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A self-designed “bottom boundary layer hydrodynamic and suspended sediment concentration (SSC) measuring system” was built to observe the hydrodynamic and the SSC processes over the intertidal mudflats at the middle part of the Jiangsu coast during August 8–10, 2013. Velocity profiles within 10 cm of the mudflat surface were obtained with a vertical resolution as fine as 1 mm. An ADCP was used to extend the profile over the full water depth with a resolution of 10 cm and the vertical SSC profile was measured at intervals using Optical Backscatter Sensors (OBS). At the same time, water levels and wave conditions were measured with a Tide and Wave Recorder. Measured data suggested that the vertical structure of velocity profiles within 10 cm above the bed maintains a logarithmic distribution during the whole tidal cycle except the slack-water periods. Shallow flows during both the early-flood period and the later-ebb period are characterized by a relatively large vertical velocity gradient and a “surge” feature. We conclude that the very shallow water stages are transient and may not contribute much to the whole water and sediment transport, while they can play a significant role in the formation and evolution of micro-topographies on tidal flats.

Laboratory and field experiments have suggested that near-bed sediment resuspension is an intermittent process influenced by turbulent coherent structures. This is in contrast to many sediment transport theories that relate sediment transport rates to mean near bed flow parameters. This study presents observations of turbulent bursting events obtained from two contrasting environments dominated by unidirectional and oscillatory flow regimes with mean currents higher than the critical value for sand transport. In agreement with previous laboratory and field experiments the data investigated in this study (both in unidirectional current and oscillatory flow environments) indicated that sediment resuspension was an event based system with sediment flux controlled by ejections and sweeps. Both the magnitude of the Reynolds stress and duration of the events contributed to sediment resuspension. Wavelet analysis confirmed that over time turbulence occurred in slowly evolving clusters that were closely followed by periods of strong resuspension near the bed, evolving from the primary leading scales at low frequencies, and decaying in time after the termination of the turbulent agitation. The results highlight the necessity of considering the instantaneous Reynolds shear stress effects in the existing sediment transport equations which are developed based on a time-averaged flow parameters and mean a critical velocity. Field observations presented in this study highlight the importance of considering turbulence events along with the concept of impulse as an essential parameter to be considered into future modelling of sediment movement under both unidirectional and oscillatory flows.

Wave-induced, steep vortex ripples are ubiquitous features in shallow coastal seas and it is therefore important to fully understand and model the sediment transport processes that occur over them. To this end, two two-dimensional vertical (2DV) models have been critically tested against detailed velocity and sediment concentration measurements above mobile ripples in regular asymmetric oscillatory flow. The two models are a k–ω turbulence-closure model and a discrete-vortex, particle-tracking (DVPT) model, while the data are obtained in the Aberdeen oscillatory flow tunnel (AOFT). The models and the data demonstrate that the time-dependent velocity and suspended sediment concentration above the ripple are dominated by the generation of lee-side vortices and their subsequent ejection at flow reversal. The DVPT model predicts the positions and strengths of the vortices reasonably well, but tends to overpredict the velocity close to the ripple surface. The k–ω model, on the other hand, underpredicts the height to which the vortices are lifted, but is better able to predict the velocity close to the bed. In terms of the cycle- and ripple-averaged horizontal velocity, both models are able to reproduce the observed offshore flow close to and below the ripple crest and the DVPT model is able to produce the onshore flow higher up. In the vicinity of the vortices, the DVPT model better represents the concentration (because of its better prediction of vorticity). The k–ω model, on the other hand, better represents the concentration close to the ripple surface and higher up in the flow (because of the better representation of the near-bed flow and background turbulence). The measured and predicted cycle- and ripple-averaged suspended sediment concentrations are in reasonable agreement and demonstrate the expected region of exponential decay. The models are able to reproduce the observed offshore cycle- and ripple-averaged suspended sediment flux from the ripple troughs upwards, and as a result, produce net offshore suspended sediment transport rates that are in reasonable agreement. The net measured offshore suspended transport rate, based on the integration of fluxes, was found to be consistent with the total net offshore transport measured in the tunnel as a whole once the onshore transport resulting from ripple migration was taken into account, as would be expected. This demonstrates the importance of models being able to predict ripple-migration rates. However, at present neither of the models is able to do so.

Instrumented bottom tripods have provided important data on sediment transport processes on continental shelves and in estuaries for four decades. Since the initial deployment in a tidal channel in Puget Sound, WA, in 1965 numerous tripods have been constructed to investigate bottom boundary layer and sediment dynamics worldwide. Tripod data have led to new understanding of near-bottom wave and current flows in the coastal ocean, and have been crucial to the development of shelf circulation and sediment transport models. Calculations of bottom stress, bottom roughness, and sediment flux that resulted directly from tripod data have been compared to bottom boundary layer model results. Where these have differed, new or revised model components have been developed to improve the skill of the models. The many discoveries that have been made from tripod experiments include dense, near-bottom fluid mud layers that transport large quantities of suspended sediment offshore into deeper regions of the continental shelf. This process has been linked to the seaward progradation of subaqueous deltas and to the boundaries of mid-shelf mud deposits off rivers with high fine-sediment discharge.

