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Abstract The seabed is sloped in shallow coastal regions, where strong nonlinear shoaling waves can generate sheet-flow sediment transport. Both wave nonlinearity and bottom slope can lead to a wave-averaged (net) sediment transport rate, but the bottom slope effect is often overlooked in existing sediment transport models. A full-scale experimental study of sheet-flow sediment transport under the oscillatory flows with nonlinear features (skewness or asymmetry) was conducted in an inclinable oscillatory water tunnel, so the bottom-slope-induced net transport rate in the context of nonlinear waves was experimentally investigated. The tests cover a variety of nonlinear wave shape, bottom slope (0 to 2. 5 ∘ ) and two sediment diameters (0.24 and 0.51 mm). Since the downslope direction is the "offshore" direction in our tests, it is found that bottom slope reduces the net "onshore" transport rate due to wave nonlinearity. The slope contribution to the net transport rate is evaluated by taking the difference between the measured net transport rate from a sloping-bed experiment and the measured net transport rate from the corresponding horizontal-bed experiment. The results suggest that slope-induced net transport rate is insensitive to wave nonlinearity, and its variation with bottom slope can be considered linear in the context of predicting net transport rate. Comparisons between this study and a preceding sinusoidal-flow study of Yuan et al. (2017b) suggest that nonlinear waves can be approximated as sinusoidal flows for estimating the slope contribution to the net transport rate. Subsequently, an empirical model for predicting net sheet-flow transport rate due to bottom slope is established based on dimensional analysis. This model can be simply added to existing sheet-flow sediment transport models to improve their applicability in coastal regions. Highlights • Presented experiments on the sheet-flow sediment transport under skewed and asymmetric oscillatory flows above a sloping bed. • Test results show that the slope-induced net transport rate is insensitive to nonlinear wave shape. • Proposed an empirical model for slope's contribution in net sheet-flow sediment transport rate. [ABSTRACT FROM AUTHOR]

A one-dimensional-vertical model is developed for net sheet-flow sediment transport rate under oscillatory boundary layer flows. This model couples the boundary-layer model proposed by Yuan and Madsen (2015) with new predictors for bedload and suspended-load transport rates. Sediment concentration is modeled by solving the advection-diffusion equation with a parameterized turbulence diffusivity and a bottom pick-up rate. The slope effect is included in the predictions of instantaneous bedload transport rate and sediment pick-up rate, so the model can rigorously account for wave nonlinearity (velocity skewness and asymmetry) together with a mild bottom slope. The model performance on net sediment transport rate is excellent for large grain sizes, except that the velocity-asymmetry effect seems to be moderately underestimated. For small grain sizes, the model performance deteriorates, but is still acceptable for many cases. This is possibly because the model does not consider turbulence damping due to stratification and underestimates the phase-lag effect. Using the validated model, it is found that a boundary layer streaming due to velocity skewness and/or asymmetry can be a major contributor to net transport rate, when significant sediment suspension occurs. A simple case study also suggests that a mild bottom slope can be equally important as the wave nonlinearity for producing net sediment transport rate. Model predictions show that wave-nonlinear and bottom-slope effects can be linearly superimposed, which provides a simple way for incorporating bottom slope into existing transport-rate formulas. [ABSTRACT FROM AUTHOR]

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 0.74 m and wave periods of 8 and 12.2 s were analyzed. Sediment flux in the sheet flow layer exceeds several hundred kg m − 2 s − 1 during both uprush and backwash. Suspended sediment flux is large during uprush and can exceed 200 kg m − 2 s − 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 12.2 s 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. [ABSTRACT FROM AUTHOR]

