Sheet Flow Layer Research Articles

Geomorphic Response of a Coastal Berm to Storm Surge and the Importance of Sheet Flow Dynamics

AbstractDuring a storm, as the beach profile is impacted by increased wave forcing and rapidly changing water levels, sand berms may help mitigate erosion of the backshore. However, the mechanics of berm morphodynamics have not been fully described. In this study, 26 trials were conducted in a large wave flume to explore the response of a near‐prototype berm to scaled storm conditions. Sensors were used to quantify hydrodynamics, sheet flow dynamics, and berm evolution. Results indicate that berm overtopping and offshore sediment transport were key processes causing berm erosion. During the morphological evolution of the beach profile, two sand bars were formed offshore that attenuated subsequent wave energy. The landward extent of that energy was confined to the seaward foreshore, inhibiting inundation of the backshore. Net offshore‐directed transport was dominant when infragravity motions increased in the swash zone. Conversely, the influence of incident‐band motions on sediment transport was relatively greater in the inner‐surf zone. Near‐bed flow velocities and sheet flow layer thicknesses were larger in the swash zone than in the inner‐surf zone. This paper also provides a valuable analysis between morphology‐estimated total sediment transport rates and rates derived from in situ measurements. Sheet flow dynamics dominated foreshore cross‐shore sediment processes, constituting the largest portion of the total sediment transport load throughout the berm erosion.

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

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

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

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

A multi-dimensional two-phase mixture model for intense sediment transport in sheet flow and around pipeline

A two-phase mixture model is developed to simulate intense sediment transport covering the bed-load layer and suspended load layer. The proposed model maintains high accuracy as an Eulerian two-phase model but requires low computational cost. The proposed model applies an analytical formula for relative velocity between phases. The dense granular flow rheology is employed to close particle stress economically. The closure of Reynolds stress considers turbulence damping and small-scale fluctuation of fluid–particle interaction and particle collision. A damping function is adopted in eddy viscosity for extra turbulence damping from inter-particle interaction. The optimal exponent of the damping function refers to sediment shape and size. The sediment diffusion includes turbulence diffusion and shear-induced self-diffusion originating from dense sediment. The proposed model is validated by several sets of sheet flow cases (Shields number Θ = 0.44–2.20 and particle Reynolds number Res = 1.6–603.0) and shows a wide applicable range and good accuracy. The small-scale fluctuation and shear-induced self-diffusion improve the computation in the lower sheet flow layer where volumetric sediment concentration is larger than 0.2. Furthermore, the proposed model shows reasonable applicability on the multi-dimensional pipeline scour development. The scour profiles are well predicted and the Brier Skill Score = 0.809. However, the proposed model does not perform the wake characteristic around the pipeline sufficiently, and slight scour difference exists between the simulation and experiment.

Numerical investigation of sheet flow driven by a near-breaking transient wave using SedFoam

Sheet flow driven by a near-breaking transient wave is numerically investigated using SedFoam, a two-phase Eulerian sediment transport model in the OpenFOAM framework. SedFoam resolves the full profile of sediment transport processes within the bottom boundary layer based on closures for intergranular stresses and fluid–particle interactions. Compared with large-scale wave flume data, good agreements are obtained for streamwise flow velocity profiles, sediment concentration profiles, and the sheet flow layer thickness. Model results show near-bed velocity and sediment profile evolution within the sheet flow layer. Intense sediment suspension trailing the wave crest generates stable density stratification that dampens near-bed turbulent kinetic energy and contributes to decreasing bed shear stress. Results suggest that the buoyant flux dominates turbulent kinetic energy dissipation after the passing of the wave crest, coinciding with the reduction of bed shear stress. The instantaneous upper bound of the sheet flow layer exists in different log-law regimes under the transient wave, giving rise to a near-bed velocity profile that is highly dependent on the variable sheet flow layer wall unit. The effect of the profile shape parameter on bedload sediment transport is studied where the bedload predicted using the time-varying optimal profile shape parameter yields good agreement compared to the directly modeled bedload. Modeled sediment transport rates demonstrate that reduced bed shear stress caused by density stratification limits bedload and results in a suspended load-dominant mode.

