Oscillating Water Tunnel Research Articles

Bed shear stress under skewed and asymmetric oscillatory flows

We present a new formulation to predict the bed shear stress under skewed/asymmetric oscillatory flows (with or without a co-linear mean current), extending the work of Nielsen (1992). The nonlinearity of the oscillatory flow is incorporated through the use of two parameters: the index of skewness or nonlinearity, and the waveform parameter. The new formulation is tested against the bed shear stress estimated from the log-fit and momentum-integral methods, using oscillatory data from oscillating water tunnel experiments. The new formulation and the momentum-integral method agree well, but differ from those with the log-fit method, possibly because both methodologies lead to different results for the phase lead between the bed shear stress and the free-stream velocity. The new bed shear stress formulation is incorporated in a quasi-steady bedload formula, and accurately reproduces net transport rates under non-linear, nonbreaking waves with and without an opposing current.

EXPERIMENTAL STUDY OF TURBULENT OSCILLATORY BOUNDARY LAYERS IN A NEW OSCILLATORY WATER TUNNEL

A new oscillatory water tunnel has been built in the Civil and Environmental Engineering Department's Hydraulic Laboratory at the National University of Singapore. It can accurately produce oscillatory flows that correspond to full-scale sea waves. Tests including pure sinusoidal waves and combined wave-current flows over smooth and rough bottoms have been performed. High quality measurements of the boundary layer flow fields are obtained using a PIV system. The PIV measured flow field is phase and spatially averaged to give a mean vertical velocity profile. It is found that the logarithmic profile can accurately approximate the near-bottom first-harmonic amplitude of sinusoidal waves and give highly accurate determinations of the hydrodynamic roughness and the theoretical bottom location. The bottom shear stress obtained from momentum integral is in general agreement with results from log-profile fitting. The current profiles of combined wave-current flows indicate a two-log-profile structure as suggested by simple combined wave-current flow theory. The difference between the two current shear velocities obtained from combined wave-current flows, as well as a small but meaningful third harmonic embedded in a pure sinusoidal wave, suggest the existence of a time-varying turbulent eddy viscosity.

Sediment transport in nonlinear skewed oscillatory flows: Transkew experiments

New experiments under sheet flow conditions were conducted in an oscillating water tunnel to study the effects of flow acceleration on sand transport. The simulated hydrodynamic conditions considered flow patterns that drive cross-shore sediment transport in the nearshore zone: the wave nonlinearities associated with velocity and acceleration skewness and a negative mean current, the undertow. Net transport rates were evaluated from the sediment balance equation and show that (1) the acceleration skewness in an oscillatory flow produces a net sediment transport in the direction of the highest acceleration; (2) the net transport in the presence of an opposing current is negative, against the direction of the highest acceleration, and reduces with an increase in flow acceleration; and (3) velocity skewness increases the values of the net onshore transport rates.

Suspension and near‐bed load sediment transport processes above a migrating, sand‐rippled bed under shoaling waves

The present study focuses on the fine‐scale flow and sand transport processes above onshore migrating ripples below skewed surface gravity waves in the shoaling zone. A set of acoustic instruments was deployed in the shoaling region of the large‐scale wave channel at Canal d'Investigacío i Experimatacío Marítima, Universitat Poltiècnica de Catalunya, Barcelona, Spain, in order to provide high‐resolution velocity and sediment concentration profiles with an acoustic concentration and velocity profiler (ACVP). Measurements are analyzed relative to the positions of the measured nonmoving sand bed and the interface separating the suspension from the near‐bed load layer. This interface is detected here by the application of a novel acoustic bed echo detection method. Furthermore, the use of the dual‐frequency inversion proposed in the work of Hurther et al. (2011) allows for the calculation of the sediment concentration profile across both the suspension and near‐bed load layers. The sand bed was covered by quasi‐two‐dimensional suborbital ripples migrating onshore. As proposed by O'Donoghue et al. (2006), the occurrence of quasi‐two‐dimensional ripples is attributed to the fine‐size sand ofD50= 250μm used in the present study under full‐scale forcing conditions. In order to determine the effect of shoaled wave skewness on the ripple vortex entrainment and sediment transport, the instantaneous and mean measurements of the flow, sediment concentration, and sediment flux along the ripple profile are discussed in terms of (1) the occurrence of ripple vortex entrainment on either side of the ripple crest; (2) the wave velocity phase lagging driven by the ripple vortex entrainment process and the turbulent bed friction effects in the wave boundary layer; (3) phase lagging between velocity and maximum concentration and sediment flux events; (4) the structure of bed friction and ripple‐driven turbulence across the suspension and the near‐bed load layers; and (5) the streaming components. The results on these aspects strongly support that the wave velocity skewness effect under shoaling waves is fairly similar to the one obtained in skewed oscillatory water tunnel flows. Furthermore, it is found that the onshore‐oriented net bed load sediment transport is at the origin of the onshore ripple migration. This flux is roughly twice as much as the opposite offshore‐oriented net suspension flux dominated by the ripple vortex entrainment processes.