Detailed measurements of bed level motions and sheet flow processes in the lower swash are presented. The measurements are obtained during a large-scale wave flume experiment focusing on swash zone sediment transport induced by bichromatic waves. A new instrument (CCM + ) provides detailed phase-averaged measurements of sheet flow concentrations, particle velocities, and bed level evolution during a complete swash cycle. The bed at the lower swash location shows a clear pattern of rapid erosion during the early uprush and progressive accretion during the middle backwash phase. Sheet flow occurs during the early uprush and mid and late backwash phases. Sheet flow sediment fluxes during these instances are highest in the pick-up layer. Sediment entrainment from the pick-up layer occurs not only during instances of high horizontal shear velocities but also in occurrence of wave–backwash interactions. As opposed to oscillatory sheet flow, the pivot point elevation of the sheet flow layer is time-varying during a swash event. Moreover, the upper sheet flow layer concentrations do not mirror the concentrations in the pick-up layer. Both differences suggest that in the lower swash zone the dynamics of the upper sheet flow layer are not only controlled by vertical sediment exchange (such as in oscillatory sheet flows) but are strongly affected by horizontal advection processes induced by the non-uniformity of the flow. [ABSTRACT FROM AUTHOR]

An experimental study is presented consisting of swash zone hydrodynamics, sediment transport flux estimates, and bed changes under a single dam-break-driven swash event over a steep (1:7), movable sand bed. The study provides resolved measurements of instantaneous water depths, cross-shore flow velocities, sheet flow and suspended sediment concentrations, and morphological response. An ensemble-averaging over 24 repetitions of the swash event allows convergence in the time-averaged measured swash quantities. The sand bed profile was re-constructed after each run with a re-smoothing procedure that allowed for minimum variations that are within the profiler system accuracy (<0.006 m). High-resolution measurements are employed to reconstruct the spatial-temporal evolution of the flow velocity field. Temporal extrapolation and spatial logarithmic/exponential fits are employed to fill velocity gaps in the measured data for the estimation of instantaneous and depth-integrated sediment flux. The spatial gradients of instantaneous sediment flux indicate that the sheet flow sediment dominates over the suspended load during the most relevant phases for sediment transport. Upon bore arrival and initial uprush, the relative contribution of the sheet flow load is larger than during the backwash phase and is decreasingly dominant onshore. Depth-integrated instantaneous sediment flux and total sediment transport estimates, based on two different approaches, are compared with equivalent total sediment transport estimates obtained from the observed morphological changes and the sediment continuity equation. The close agreement between the various approaches indicates that existing models and empirical estimates for the sheet load concentration profile can be used as first order estimates for sediment transport in the swash zone. Novel laboratory experiments of instantaneous sediment transport flux in the swash zone Suspended and sheet flow contributions estimated across the swash zone Relative contribution of the sheet flow load decreases landward Existing spatial approximations and temporal extrapolation techniques for the estimation of swash zone sediment transport are tested New bench mark data set for testing time-/phase-resolving hydro-morphodynamical swash models [ABSTRACT FROM AUTHOR]

Abstract: Near-bed oscillatory flows with acceleration skewness are characteristic of steep and breaking waves in shallow water. In order to isolate the effects of acceleration skewness on sheet flow sand transport, new experiments are carried out in the Aberdeen Oscillatory Flow Tunnel. The experiments have produced a dataset of net transport rates for full-scale oscillatory flows with varying degrees of acceleration skewness and three sand sizes. The new data confirm previous research that net transport in acceleration-skewed flow is non-zero, is always in the direction of the largest acceleration and increases with increasing acceleration skewness. Large transport rates for the fine sand conditions suggest that phase lag effects play an important role in augmenting positive net transport. A comparison of the new experimental data with a number of practical sand transport formulations that incorporate acceleration skewness shows that none of the formulations performs well in predicting the measured net transport rates for both the fine and the coarser sands. The new experimental data can be used to further develop practical sand transport formulations to better account for acceleration skewness. [Copyright &y& Elsevier]