Near‐Bed Sediment Transport During Offshore Bar Migration in Large‐Scale Experiments

AbstractThis study presents novel insights into hydrodynamics and sediment fluxes in large‐scale laboratory experiments with bichromatic wave groups on a relatively steep initial beach slope (1:15). An Acoustic Concentration and Velocity Profiler provided detailed information of velocity and sand concentration near the bed from shoaling up to the outer breaking zone including suspended sediment and sheet flow transport. The morphological evolution was characterized by offshore migration of the outer breaker bar. Decomposition of the total net transport revealed a balance of onshore‐directed, short wave‐related and offshore‐directed, current‐related net transport. The short wave‐related transport mainly occurred as bedload over small vertical extents. It was linked to characteristic intrawave sheet flow layer expansions during short wave crests. The current‐related transport rate featured lower maximum flux magnitudes but occurred over larger vertical extents. As a result, it was larger than the short wave‐related transport rate in all but one cross‐shore position, driving the bar’s offshore migration. Net flux magnitudes of the infragravity component were comparatively low but played a nonnegligible role for total net transport rate in certain cross‐shore positions. Net infragravity flux profiles sometimes featured opposing directions over the vertical. The fluxes were linked to a standing infragravity wave pattern and to the correlation of the short wave envelope, controlling suspension, with the infragravity wave velocity.

Emplacement of massive deposits by sheet flow

AbstractTurbidity current and coastal storm deposits are commonly characterized by a basal sandy massive (structureless) unit overlying an erosional surface and underlying a parallel or cross‐laminated unit. Similar sequences have been recently identified in fluvial settings as well. Notwithstanding field, laboratory and numerical studies, the mechanisms for emplacement of these massive basal units are still under debate. It is well accepted that the sequence considered here can be deposited by waning‐energy flows, and that the parallel‐laminated units are deposited under transport conditions corresponding to upper plane bed at the dune–antidune transition. Thus, transport conditions that are more intense than those at the dune–antidune transition should deposit massive units. This study presents experimental, open‐channel flow results showing that sandy massive units can be the result of gradual deposition from a thick bedload layer of colliding grains called sheet flow layer. When this layer forms with relatively coarse sand, the non‐dimensional bed shear stress associated with skin friction, the Shields number, is larger than a threshold value approximately equal to 0·4. For values of the Shields number smaller than 0·4 the sheet flow layer disappeared, sediment was transported by a standard bedload layer one or two grain diameters thick, and the bed configuration was characterized by downstream migrating antidunes and washed out dunes. Parallel laminae were found in deposits emplaced with standard bedload transport demonstrating that the same dilute flow can gradually deposit the basal and the parallel‐laminated unit in presence of traction at the depositional boundary. Further, the experiments suggested that two different types of upper plane bed conditions can be defined, one associated with standard bedload transport at the dune–antidune transition, and the other associated with bedload transport in sheet flow mode at the transition between upstream and downstream migrating antidunes.

A numerical study of sheet flow driven by velocity and acceleration skewed near-breaking waves on a sandbar using SedWaveFoam

A new methodology capable of concurrently resolving free surface wave field, bottom boundary layer, and sediment transport processes throughout the entire water column was recently developed in the OpenFOAM framework, called SedWaveFoam. In this study, SedWaveFoam is validated with large wave flume data for sheet flow driven by near-breaking waves. Good agreements are obtained for free surface elevation, flow velocity, turbulence kinetic energy, sediment concentration, and sheet flow sediment fluxes. Model results are used to investigate the joint effects of velocity skewness, acceleration skewness, and progressive wave streaming on sheet flow sediment transport. SedWaveFoam results are contrasted with rigid-lid one-dimensional-vertical model results to isolate the effect of the free surface. Onshore directed near-bed flow velocity and sediment flux are enhanced due to the presence of the free surface via progressive wave streaming. However, the enhancement of net onshore sediment transport for the near-breaking condition with both high velocity and acceleration skewness is several factors greater than that found in the nonbreaking condition with only high velocity skewness. Model results suggest that the large horizontal pressure gradient, which has a Sleath parameter exceeding 0.2, may play a key role. Momentary bed failure is identified via near-bed instability of the sheet flow layer, associated with a large bed shear stress and horizontal pressure gradient. Instantaneous near-bed vortices due to the near-bed instability correspond to the increase of horizontal pore pressure gradient during the wave crest, consistent with measured data. Model inter-comparison suggests that a two-dimensional model is crucial to capture the effect of momentary bed failure that increases sediment suspension during wave crest passage and net onshore sediment transport.