PREDICTION OF NET BEDLOAD TRANSPORT RATES OBTAINED IN OSCILLATING WATER TUNNELS AND APPLICABILITY TO REAL SURF ZONE WAVES

Experimental studies of sediment transport rates due to nearshore waves are often conducted in oscillating water tunnels (OWTs). In an OWT, the oscillatory motion produced by the piston propagates almost instantaneously along the entire tunnel. Consequently, unlike the wave motion in the sea or in a wave flume, flow in an OWT is uniform along the tunnel, and second-order wave propagation effects (such as Longuet-Higgins's streaming) are absent. The effect of these hydrodynamic differences between OWT and sea waves on sediment transport rates has generally been neglected. In this paper we present a simple, practical formulation to evaluate bed shear stresses and bedload transport rates due to asymmetric and skewed waves plus a current in an OWT, based on fitting the exact results of a rigorous, analytical model of the OWT wave-current boundary layer. By then accounting for real wave effects we find that wave propagation significantly affects the predicted period-averaged net sediment transport rates. Such real wave effects can therefore not be neglected when comparing nearshore transport models with OWT data.

Boundary-layer hydrodynamics and bedload sediment transport in oscillating water tunnels

Oscillating water tunnels are experimental facilities commonly used in coastal engineering research. They are intended to reproduce near-bed hydrodynamic and sediment transport phenomena at a realistic scale. In an oscillating water tunnel, a piston generates an oscillatory motion that propagates almost instantaneously to the whole tunnel; consequently, flow is uniform along the tunnel, unlike the propagating wave motion in the sea or in a wave flume. This results in subtle differences between the boundary-layer hydrodynamics of an oscillating water tunnel and of a propagating wave, which may have a significant effect in the resulting sediment transport. In this paper, we present a zeroth-order analytical model of the turbulent boundary-layer hydrodynamics in an oscillating water tunnel. By using a time-varying eddy viscosity and by accounting for the constraints arising from the tunnel's geometry, the model predicts the oscillating water tunnel hydrodynamics and yields analytical expressions to compute bed shear stresses for asymmetric and skewed waves, both in the absence or presence of an imposed current. These expressions are applied to successfully quantify bedload sediment transport in oscillating water tunnel experiments.

Coherent structures in wave boundary layers. Part 1. Oscillatory motion

This work concerns oscillatory boundary layers over smooth beds. It comprises combined visual and quantitative techniques including bed shear stress measurements. The experiments were carried out in an oscillating water tunnel. The experiments reveal two significant coherent flow structures: (i) Vortex tubes, essentially two-dimensional vortices close to the bed extending across the width of the boundary-layer flow, caused by an inflectional-point shear layer instability. The imprint of these vortices in the bed shear stress is a series of small, insignificant kinks and dips. (ii) Turbulent spots, isolated arrowhead-shaped areas close to the bed in an otherwise laminar boundary layer where the flow ‘bursts’ with violent oscillations. The emergence of the turbulent spots marks the onset of turbulence. Turbulent spots cause single or multiple violent spikes in the bed shear stress signal, which has profound implications for sediment transport (in both the laboratory and the field). The experiments also show that similar coherent flow structures exist in the case of combined oscillatory flow and current.

