bottom profiles in subglacial channels and potential cross-section constrictions after channel confluences. Both landforms are present in the bedrock channels of the Labyrinth. We therefore conclude that these geomorphic features are a possible signature of bedrock abrasion, rather than glacial scour, and reflect the complex interplay between transport rate, sediment load, and transport capacity in subglacial channels.

Quantitative Study on Bed Load Proppant Transport during Slickwater Hydraulic Fracturing

Abstract Bed load proppant transport is a significant phenomenon during slickwater hydraulic fracturing. However, the mechanism of bed load proppant transport is still unclear. In the present study, the proppant transport process during slickwater hydraulic fracturing was simulated with a coupled computational fluid dynamics- (CFD-) discrete element method (DEM) model, and the mechanism of the bed load proppant transport was analyzed. A model for calculating the mass flux of the bed load layer was proposed and verified with experimental results from the literature. The results show that bed load migration is an essential mechanism of proppant transport. When the shear force of fluid acting on the surface of the sand bank reaches the critical Shields number, the proppant in the upper layer of the sand bank begins to migrate in the form of bed load. The movement of the bed load layer increases the time for the sand bank to reach the equilibrium height. In addition, the mass flux of the bed load layer significantly affects the equilibrium height of the sand bank. The mass flux of the bed load layer decreases, and the equilibrium height increases as the proppant density, proppant diameter, or rolling friction coefficient and static friction coefficient of the proppant increase, but the mass flux of the bed load layer increases, and the equilibrium height decreases as the fluid viscosity increases.

Assessing and Modelling the Interactions of Instrumented Particles with Bed Surface at Low Transport Conditions

Sediment transport at near threshold to low transport stages (below the continuous transport) can still be affected by flow turbulence and its dynamics can benefit from further comprehensive studies. This study uses an instrumented particle embedded with micro electromechanical sensors (MEMS) to allow tracking the motions and forces acting on it, leading to and during its transport. Instrumented particle transport experiments were carried out at laboratory flume under a range of flow conditions. The probability distributions functions (PDFs) of bed load particle instantaneous velocities, hop distances and associated travel times (measured from start to stop of transport) were obtained for all the performed experiments with varying flow rates and particle density. The modelled distributions are useful and enable a deeper understanding of bed load sediment transport dynamics from a Lagrangian perspective. Furthermore, the results analyzed from the instrumented particle (including the particle’s transport mode) were validated using visual particle tracking methods (top and side cameras). The findings of this study demonstrate that for the range of turbulent flows trialed herein, the instrumented particle can be a useful, accessible, and low-cost tool for obtaining particle transport dynamics, having demonstrated satisfactory potential for field deployment in the near future.

Rock mound to reduce wave overwash and crest lowering of a sand barrier

ABSTRACT The quantitative understanding of sand transport in the vicinity of porous coastal structures is still rudimentary. In this study, irregular wave overwash and landward migration of a narrow sand barrier were investigated in a laboratory experiment for three different barriers with no structure, a rock mound, and a rock cover. The rock mound consisting of three layers of stable stones reduced the landward migration and crest lowering of the sand barrier but sand transport through the porous structure was appreciable. The rock cover consisting of a single layer of stable stones was not effective in reducing the barrier deformation because of the stone settlement and spreading and the exposure of underlying sand to direct wave action. The cross-shore model CSHORE was calibrated for a rock structure placed directly on an eroding sand barrier. The mean and standard deviation of the free surface elevation and cross-shore velocity were predicted within errors of about 20 except for small transmitted waves. The bed load parameter was increased by a factor of five to reproduce the degree of the profile deformation of the sand barrier without and with the rock mound or cover.

Content of heavy metals in bed loads of gully systems within the limits of the Saratov city in summer season

The article presents the results of the analysis of the chemical composition of bed loads (layer of 0–1 cm) entering through streams in gullies from the limits of the Saratov city to the Volgograd Reservoir. The content of six heavy metals (Ni, Cu, Cd, Cr, Pb, Zn) having the property of accumulation and high toxicity were analysed. The most polluted sediments are found in gully systems, the drainage basins of which are located in industrial zones. The results demonstrate that in the absence of surface runoff treatment systems contaminated waters as well as soils enter water bodies from urbanized areas.

An Estimation of Annual Sediment Load for Egyptian Irrigation Canals.(Dept.C)

Erosion, transport, and deposition processes in alluvial channel are considered one of the complex phenomena due to the limited knowledge in the theory of interaction between water and sediment flow, and in predicting the behaviour of alluvial water courses. In the last thirty years although experience in sediment transport in both practice and theory as well as the increasing capacity of digital computers and the numerical techniques have provided a substantial aid in this field, but the mechanics of sediment transportation is still considerably far from being satisfactory. Four conventional bed load formulae have been applied to some selected Egyptian irrigation canals. Located in the Delta region. Limitations of each formula have been discussed, and the most appropriate formula has been used for an estimation of the annual bed load for these canals.

Asymptotic derivation and simulations of a non-local Exner model in large viscosity regime

The present paper deals with the modeling and numerical approximation of bed load transport under the action of water. A new shallow water type model is derived from the stratified two-fluid Navier–Stokes equations. Its novelty lies in the magnitude of a viscosity term that leads to a momentum equation of elliptic type. The full model, sediment and water, verifies a dissipative energy balance for smooth solutions. The numerical resolution of the sediment layer is not trivial since the viscosity introduces a non-local term in the model. Adding a transport threshold makes the resolution even more challenging. A scheme based on a staggered discretization is proposed for the full model, sediment and water.