Sand transport processes and bed level changes induced by two alternating laboratory swash events

Sand transport processes and net transport rates are studied in a large-scale laboratory swash zone. Bichromatic waves with a phase modulation were generated, producing two continuously alternating swash events that have similar offshore wave statistics but which differ in terms of wave-swash interactions. Measured sand suspension and sheet flow dynamics show strong temporal and spatial variability, related to variations in flow velocity and locations of wave capture and wave-backwash interactions. Suspended and sheet flow layer transport rates in the lower swash zone are generally of same magnitude, but sheet flow exceeds the suspended load transport by up to a factor four during the early uprush. The bed level near the inner surf zone is relatively steady during a swash cycle, but changes of O(cm/s) are measured near the mid swash zone where wave-swash interactions lead to strongly non-uniform flows. The two alternating swash events produce a dynamic equilibrium, with bed level changes up to a few mm induced by single swash events, but with net morphodynamic change over multiple events that is two orders of magnitude lower. Most of the intra-swash and the single-event-averaged bed level changes in the swash zone are caused by a redistribution of sediment within the swash. The transport of sediment across the surf-swash boundary is minor at intra-swash time scale, but becomes increasingly significant at swash-averaged time scales or longer (i.e., averaged over multiple swash events).

Wave Boundary Layer Hydrodynamics and Sheet Flow Properties Under Large‐Scale Plunging‐Type Breaking Waves

AbstractWave boundary layer (WBL) dynamics are measured with an Acoustic Concentration and Velocity Profiler (ACVP) across the sheet flow‐dominated wave‐breaking region of regular large‐scale waves breaking as a plunger over a developing breaker bar. Acoustic sheet flow measurements are first evaluated quantitatively in comparison to Conductivity Concentration Meter (CCM+) data used as a reference. The near‐bed orbital velocity field exhibits expected behaviors in terms of wave shape, intrawave WBL thickness, and velocity phase leads. The observed fully turbulent flow regime all across the studied wave‐breaking region supports the model‐predicted transformation of free‐stream velocity asymmetry into near‐bed velocity skewness inside the WBL. Intrawave concentration dynamics reveal the existence of a lower pickup layer and an upper sheet flow layer similar to skewed oscillatory sheet flows, and with similar characteristics in terms of erosion depth and sheet flow layer thickness. Compared to the shoaling region, differences in terms of sheet flow and hydrodynamic properties of the flow are observed at the plunge point, attributed to the locally enhanced wave breaker turbulence. The ACVP‐measured total sheet flow transport rate is decomposed into its current‐, wave‐, and turbulence‐driven components. In the shoaling region, the sand transport is found to be fully dominated by the onshore skewed wave‐driven component with negligible phase lag effects. In the outer surf zone, the total net flux exhibits a three‐layer vertical structure typical of skewed oscillatory sheet flows. However, in the present experiments this structure originates from offshore‐directed undertow‐driven flux, rather than from phase lag effects.

On Bedload and Suspended Load Measurement Performances in Sheet Flows Using Acoustic and Conductivity Profilers

AbstractIntense sediment transport experiments were performed in a gravity‐driven open‐channel flow with two sizes of uniformly distributed nonspherical acrylic particles having median diameters of 1.0 and 3.0 mm and a maximum packing volumetric concentration (ϕ) of 0.55. The flow conditions were adapted to each particle size to ensure similar sediment transport flow regimes as sheet flow corresponding to Shields numbers slightly above unity and a suspension number, ratio of settling velocity to friction velocity, near unity for the two experiments. An acoustic scattering‐based system (Acoustic Concentration and Velocity Profiler) and conductivity probes (Conductivity Concentration Profiler [CCP]) with two different vertical resolutions, 1 mm (CCP1mm) and 2 mm (CCP2mm) were used to measure instantaneous concentration profiles across the bedload and suspension layers. Measured concentration profiles, bed interface position, and sheet flow layer thicknesses are compared between the two techniques. The capabilities and limitations of both technologies are outlined. Average volumetric sediment concentration profiles were overestimated by 10% with the Acoustic Concentration and Velocity Profiler in the dense sheet layer when ⟨ϕ(z)⟩≳ 0.35, and by 100% with the CCP in the more diluted region when ⟨ϕ(z)⟩≲ 0.015 and ⟨ϕ(z)⟩≲ 0.20 for CCP1mm and CCP2mm, respectively. Good agreement is found between the three systems in terms of average and time‐resolved bed level position and sheet layer thickness, validating the different bed interface detection methods on data from the two sensors.