Abstract: A 1DV-RANS diffusion model is used to study sand transport processes in oscillatory flat-bed/sheet flow conditions. The central aim is the verification of the model with laboratory data and to identify processes controlling the magnitude and direction (‘onshore’/‘offshore’) of the net time-averaged sand transport. The model is verified with a large series of measured net sand transport rates, as collected in different wave tunnels for a range of wave-current conditions and grain sizes. Although not all sheet flow details are represented in the 1DV-model, it is shown that the model is able to give a correct representation of the observed trends in the data with respect to the influence of the velocity, wave period and grain diameter. Also detailed mean sediment flux profiles in the sheet flow layer are well reproduced by the model, including the direction change from ‘onshore’ to ‘offshore’ due to a difference in grain size from 0.34mm (medium sand) to 0.13mm (fine sand). A model sensitivity study with a selected series of net transport data shows that the stirring height of the suspended sediment ε s /w s strongly controls the magnitude and direction of the net sediment transport. Inclusion of both hindered settling and density stratification appears to be necessary to correctly represent the sand fluxes for waves alone and for waves + a superimposed current. The best agreement with a large dataset of net transport measurements is obtained with the 1DV-RANS model in its original settings using a Prandtl–Schmidt number σ ρ =0.5. [Copyright &y& Elsevier]

Abstract: To predict sediment transport under oscillatory sheet flow condition, especially for fine sand, is still a challenging research subject in coastal engineering. This paper describes a newly-developed numerical model based on two-phase theory with the use of a one-equation turbulence closure, and its applications in predicting fine sediment suspension in near-prototype oscillatory sheet flow conditions. Model results were compared with comprehensive laboratory measurements of flow velocity and sediment concentration under both symmetrical and asymmetrical oscillatory sheet flows from a large-scale water tunnel. Good agreements between the model results and measurements were achieved and the results demonstrated that the model is capable of reproducing detailed characteristics of sediment entrainment process in the sheet flow regime. The comparisons also revealed the fact that the concentration peaks at flow reversal is associated with the strong vertical sediment transport flux in the pickup layer, which has been widely observed in many laboratory experiments. The effects of flow reversal events on total sediment transport were also discussed. [Copyright &y& Elsevier]

Abstract: A large number of studies have been done dealing with sinusoidal wave boundary layers in the past. However, ocean waves often have a strong asymmetric shape especially in shallow water, and net of sediment movement occurs. It is envisaged that bottom shear stress and sediment transport behaviors influenced by the effect of asymmetry are different from those in sinusoidal waves. Characteristics of the turbulent boundary layer under breaking waves (saw-tooth) are investigated and described through both laboratory and numerical experiments. A new calculation method for bottom shear stress based on velocity and acceleration terms, theoretical phase difference, φ and the acceleration coefficient, a c expressing the wave skew-ness effect for saw-tooth waves is proposed. The acceleration coefficient was determined empirically from both experimental and baseline k–ω model results. The new calculation has shown better agreement with the experimental data along a wave cycle for all saw-tooth wave cases compared by other existing methods. It was further applied into sediment transport rate calculation induced by skew waves. Sediment transport rate was formulated by using the existing sheet flow sediment transport rate data under skew waves by Watanabe and Sato [Watanabe, A. and Sato, S., 2004. A sheet-flow transport rate formula for asymmetric, forward-leaning waves and currents. Proc. of 29th ICCE, ASCE, pp. 1703–1714.]. Moreover, the characteristics of the net sediment transport were also examined and a good agreement between the proposed method and experimental data has been found. [Copyright &y& Elsevier]

Abstract: For the general purposes of morphodynamic computations in coastal zones, simple formula-based models are usually employed to evaluate sediment transport. Sediment transport rates are computed as a function of the bottom shear stress or the near bed flow velocity and it is generally assumed that the sediment particles react immediately to changes in flow conditions. It has been recognized, through recent laboratory experiments in both rippled and plane bed sheet flow conditions that sediment reacts to the flow in a complex manner, involving non-steady processes resulting from memory and settling/entrainment delay effects. These processes may be important in the cross-shore direction, where sediment transport is mainly caused by the oscillatory motions induced by surface short gravity waves. The aim of the present work is to develop a semi-unsteady, practical model, to predict the total (bed load and suspended load) sediment transport rates in wave or combined wave-current flow conditions that are characteristic of the coastal zone. The unsteady effects are reproduced indirectly by taking into account the delayed settling of sediment particles. The net sediment transport rates are computed from the total bottom shear stress and the model takes into account the velocity and acceleration asymmetries of the waves as they propagate towards the shore. A comparison has been carried out between the computed net sediment transport rates with a large data set of experimental results for different flow conditions (wave-current flows, purely oscillatory flow, skewed waves and steady currents) in different regimes (plane bed and rippled bed) with fine, medium and coarse uniform sand. The numerical results obtained are reasonably accurate within a factor of 2. Based on this analysis, the limits and validity of the present formulation are discussed. [Copyright &y& Elsevier]