Coherent structures in wave boundary layers. Part 2. Solitary motion

This study continues the investigation of wave boundary layers reported by Carstensen, Sumer & Fredsøe (J. Fluid Mech., 2010, part 1 of this paper). The present paper summarizes the results of an experimental investigation of turbulent solitary wave boundary layers, simulated by solitary motion in an oscillating water tunnel. Two kinds of measurements were made: bed shear stress measurements and velocity measurements. The experiments show that the solitary-motion boundary layer experiences three kinds of flow regimes as the Reynolds number is increased: (i) laminar regime; (ii) laminar regime where the boundary-layer flow experiences a regular array of vortex tubes near the bed over a short period of time during the deceleration stage; and (iii) transitional regime characterized with turbulent spots, revealed by single/multiple, or, sometimes, quite dense spikes in the bed shear stress traces. Supplementary synchronized flow visualization tests confirmed the presence of the previously mentioned flow features. Information related to flow resistance are also given in the paper.

A model of average velocity in oscillating turbulent boundary layers

A model to calculate the vertical distribution of the average velocity in the oscillating turbulent boundary layer forced by the overlaying oscillating free flow is discussed. The model is founded on the Rotationally Anisotropic Turbulence theory. The oscillating boundary layer is treated within the approximation of average velocity depending on the time and vertical coordinate only. The description results in an analytical expression for the flow velocity analogous to the expression for the flow velocity in the boundary layer of oscillating pipe flow. The resulting expressions for the vertical velocity profile are compared with respective laboratory data obtained in two tests of an oscillatory water tunnel experiment. It is shown that similar to the description of oscillating flow in tubes, the outcome of the suggested model with the appropriate values of the model parameters matches the measured velocity data reasonably well.

Computational Mechanics of Vertical Sorting of Sediment in Sheetflow Regime by 3D Granular Material Model

In a beach morphological process, such as a formation of a sand bar and a beach cusp, sediment sorting plays an important role. To describe a mechanism of sediment sorting directly, the movable bed simulator, which is a granular material model based on the distinct element method is used. In this paper, a hydraulic experiment using an oscillatory water tunnel is conducted to track motion of sediment particles under vertical sorting in sheetflow regime. The three-dimensional movable bed simulator is driven to track detailed motion of individual sediment particles numerically. Performance of the movable bed simulator as a tool of computational dynamics is shown by comparing simulated physical properties, such as series of instantaneous images, velocity profile and number-density distribution of sediment particles, with the result of the experiment.

Sand motion induced by oscillatory flows: Sheet flow and vortex ripples

A large series of field-scale experiments on turbulent sand-laden flows, conducted in preceding years in the LOWT and AOFT large oscillating water tunnels are reviewed and reanalysed. Using the combined experimental data sets, new insights are obtained on the detailed sand transport processes occurring in sheet-flow and ripple regime conditions. For sheet flow (i) new equations are presented relating maximum erosion depth and sheet-flow layer thickness to the maximum Shields parameter; (ii) detailed analysis of sediment flux data shows the dominance of the current-related flux in the sheet-flow layer and the different characters of the current-related flux for fine and medium sands; (iii) a RANS-diffusion type model is shown to reproduce important trends in net transport rate related to grain size, velocity and wave period and to predict the magnitude of net transport rate to within a factor 2 of measured values. For the ripple regime it is shown that (i) asymmetric waves generate negative (‘offshore’) streaming and the current-related suspended sediment flux associated with this streaming appears to be of the same order of magnitude as the wave-related suspended sediment flux; (ii) time-averaged near-bed transport and time-averaged suspended transport appear to be of about equal magnitude but of opposite sign, and are concentrated on the ‘onshore’ flank of the ripple for asymmetric wave conditions; (iii) near-bed transport along the onshore flank is generated by sand transported over the ripple crest during the ‘onshore’ half-cycle. Net sand transport under asymmetric waves can be ‘onshore’ directed or ‘offshore’ directed, depending on the degree of unsteadiness in the sand flux behaviour during the wave cycle. Dimensionless phase-lag parameters are presented, for sheet flow and ripples, which can discriminate between predominantly quasi-steady behaviour (resulting in ‘onshore’ transport) and predominantly unsteady behaviour (resulting in ‘offshore transport’).