The influence of wave groups and wave-swash interactions on sediment transport and bed evolution in the swash zone

Large scale laboratory measurements of sediment dynamics in the swash zone are presented. Two bichromatic wave group conditions were generated, having the same energy content but different wave group period (Tg=15.0 s and 27.7 s). For the shortest wave group, due to bore focusing, the shoreline fluctuates predominantly at the Tg time scale, showing a large runup and the presence of wave–swash interactions with strong momentum exchange. In contrast, for the longer wave groups, the swash excursion is dominated by the individual waves. The uprush generally promotes onshore sediment advection with consequent erosion at the rundown location but accretion close to the runup. On the contrary, the backwash promotes seaward sediment advection and accretion at the rundown location. The presence of repeated wave–swash interactions modifies these patterns slightly. A wave overrunning a previous uprush promotes a reduction in onshore sediment advection while weak wave–backwash interactions reduce seaward advection. Consequently, the measured sediment dynamics shows stronger intra–swash cross–shore sediment advection for the swash events produced by the short wave groups. Measurements of the sheet flow layer near the shoreline show that for the shortest wave groups the vertical structure of the concentration is influenced by horizontal advection, leading to large sheet-flow layer thickness. However, for the longer wave groups, the local vertical exchange of sediment in the sheet–flow layer is dominant, with the presence of a pick–up and mirroring upper layer similar to oscillatory sheet–flow measurements. These results reaffirm the important effects of the wave group structure and the wave–swash interactions on the swash zone sediment dynamics and beach face evolution.

Intense Granular Sheetflow in Steep Streams

AbstractQuantifying sediment transport rates in mountainous streams is important for hazard prediction, stream restoration, and landscape evolution. While much of the channel network has steep bed slopes, little is known about the mechanisms of sediment transport for bed slopes between 10% < S < 30%, where both fluvial transport and debris flows occur. To explore these slopes, we performed experiments in a 12‐m‐long sediment recirculating flume with a nearly uniform gravel bed. At 20% and 30% bed gradients, we observed a 4‐to‐10 particle‐diameter thick, highly concentrated sheetflow layer between the static bed below and the more dilute bedload layer above. Sheetflow thickness increased with steeper bed slopes, and particle velocities increased with bed shear velocity. Sheetflows occurred at Shields stresses close to the predicted bedload‐to‐debris flow transition, suggesting a change of behavior from bedload to sheetflow to debris flow as the bed steepens.

Sheet flow hydrodynamics over a non-uniform sand bed channel

The current study experimentally investigates the flow characteristics and temporal variations in the sheet flow profile of a non-uniform sand bed channel. Experiments were done to explore turbulent structures in the presence of a sheet flow layer with and without seepage. The turbulent events, such as stream wise velocity, Reynolds shear stresses, and turbulence intensities were found to be increasing and vertical velocity was found decreasing with a sheet layer. The presence of a sheet layer also effects the turbulent energy production and energy dissipation. All the turbulence parameters with and without a sheet layer have also been influenced by the presence of downward seepage. The rate of sheet flow movement is increased with seepage, owing to increased turbulence with seepage. The current study used wavelet analysis on temporally lagged spatial bed elevation profiles obtained from a set of laboratory experiments and synchronized the wavelet coefficients with bed elevation fluctuation at different spatial scales. A spatial cross correlation analysis at multiple scales, based on the wavelet coefficients, has been done on these bed elevation datasets to observe the effect of downward seepage on the dynamic behavior of sheet flow at different length scales. It is found that seepage increases average bed celerity and also increases the celerity of sheet flow of similar length scales. This increase in the celerity has been hypothesized as the increase of sheet flow movement as well as the increase in turbulent parameters with seepage, which destabilizes the bed particles resulting in a disruption in the continuous propagation pattern of the sheet flow. The increase of sheet flow celerity with seepage is confirmed from the saturation level of the wavelet power spectra of the bed elevation series. The presence of seepage also affects the non-uniformity of collective sheet material.