Abstract: The effect of acceleration skewness on sheet flow sediment transport rates is analysed using new data which have acceleration skewness and superimposed currents but no boundary layer streaming. Sediment mobilizing forces due to drag and to acceleration (∼ pressure gradients) are weighted by cosine and sine, respectively, of the angle φ τ · φ τ =0 thus corresponds to drag dominated sediment transport, , while φ τ =90° corresponds to total domination by the pressure gradients, . Using the optimal angle, φ τ =51° based on that data, good agreement is subsequently found with data that have strong influence from boundary layer streaming. Good agreement is also maintained with the large body of U-tube data simulating sine waves with superimposed currents and second-order Stokes waves, all of which have zero acceleration skewness. The recommended model can be applied to irregular waves with arbitrary shape as long as the assumption negligible time lag between forcing and sediment transport rate is valid. With respect to irregular waves, the model is much easier to apply than the competing wave-by-wave models. Issues for further model developments are identified through a comprehensive data review. [Copyright &y& Elsevier]

Abstract: Oscillatory flow tunnel measurements of velocities and fluxes for sands in oscillatory sheet flow conditions are presented. The experiments involved a range of well-sorted and graded sands in two asymmetric flows. Velocities were measured using an ultrasonic velocity profiler (UVP) capable of measuring deep within the sheet flow layer. Velocity profiles are found to be similar for the same flow but different sand beds and display expected features of oscillatory boundary layer flow. The near-bed velocity leads the main flow velocity by approximately 21° and a small offshore-directed current is generated near the bed. Measures of the boundary layer thickness are in good agreement with those predicted using an equation formulated for the boundary layer thickness over fixed beds. Velocity data have been combined with concentration data to produce time-dependent sand flux profiles covering the sheet flow and suspension regions. There are fundamental differences in the transport processes of sands of different size and grading, caused by unsteady effects which dominate in the case of fine sand and are largely absent in the case of coarse sand. (1) Time-averaged flux is onshore-directed (positive) in the case of coarse sand and is confined to a region immediately above the bed; in contrast, time-averaged flux in the case of fine sand extends high above the bed, is offshore-directed (negative) in the sheet flow layer and becomes onshore-directed in the suspension layer. (2) Net transport in the case of coarse sand is directed onshore. As the percentage of fine sand in the bed increases, offshore transport becomes increasingly dominant as the percentage of fine sand increases. (3) Net transport in the suspension layer is onshore while net transport in the sheet-flow layer may be onshore or offshore depending on sand size and grading. [Copyright &y& Elsevier]

New data on the settling velocity of artificial sediments and natural sands at high concentrations are presented. The data are compared with a widely used semiempirical Richardson and Zaki equation (Trans. Inst. Chem. Eng. 32 (1954) 35), which gives an accurate measure of the reduction in velocity as a function of concentration and an experimentally determined empirical power n. Here, a simple method of determining n is presented using standard equations for the clear water settling velocity and the seepage flow within fixed sediment beds. The resulting values for n are compared against values derived from new and existing laboratory data for beach and filter sands. For sands, the appropriate values of n are found to differ significantly from those suggested by Richardson and Zaki for spheres, and are typically larger, corresponding to a greater reduction in settling velocity at high concentrations. For fine and medium sands at concentrations of order 0.4, the hindered settling velocity reduces to about 70% of that expected using values of n derived for spheres. At concentrations of order 0.15, the hindered settling velocity reduces to less than half of the settling velocity in clear water. These reduced settling velocities have important implications for sediment transport modelling close to, and within, sheet flow layers and in the swash zone. [Copyright &y& Elsevier]