Seabed shear stress and bedload transport due to asymmetric and skewed waves

A simple conceptual formulation to compute seabed shear stress due to asymmetric and skewed waves is presented. This formulation generalizes the sinusoidal wave case and uses a variable friction factor to describe the physics of the boundary layer and to parameterize the effects of wave shape. Predictions of bed shear stresses agree with numerical computations using a standard boundary layer model with a k– ε turbulence closure. The bed shear stress formulation is combined with a Meyer-Peter and Müller-type formula to predict sheet flow bedload transport under asymmetric and skewed waves for a horizontal or sloping bed. The predictions agree with oscillatory water tunnel measurements from the literature.

Formula for predicting bedload transport rate in oscillatory sheet flows

At high bed shear stress sheet flows often occur in coastal waters in which high-concentration bedload sediments are transported in a thin layer near the bed. This paper firstly constructs a theoretical model (partial differential equations, PDEs) for the intense transport of non-cohesive bedload sediments by unidirectional currents and then seeks a special solution to the PDEs to determine the thickness of the bedload particle–water mixture, which could serve as the “reference height” that is often invoked in numerical computation and simulation of suspended sediment transport in turbulent flows. Moreover, a modified formula is presented to determine the “reference concentration”. Using a “ uch” approach the present study derives a 1 D formula for predicting bedload transport rate in sheet flows driven by asymmetric waves, with the help of a novel formula for evaluating wave friction factor. The new bedload formula can generically take into account slope angle (positive and negative), wash load concentration in the driving water flow and other factors that affect bedload transport rate. It compares well with measured data in a large-scale wave flume [Dohmen-Janssen, C.M., Hanes, D.M., 2002. Sheet flow dynamics under monochromatic non-breaking waves. Journal of Geophysical Research, 107(C10), 1301–1321], a large-scale oscillatory water tunnel [ Hassan, W.N., Ribberink, J.S., 2005. Transport processes of uniform and mixed sands in oscillatory sheet flow. Coastal Engineering, 52, 745–770] and in a swash zone of natural beach [Masselink, G., Hughes, M.G., 1998. Field investigation of sediment transport in the swash zone. Continental Shelf Research, 18, 1179–1199].

Phytoplankton deposition to permeable sediments under oscillatory flow: Effects of ripple geometry and resuspension

Pressure-gradient driven, advective flows through permeable sediments are recognized as an important mechanism by which dissolved and particulate materials are exchanged between the near-bottom water and the seabed. Here, we focus on the deposition of particulate organic matter in wave-formed, rippled beds representative of permeable coastal ocean sediments. In addition, we examine the release of previously deposited material when sediment transport is initiated. Laboratory experiments were conducted in an oscillatory water tunnel using cultured diatoms (4–6 μm dia.) as a tracer for particulate organic matter. Using different ripple geometries on a coarse sand bed (median dia.=575 μm) and following deposition under controlled flow conditions, we cored extensively to determine the distribution and flux of diatoms to the bed. Results showed greater diatom deposition (3.5×) and penetration depths (4–5 cm) in ripple troughs in comparison to ripple crests (<2 cm), qualitatively in agreement with past model predictions and experimental results. Both this pattern and the total diatom flux were consistent over a 3-fold variation in ripple wavelength (17–50 cm) and a 4-fold variation in ripple height (1.4–7.7 cm). The total diatom flux was only marginally enhanced by the presence of ripples (∼1.6×) compared to a flat bed. Our data suggest that the total diatom flux may be less controlled by rippled bed topography than solute exchanges, including but not limited to advective porewater flows. Repeatability between replicate depositional runs allowed us to conduct resuspension experiments in which rippled beds previously impregnated with diatoms were eroded and allowed to establish a new ripple configuration. Approximately half (45–60%) of the embedded diatoms were released during reconfiguration of the ripples, a process that lasted 20 min. Remaining diatoms were buried comparatively deeper in isolated pockets, the location a function of ripple position both before and after sediment resuspension. Our results clearly indicate that small particle deposition to a rippled bed is a simple pattern predicted by models and prior experiments but that the total flux is not strongly determined by ripple geometry. However, the net depositional pattern in the real ocean will be determined by the combined effects of local sediment erosion and deposition, and thus these two processes act in tandem to lead to substantial, ripple-scale heterogeneity of organic matter input to shelf sediments in both the horizontal and vertical dimensions.