On bedload measurement performances of high-resolution acoustic (ACVP) and conductivity (CCP) profilers

Intense sediment transport experiments were performed in a gravity driven open-channel flow with two sizes of non-spherical acrylic particles of diameter dp=1.0mm and dp=3.0 mm, and a maximum packing volumetric concentration of 0.55 m3/m3. Two flow conditions were adapted to ensure the same sediment transport regime for particle sizes as sheet flows (Shields numbers above unity) with an equi-repartition of the total net sediment transport rate between the suspended load and the bedload (Suspension number around unity). An acoustic scattering-based system, the Acoustic Concentration and Velocity Profiler (ACVP) and electric conductivity probes, the Conductivity Concentration Profiler (CCP) with two different vertical resolutions of 1mm (CCP1mm) and 2 mm (CCP2mm),were used to measure time-resolved and averaged concentration profiles across the bed-load and suspension layers. A detailed comparative analysis of concentration and sheet flow layer thickness measurements obtained with the two systems across both the suspension and the bedload layers is presented. The capabilities and limitations of the two flow measurement technologies are outlined. Average sediment concentration profiles were overestimated by 10% with the ACVP in the dense sheet layer when ⟨φ(z)⟩ ≳ 0.35, and by 100% with the CCP in the more diluted region when ⟨φ(z)⟩ ≲ 0.015 and ⟨φ(z)⟩ ≲ 0.20 for CCP1mmand CCP2mm, respectively. Good agreement is found elsewhere between the three systems in terms of average and time-resolved concentration as well as bed level position and sheet flow layer thickness.

An Eulerian two-phase model for steady sheet flow using large-eddy simulation methodology

Abstract A three-dimensional Eulerian two-phase flow model for sediment transport in sheet flow conditions is presented. To resolve turbulence and turbulence-sediment interactions, the large-eddy simulation approach is adopted. Specifically, a dynamic Smagorinsky closure is used for the subgrid fluid and sediment stresses, while the subgrid contribution to the drag force is included using a drift velocity model with a similar dynamic procedure. The contribution of sediment stresses due to intergranular interactions is modeled by the kinetic theory of granular flow at low to intermediate sediment concentration, while at high sediment concentration of enduring contact, a phenomenological closure for particle pressure and frictional viscosity is used. The model is validated with a comprehensive high-resolution dataset of unidirectional steady sheet flow (Revil-Baudard et al., 2015, Journal of Fluid Mechanics, 767, 1–30). At a particle Stokes number of about 10, simulation results indicate a reduced von Karman coefficient of κ ≈ 0.215 obtained from the fluid velocity profile. A fluid turbulence kinetic energy budget analysis further indicates that the drag-induced turbulence dissipation rate is significant in the sheet flow layer, while in the dilute transport layer, the pressure work plays a similar role as the buoyancy dissipation, which is typically used in the single-phase stratified flow formulation. The present model also reproduces the sheet layer thickness and mobile bed roughness similar to measured data. However, the resulting mobile bed roughness is more than two times larger than that predicted by the empirical formulae. Further analysis suggests that through intermittent turbulent motions near the bed, the resolved sediment Reynolds stress plays a major role in the enhancement of mobile bed roughness. Our analysis on near-bed intermittency also suggests that the turbulent ejection motions are highly correlated with the upward sediment suspension flux, while the turbulent sweep events are mostly associated with the downward sediment deposition flux.

Bedload and suspended load contributions to breaker bar morphodynamics

This study presents measurements of sheet flow processes, grain sorting, and bedload plus suspended load transport rates around a medium-sand breaker bar in a large-scale wave flume. The results offer insights in effects of wave breaking on bedload and grain sorting processes and in the quantitative contributions by bedload and suspended transport to breaker bar morphodynamics. Sheet flow layer dynamics are highly similar to observations under non-breaking waves, revealing clear effects by velocity asymmetry but no evident effects by breaking-generated turbulence, bed slope, or the cross-shore non-uniform flow. The sheet flow layer thickness can be predicted using existing empirical formulations based on local hydrodynamic forcing. At locations covering the shoaling region up to the bar crest the cross-shore variation in bedload transport rates is explained by variations in wave shape (i.e. velocity skewness and asymmetry). At locations between bar crest and bar trough, bedload transport rate magnitudes correlate positively with bed slope and turbulent kinetic energy. Bedload and suspended load transport rates are of similar magnitude but of opposite sign. Bedload transport is onshore-directed and dominates in the shoaling zone, but after wave breaking, the offshore-directed suspended sediment transport increases in magnitude and exceeds bedload transport rates in the breaking and inner surf zones. Bedload and suspended load transport contribute notably differently to bed profile evolution: bedload transfers sand grains from the offshore slope to the bar crest and additionally leads to erosion of the shoreward bar slope and deposition at the bar trough, while suspended load transport induces an opposite pattern of erosion at the bar trough and accretion at the bar crest. Grain size analysis of suspended sediment samples reveals size-selective entrainment and vertical size segregation in the inner surf zone, but suggest size-indifferent entrainment and vertical mixing by energetic vortices in the breaking region. Size-selective transport by bedload and suspended load leads to a cross-shore coarsening of the bed from shoaling to inner surf zone, with local additional sorting mechanisms around the breaker bar due to bed slope effects.