In wave-generated sheet flow conditions, oscillatory near-bed flow is large, ripples are washed out and sand is mainly transported in a thin layer, a few centimetres thick, close to and below the no-flow bed level. New experiments in the Aberdeen Oscillatory Flow Tunnel have produced a comprehensive dataset of transport processes for well-sorted and graded sands in large-scale sinusoidal and asymmetric oscillatory sheet flow conditions. In this paper, detailed time- and height-varying concentration measurements are analysed. The range and level of detail in the measurements make it possible to quantitatively analyse concentration behaviour more rigorously than before. A new empirical equation is presented which characterises the time-dependent concentration profile in the sheet flow layer. The equation involves two time-dependent parameters: erosion depth and reference concentration. The dependence of both parameters on flow and bed conditions is analysed. New results are presented which make it possible to estimate time-dependent erosion depth, reference concentration and, therefore, concentration profile. The effects of grading on sheet flow concentrations are analysed by comparing results from the large range of sand beds tested. [Copyright &y& Elsevier]

A simple, quasi-steady, one-dimensional, vertical (1DV) model of unsteady sheet flow is presented. The central aim is to provide greater realism in the near-bed, high-concentration layer, than is possible using models based on the classical reference-concentration approach. This is achieved by tracking erosion and deposition at the bottom of the mobile sediment layer in relation to the amount of sediment present in the sheet-flow and suspension layers. The model relies on empirical assumptions for the time-varying sheet-flow layer thickness (δ) and time-varying equivalent bed roughness (ks). The formulations adopted yield realistic instantaneous vertical profiles of velocity and sediment concentration from the stationary bed, through the high-concentration sheet-flow layer up into the outer suspension layer. The suspension layer itself is modelled using a standard k–&epsiv; turbulence-closure scheme, together with the sediment continuity equation. Matching conditions are applied at the interface between the sheet-flow and suspension layers. Preliminary results presented here include initial validation comparisons with the data set of Horikawa et al. [Horikawa, K., Watanabe, A., Katori, S., 1982. Sediment transport under sheet flow conditions. Proceedings of the 18th International Conference on Coastal Engineering, Cape Town. ASCE, Reston VA, USA, 1335–1352.]. Further comparisons with the experiments of Dohmen-Janssen et al. [J. Geophys. Res. 106 (2001) 27103] cover a wide range of wave–current conditions and sand grain sizes. The results are satisfactory with respect to the measured velocity and concentration profiles, apart from cases involving fine sand for which the effects of flow unsteadiness are not accounted for in the formulation. In addition, the new model provides satisfactory predictions of sand transport rates. [Copyright &y& Elsevier]