Modelling and measurement of sand transport processes over full-scale ripples in oscillatory flow

A new series of laboratory experiments was performed in the Aberdeen Oscillatory Flow Tunnel (AOFT) and the Large Oscillating Water Tunnel (LOWT) to investigate time-averaged suspended sand concentrations and transport rates over rippled beds in regular and irregular oscillatory flow. The wave-induced oscillatory near-bed flows were simulated at full-scale. Five series of experiments were carried out. During the two AOFT experimental series, ripple dimensions, ripple migration rates and net sand transport rates were measured under regular and irregular asymmetric flow for two different sand types. The three LOWT experimental series focussed on measurements of the ripple dimensions, ripple migration rates, time-averaged suspended sand concentrations and net sand transport rates under regular asymmetric and irregular weakly asymmetric flow for two different sand types. From analysis of new and other full-scale data, it is concluded that the lower part of the time- and bed-averaged concentration profile (up to two times the ripple height above the ripple crest level) has an exponential profile. A new reference concentration formula is proposed based on the formula of Bosman and Steetzel [Bosman, J.J., Steetzel, H.J., 1986. Time- and bed-averaged concentration under waves. Proc. 20th ICCE Taipei, ASCE, pp. 986–1000], which includes the grain-size influence. Furthermore, it is shown that the concentration decay length is strongly related to the ripple height and that the simple formula R c = 1.27 η gives good agreement with the data. A new transport model is proposed for the wave-related net transport over full-scale ripples based on a modified half wave cycle concept of Dibajnia and Watanabe [Dibajnia, M., Watanabe, A., 1992. Sheet flow under nonlinear waves and currents. Proc. 23rd ICCE Venice, ASCE, pp. 2015–2028; Dibajnia, M., Watanabe, A., 1996. A transport rate formula for mixed sands. Proc. 25th ICCE Orlando, ASCE, pp. 3791–3804]. The magnitudes of the half wave cycle transport contributions are related to the grain-related Shields parameter, the degree of wave asymmetry and a newly defined vortex suspension parameter P, which is the ratio between the ripple height and the median grain-size. The new model has been calibrated using transport data from the new regular flow experiments and has subsequently been validated using other data, including measurements from irregular flow experiments. The new model is seen to perform better overall than existing practical models for ripple regime net sand transport.

Transport processes of uniform and mixed sands in oscillatory sheet flow

New experiments were carried out in the Large Oscillating Water Tunnel of WL|Delft Hydraulics (scale 1:1) using asymmetric 2nd-order Stokes waves. The main aim was to gain a better understanding of size-selective sediment transport processes under oscillatory plane-bed/sheet-flow conditions. The new data show that for uniform sand sizes between 0.2 < D < 1.0 mm, measured net transport rates are hardly affected by the grain size and are proportional to the third-order velocity moment. However for finer grains ( D = 0.13 mm) net sand transport rates change from the ‘onshore’ direction into the ‘offshore’ direction in the high velocity range. A new measuring technique for sediment concentrations, based on the measurement of electro-resistance (see [McLean, S.R., Ribberink, J.S., Dohmen-Janssen, C.M. and Hassan, W.N.M., 2001. Sediment transport measurements within the sheet flow layer under waves and currents. J. Waterw., Port, Coast., Ocean Eng., ISSN 0733-950X]), was developed further for the improved measurement of sediment dynamics inside the sheet-flow layer. This technique enabled the measurements of particle velocities during the complete wave cycle. It is observed that for long period waves ( T = 12.0 s), time-dependent concentrations inside the sheet-flow layer are nearly in phase with the time-dependent flow velocities. As the wave period decreases, the sediment entrainment from the bed as well as the deposition process back to the bed lags behind the wave motion more and more. The new data show that size-gradation has almost no effect on the net total transport rates, provided the grain sizes of the sand mixture are in the range of 0.2 < D < 1.0 mm. However, if very fine grains ( D = 0.13 mm) are present in the mixture, net total transport rates of graded sand are generally reduced in comparison with uniform sand with the same D 50. The transport rates of individual size fractions of a mixture are strongly influenced by the presence of other fractions in a mixture. Fine particles in sand mixtures are relatively less transported than in that uniform sand case, while the opposite occurs for coarse fractions in a mixture. The relative contribution of the coarse grains to the net total transport is therefore larger than would be expected based on their volume proportion in the original sand mixture. This partial transport behaviour is opposite to what is generally observed in uni-directional (e.g. river) flows. This is caused by vertical sorting of grain sizes in the upper bed layer and in the sheet flow and suspension layers. Kinematic sorting is believed to be responsible for the development of a coarse surface layer on top of a relatively fine sub-layer, providing in this way a relatively large flow exposure for the coarser sizes. Furthermore fine grains are suspended more easily than coarse grains to higher elevations in the flow where they are subject to increasing phase-lag effects (settling lags). The latter also leads to reduced net transport rates of these finer sizes.