Mobile bed thickness in skewed asymmetric oscillatory sheet flows

A new instantaneous mobile bed thickness model is presented for sediment transport in skewed asymmetric oscillatory sheet flows. The proposed model includes a basic bed load part and a suspended load part related to the Shields parameter, and takes into account the effects of mass conservation, phase-lag, and asymmetric boundary layer development, which are important in skewed asymmetric flows but usually absent in classical models. The proposed model is validated by erosion depth and sheet flow layer thickness data in both steady and unsteady flows, and applied to a new instantaneous sediment transport rate formula. With higher accuracy than classical empirical models in steady flows, the new formula can also be used for instantaneous sediment transport rate prediction in skewed asymmetric oscillatory sheet flows.

Sediment micromechanics in sheet flows induced by asymmetric waves: A CFD–DEM study

Understanding the sediment transport in oscillatory flows is essential to the investigation of the overall sediment budget for coastal regions. This overall budget is crucial for the prediction of the morphological change of the coastline in engineering applications. Since the sediment transport in oscillatory flows is dense particle-laden flow, appropriate modeling the particle interaction is critical. Although traditional two-fluid approaches have been applied to the study of sediment transport in oscillatory flows, the approaches do not capture the interaction of the particles. The study of the motion of individual sediment particles and their micromechanics (e.g., packing and contact force) in oscillatory flows is still lacking. In this work, a parallel CFD–DEM solver SediFoam that can model the inter-particle collision is applied to study the granular micromechanics of sediment particles in oscillatory flows. The results obtained from the CFD–DEM solver are validated by using the experimental data of coarse and medium sands. The comparison with experimental results suggests that the flow velocity, the sediment flux and the net sediment transport rate predicted by SediFoam are satisfactory. Moreover, the micromechanic quantities of the sediment bed are presented in detail, including the Voronoi concentration, the coordination number, and the particle interaction force. It is demonstrated that the variation of these micromechanic quantities at different phases in the oscillatory cycle is significant, which is due to different responses of the sediment bed. To investigate the structural properties of the sediment bed, the correlation of the Voronoi volume fraction and coordination number is compared to the results from the fluidized bed simulations. The consistency in the comparison indicates the structural micromechanics of sediment transport and fluidized bed are similar despite the differences in flow patterns. From the prediction of the CFD–DEM model, we observed that the coordination number in rapid sheet flow layer is larger than one, which indicates that a typical particle in the sediment layer is in contact with more than one particles, and thus the binary collision model commonly used in two-fluid approaches may underestimate the contact between the particles.

Sediment transport partitioning in the swash zone of a large-scale laboratory beach

Swash zone sheet flow and suspended sediment transport rates are estimated on a coarse sand beach constructed in a large-scale laboratory wave flume. Three test cases under monochromatic waves with wave heights of 0.74m and wave periods of 8 and 12.2s were analyzed. Sediment flux in the sheet flow layer exceeds several hundred kgm−2s−1 during both uprush and backwash. Suspended sediment flux is large during uprush and can exceed 200kgm−2s−1. Instantaneous sediment flux magnitudes in the sheet layer are nearly always larger than those for suspended sediment flux. However, sediment transport rates, those integrated over depth, indicate that suspended load transport is dominant during uprush for all cases and during the early stages of backwash except in the case for the 12.2s wave case when the foreshore was steeper. Results could not be obtained for an entire swash event and were particularly truncated during backwash when water depths fell below the elevation of the lowest current meter.