A simple method is provided for calculating transport rates of not too fine (d50≥0.20 mm) sand under sheet flow conditions. The method consists of a Meyer-Peter-type transport formula operating on a time-varying Shields parameter, which accounts for both acceleration-asymmetry and boundary layer streaming. While velocity moment formulae, e.g.., Qs=Constant×u∞3, calibrated against U-tube measurements, fail spectacularly under some real waves (Ribberink, J.S., Dohmen-Janssen, C.M., Hanes, D.M., McLean, S.R., Vincent, C., 2000. Near-bed sand transport mechanisms under waves. Proc. 27th Int. Conf. Coastal Engineering, Sydney, ASCE, New York, pp. 3263–3276, Fig. 12), the new method predicts the real wave observations equally well. The reason that the velocity moment formulae fail under these waves is partly the presence of boundary layer streaming and partly the saw-tooth asymmetry, i.e., the front of the waves being steeper than the back. Waves with saw-tooth asymmetry may generate a net landward sediment transport even if u∞3=0, because of the more abrupt acceleration under the steep front. More abrupt accelerations are associated with thinner boundary layers and greater pressure gradients for a given velocity magnitude. The two “real wave effects” are incorporated in a model of the form Qs(t)=Qs[θ(t)] rather than Qs(t)=Qs[u∞(t)], i.e., by expressing the transport rate in terms of an instantaneous Shields parameter rather than in terms of the free stream velocity, and accounting for both streaming and accelerations in the θ(t) calculations. The instantaneous friction velocities u*(t) and subsequently θ(t) are calculated as follows. Firstly, a linear filter incorporating the grain roughness friction factor f2.5 and a phase angle ϕτ is applied to u∞(t). This delivers u*(t) which is used to calculate an instantaneous grain roughness Shields parameter θ2.5(t). Secondly, a constant bed shear stress is added which corresponds to the “streaming related bed shear stress” −ρ(u˜w˜)∞. The method can be applied to any u∞(t) time series, but further experimental validation is recommended before application to conditions that differ strongly from the ones considered below. The method is not recommended for rippled beds or for sheet flow with typical prototype wave periods and d50<0.20 mm. In such scenarios, time lags related to vertical sediment movement become important, and these are not considered by the present model. [Copyright &y& Elsevier]

Sheet flow corresponds to the high velocity regime when small bed ripples are washed out and sand is transported in a thin layer close to the bed. Therefore, it is often assumed that sand transport in oscillatory sheet flow behaves quasi-steady: time-dependent transport rates are assumed to be instantaneously related to the near-bed orbital velocity. However, new experimental results show that even in sheet flow, phase lags between sediment concentration and near-bed velocity can become so large that they lead to a reduction of the net (wave-averaged) transport rate. A phase lag parameter is defined, which shows that phase lags become important for fine sand, high velocities and short wave periods. A semi-unsteady model is developed that includes the effects of phase lags on the net transport rate.New experiments were carried out in a large oscillating water tunnel with three different sands (D50=0.13, 0.21 and 0.32 mm) for a range of prototype, combined wave–current flow conditions. Measured net transport rates were compared with predictions of a quasi-steady model and the new semi-unsteady model. This comparison indicates that net transport rates are reduced if phase lags become important: the quasi-steady model overestimates the net transport rates and the semi-unsteady model gives better agreement with the data. Time-dependent measurements of velocities and concentrations show that the reduced net transport rates can indeed be explained by phase lag effects, because for tests with smaller net transport rates than predicted by the quasi-steady model, considerable phase lags between velocities and concentrations were observed, even inside the sheet flow layer. Verification of the semi-unsteady model against a larger data set confirms the occurrence of phase lag effects in oscillatory sheet flow. Also for the larger data set the semi-unsteady model yields better agreement with the data than the quasi-steady model. [Copyright &y& Elsevier]

In the past few years, two-phase models have been developed to describe the detail behaviour of fluid/sediment interactions and transport under the sheet flow conditions. Due to the complexity of the governing equations and uncertainties in the formulations of various stress terms, few complete solutions of these equations are known and the validations are thus far limited to only a few experimental data. In this paper, the numerical predictions of the behaviour of sheet flows using an improved version of an earlier two-phase flow model [Coastal Eng. 36(2) (1999) 87] are described. Although the general structure of the model was retained, a number of improvements had been made to give better account the underlying physics of the flow in areas very close to the stationary bed. All key flow parameters have been predicted and analysed in order to gain insight into the processes. Calculated time-dependent as well as time-averaged concentrations are compared with experimental data from purely oscillatory flows and oscillatory flow plus a current. Good qualitative agreements between predictions and measurements were achieved for the time-dependent concentrations while the time-averaged concentrations are quantitatively accurate as well. [Copyright &y& Elsevier]