Effect of externally generated turbulence on wave boundary layer

This experimental study deals with the effect of externally generated turbulence on the oscillatory boundary layer to simulate the turbulence in the wave boundary layer (WBL) under broken waves in the swash zone. The subject has been investigated experimentally in a U-shaped, oscillating water tunnel with a smooth bottom. Turbulence was generated ‘externally’ as the flow in the oscillator was passed through a series of grids that extended from the cover of the water tunnel to about mid-depth. Two different types of grid porosities were used. Direct measurements of the bed shear stress and velocity measurements were carried out. For the velocity measurements, mean and turbulence properties were measured in both the streamwise direction and in the direction perpendicular to the bed. A supplementary measurement for the undisturbed (without grids) case was also carried out, for comparison with the grid results. The mean and turbulence quantities in the outer flow region are increased substantially with the introduction of the grids. It is shown that the externally generated turbulence is able to penetrate the bed boundary layer, resulting in an increase in the bed shear stress, and therefore the friction coefficient. Other features related to the bed shear stress, such as transition, the friction factor and phase lead, are discussed. The range of the Reynolds number studied is Re=1×10 4–2×10 6.

Gradation effects in sediment transport

To determine the effects of grain size and density gradation in oscillatory sheet-flow, experiments are conducted in an oscillating water tunnel. A formal derivation of a schematised transport model shows that the transport rates per sediment fraction can be determined with and without the assumption of an active layer. A technique that measures sediment composition from natural occurring radionuclides is used to determine the transport rates without that assumption of an active layer. The measurements on total sediment transport rates and suspended sediment concentrations show that effects of gradation are present. The effects of size gradation are mainly in the increased transport rates of the coarse fraction with respect to the uniform coarse material and the availability at the bed under similar conditions. The experiments on density graded sediments indicate that total mass-transport rates are larger than the mass-transport rates of quartz material, whilst the transport rates of the quartz fraction show that this fraction is not hindered by the availability at the bed.

Sheet flow dynamics under monochromatic nonbreaking waves

For the first time, detailed measurements of sediment concentrations and grain velocities inside the sheet flow layer under prototype surface gravity waves have been carried out in combination with measurements of suspension processes above the sheet flow layer. Experiments were performed in a large‐scale wave flume using natural sand. Sand transport under high waves in shallow water is mainly contained within the so‐called “sheet flow layer,” a thin layer (10–60 grain diameters) in which the volume concentration of sand decreases by an order of magnitude from a value near 0.6 at the stationary bed. The thickness of the layer varies over a wave cycle and the maximum thickness increases with increasing peak Shields stress. The concentrations within the sheet flow layer vary approximately synchronously with the orbital velocity measured by an Acoustic Doppler Velocimeter (ADV) located 0.1 m above the bed, with typical phase lags of 0–π/5. In contrast, the suspended sediment concentrations a few centimeters and higher above the bed exhibit larger phase lags. Grain velocities were successfully measured in the middle and upper portions of the sheet flow layer around the time of their maximums. These velocities increased weakly with elevation from approximately 50% to 70% of the velocity outside the wave boundary layer. The observations are compared to previous experimental work and are found to be mainly consistent with observations in steady unidirectional flows and in oscillating water tunnels (OWTs), although differences in the suspended sediment concentration and the total sediment transport rate are apparent. Observations are also compared to two very different models: a 1DV suspension model for oscillatory flow with enhanced boundary roughness and a two‐phase collisional grain flow model for steady unidirectional flow. While the suspension model describes the velocity profile fairly well and the collisional model describes the concentration profile well, neither model accurately predicts both the velocity and the concentration and therefore the sediment flux over the full vertical extent of the sheet flow.

Phase lags in oscillatory sheet flow: experiments and bed load modelling

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 ( D 50=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.