Bed Load Transport Estimations in Goodwin Creek Using Neural Network Methods

Equations used for calculating bed load transport rates generally assume steady flow conditions. This assumes that the relationship of bed load transport as a function of water discharge, or some other flow parameter such as flow depth or boundary shear stress, is single-valued. One of the reasons for adopting such an approach is that almost all the pertinent laboratory data on bed load transport have been obtained from experiments performed under steady flow conditions. Similarly, the scarcity of accurate bed load field data obtained during the passage of floods is attributed to the difficulties, which at times can become life threatening, encountered under such conditions. Provision of data under difficult conditions may lead to inability to provide data in some cases and interruption in data continuity. It is extremely difficult to make predictions using classical statistical science in discontinuous or lack of data situations. Artificial neural networks (ANN) is a usefull tool to use in prediction inefficient or data conditions. In this study two Artificial Neural Network (ANN) methods, radial basis functions and generalized regression neural network are employed to estimate the bed load data. It was seen that the ANN estimations are more satisfactory compared to those of the conventional statistical methods results. It was shown that ANN estimations for gravel bed load data are more successful than the sand load data.

Numerical Simulation on Proppant Transport in Fracture Junctions Affected by the Fracture Scale

Abstract The proppant distribution significantly affects the conductivity of fracture networks. However, the law of proppant transport in fracture networks is still unclear, and the influence of fracture scale on the proppant distribution has not been determined. Thus, in the present study, the influence of fracture scale was investigated, and the influences of approaching angle and width ratio on fluid split ratio were analyzed. An Eulerian–Eulerian model was utilized to simulate suspended proppant and bed load proppant migration in fracture junctions. Then, a sensitivity analysis was carried out to evaluate the parameters that may affect the proppant distribution pattern, such as injection velocity, fluid viscosity, and proppant density. The results show that the approaching angle and width ratio significantly influence the fluid split ratio in a small-scale fracture. Moreover, the effect of the approaching angle decreases with an increase in the fracture scale. The split ratio of suspended proppant increases with increasing sand ratio, fluid split ratio, and width ratio. The split ratio of bed load proppant increases with increasing injection rate, fluid viscosity, width ratio, fluid split ratio, and decreasing proppant diameter. In small-scale fracture junctions, the approaching angle affects the split ratio of suspended proppant or bed load proppant by influencing the fluid split ratio; however, the effect is inconspicuous in large-scale fractures. The increase in fluid split ratio with the fracture scale leads to an increase in the split ratio of suspended proppant or bed load proppant.

The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

AbstractThe bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large‐scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large‐scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood‐ and ebb‐oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune‐induced directional bed roughness in numerical models of estuarine and tidal environments.

Numerical investigation of local scour around submerged pipeline in shoaling conditions

A two-dimensional numerical model is employed to predict local scour around submarine pipelines under water waves in shoaling condition. The motion of water under waves is simulated by solving the Reynolds Averaged Navier-Stokes (RANS) equations. The wave surface is traced by the Arbitrary Lagrangian-Eulerian (ALE) method. The evolution of the seabed surface near the pipeline is predicted by solving the conservation of the sediment mass, which transport in the water in the forms of bed load and suspended load. The main aim of this study is to investigate the effect of the seabed slope on the scour profiles and scour depth. To achieve this aim, numerical simulations of scour around a pipeline on a flat seabed and on a slope seabed with a slope angle of 5°, 10° and 15° are conducted for various wave conditions. The numerical results show that present numerical results of equilibrium scour depth below pipeline agree the values calculated by the suggested formula quite well for β = 0° where β is the slope angle. As β≠0°, the equilibrium scour depth agrees with the empirical formula as KC < 8 and is greater than the latter for KC > 8.

An improved formula for incipient sediment motion in vegetated open channel flows

Quantifying incipient sediment motion in vegetated open channel flow is pivotal for estimating bed load transport and the aquatic ecological environment in rivers. A new formula is developed to predict the critical flow velocity for incipient sediment motion in the presence of emergent vegetation, by incorporating the influence of vegetation drag that characterizes the effects of mean flow and turbulence on sediment movement. The proposed formula is shown to agree with existing experimental data. Moreover, the proposed formula is extended to scenarios with submerged vegetation, suggesting that the vegetation drag may be the inherent impact factor for incipient sediment motion in vegetated open channel flow.

Three-dimensional numerical simulation of wave-induced scour around a pile on a sloping beach

A fully-coupled (hydrodynamic and morphologic) numerical model based on the open-source CFD package OpenFOAM is presented and utilized to simulate breaking wave induced scour around a pile on a sloping beach. The hydrodynamic model is based on RANS equations together with k-ω turbulence closure and volume of fluid method for capturing the free surface. These are then coupled with both bed load and suspended load transport descriptions, which drive resultant morphology of the bed. The model is validated against large-scale laboratory experiments involving interaction of breaking solitary waves with a pile and induced flow and scour. The flow features and hydrodynamic characteristics around the pile are described in detail, and the scouring mechanism at different locations of the pile is thoroughly analyzed. The simulated flow field confirms the formation of horseshoe vortex in front of the pile induced by plunging breaking wave. The temporal variation of scour depth at the side of the pile is satisfactorily simulated while those at the offshore and onshore are under-predicted. The scour depth at the side of the pile increases twice during run-up and draw-down. The scour at the onshore location of the pile starts to increase rapidly at the end of wave drawdown.