Time series of vertical sediment fluxes are derived from concentration time series in sheet flow under waves. While the concentrations C(z,t) vary very little with time for &z.sfnc;z&z.sfnc;<10d50, the measured vertical sediment fluxes Qzs(z,t) vary strongly with time in this vertical band and their time variation follows, to some extent, the variation of the grain roughness Shields parameter θ2.5(t). Thus, sediment distribution models based on the pickup function boundary condition are in some qualitative agreement with the measurements. However, the pickup function models are only able to model the upward bursts of sediment during the accelerating phases of the flow. They are, so far, unable to model the following strong downward sediment fluxes, which are observed during the periods of flow deceleration. Classical pickup functions, which essentially depend on the Shields parameter, are also incapable of modelling the secondary entrainment fluxes, which sometimes occur at free stream velocity reversal. The measured vertical fluxes indicate that the effective sediment settling velocity in the high [(0.3

The present study is focused on sand transport in combined wave–current flow (at any angle between them) in the intensive flow regime. The present model is based on the 1DV approximation. The mathematical method for the continuous description of processes from the immobile bed to the formation of sheet flow and suspended load is presented. The way of modelling of sediment–flow interaction is proposed, in which the near-bed flow is divided into three layers, as done previously by Kaczmarek and Ostrowski [Kaczmarek, L.M., Ostrowski, R., 1998. Modelling of a three-layer sediment transport system in oscillatory flow. Proc. 26th ICCE, ASCE, 2559–2572] for pure oscillatory flow. Measured by Katopodi et al. [Katopodi, I., Ribberink, J.S., Ruol, P., Lodahl, C., 1994. Sediment transport measurements in combined wave–current flows. Proc. Coastal Dynamics ''94, ASCE, 837–851] and Dohmen-Janssen [Dohmen-Janssen, M., 1999. Grain size influence on sediment transport in oscillatory flow. FEBODRUK BV, ISBN 90-9012929-4, Enschede, The Netherlands], the time-averaged velocity profiles are compared with results provided by the present model for the respective conditions. The laboratory data of combined wave–current flow were further used to compare the model results for instantaneous and time-averaged concentration in the bedload (pick-up) layer and the contact load (upper sheet flow) layer, as well as for net sediment transport rate in the case of unidirectional (γ=0) wave–current interaction. The model cases for non-zero wave–current angle were tested in the context of longshore sediment transport rate against field radio-isotopic tracer data of Lubiatowo (Poland) by Pruszak and Zeidler [Pruszak, Z., Zeidler, R.B., 1994. Sediment transport in various time scales. Proc. 24th ICCE, ASCE, 2513–2526] and field luminescent tracer data of Ancao Peninsula (Portugal), by Balouin and Howa [Balouin, Y., Howa, H., 2000. Correspondence contact with University of Bordeaux].Generally, the agreement between the model results and the experimental data was found satisfactory in the context of the apparent roughness and time-averaged velocity profiles above the bed boundary layer. Further, the anti-phase behaviour of measured bedload concentration with respect to sediment concentration in the contact load layer and the asymmetric effects resulting from wave–current interaction are well represented by the model. Consequently, the present theoretical approach provides a useful tool for predictions of sand transport under wave–current flow within an accuracy factor of 1.5 either way. [Copyright &y& Elsevier]

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). •Large-scale wave flume experiments involving two alternating bichromatic wave induced swash events. •Sediment mobilization as sheet load and suspended load increases substantially from inner surf to lower swash zone. •Sheet flow transport dominates the total transport during the early uprush and during instants of strong wave-backwash interactions. •Single swash events can produce much greater net transport rates and bed level changes than the overall trend over multiple events. •The sediment exchange across the surf-swash boundary becomes increasingly significant when integrated over larger time scales. [ABSTRACT FROM AUTHOR]

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. • A free surface resolving Eulerian two-phase flow model is validated for sheet flow driven by near-breaking waves on a sandbar. • Onshore-directed sediment flux is enhanced due to progressive wave streaming effect for both velocity and acceleration skewed waves. • The enhancement of onshore transport is more significant for acceleration-skewed waves due to momentary bed failure. [ABSTRACT FROM AUTHOR]