Field observations of instantaneous water surface slopes in the swash zone are presented. For free-surface flows with a hydrostatic pressure distribution the surface slope is equivalent to the horizontal pressure gradient. Observations were made using a novel technique which in its simplest form consists of a horizontal stringline extending seaward from the beach face. Visual observation, still photography or video photography is then sufficient to determine the surface slope where the free-surface cuts the line or between reference points in the image. The method resolves the mean surface gradient over a cross-shore distance of 5 m or more to within +/- 0.001, or 1/20th -1/100th of typical beach gradients. In addition, at selected points and at any instant in time during the swash cycle, the water surface slope can be determined exactly to be dipping either seaward or landward. Close to the location of bore collapse landward dipping water surface slopes of order 0.05-0.1 occur over a very small region (order 0.5 m) at the blunt or convex leading edge of the swash. In the middle and upper swash the water surface slope at this leading edge is usually very close to horizontal or slightly seaward. Behind the leading edge, the water surface slope was observed to be very close to horizontal or dipping seaward at all times throughout the swash uprush. During the backwash the water surface slope was observed to be always dipping seaward, approaching the beach slope, and remained seaward until a new uprush edge or incident bore passed any particular cross-shore location of interest. The observations strongly Suggest that the swash boundary layer is subject to an adverse pressure gradient during uprush and a favourable pressure gradient during the backwash. Furthermore, assuming Euler's equations are a good approximation in the swash, the observations also show that the total fluid acceleration is negative (offshore) for almost the whole of the uprush and for the entire backwash. The observations are contrary to recent work suggesting significant shoreward directed accelerations and pressure gradients occur in the swash (i.e., delta u/delta t > 0 similar to delta p/delta x < 0), but consistent with analytical and numerical solutions for swash uprush and backwash. The results have important implications for sediment transport modelling in the swash zone.

The boundary layer flow and the net sediment transport beneath asymmetrical waves have been examined by a turbulence boundary layer model in conjunction with a diffusion model for suspended sediments based on the linearized boundary layer equations with horizontally uniform forcing. The turbulence closure is provided by a high Reynolds number k – e model. The flow model and sediment transport model have been verified against existing laboratory measurements for sheet flow conditions. Overall, these comparisons show that the mean (wave-averaged) velocity profile, sediment flux and total sediment transport (including mean bedload transport) are well predicted by the present model. A further investigation of the boundary layer structure shows the mechanisms causing the near-bed backflow (opposing the drift direction of the outer wave velocity). The effect of roughness and asymmetry on the mean boundary layer flow and sediment transport beneath asymmetrical waves have been investigated. The wave friction factor, the mean bedload transport and the mean velocity profiles are given for a wide range of near-bed wave excursion to bed roughness ratios. Moreover, for asymmetrical waves with the same mean drift velocity, but with different asymmetry, the wave with strongest asymmetry induces the largest mean sediment transport. This demonstrates the importance of wave asymmetry.

A numerical modelling study on the effects of sediment-induced stratification on tidal currents is conducted for an idealized tidal estuary with a muddy seabed. The numerical model is a three-dimensional oceanic sediment transport model capable of predicting dynamics of Newtonian fluid mud layers due to coupling of the sea water density and suspended sediment concentration. The model uses a reparameterized bottom drag coefficient C d that incorporates a linear stability function of flux Richardson number R f . The study has shown that the sediment-induced stratification in the bottom boundary layer (BBL) reduces the vertical eddy viscosity and bottom shear stress in comparison with the model prediction in a neutrally stratified BBL. In response to these apparent reductions, the tidal current shear in the water column is increased in the estuary.

Concurrent video images of sand ripples and current meter measurements of directional wave spectra are analyzed to study the relations between waves and wave-generated sand ripples. The data were collected on the inner shelf off Huntington Beach, California, at 15 m water depth, where the sea floor is comprised of well-sorted very fine sands (D50=92 μm), during the winter of 2002. The wave climate, which was controlled by southerly swells (12–18 s period) and westerly wind waves (5–10 s period), included three wave types: (A) uni-modal, swells only; (B) bi-modal, swells dominant; and (C) bi-modal, wind-wave dominant. Each wave type has distinct relations with the plan-view shapes of ripples that are classified into five types: (1) sharp-crested, two-dimensional (2-D) ripples; (2) sharp-crested, brick-pattern, 3-D ripples; (3) bifurcated, 3-D ripples; (4) round-crested, shallow, 3-D ripples; and (5) flat bed. The ripple spacing is very small and varies between 4.5 and 7.5 cm. These ripples are anorbital as ripples in many field studies. Ripple orientation is only correlated with wave directions during strong storms (wave type C). In a poly-modal, multi-directional spectral wave environment, the use of the peak parameters (frequency, direction), a common practice when spectral wave measurements are unavailable, may lead to significant errors in boundary layer and sediment transport calculations.

The suspended sediment concentration under large random waves alone as well as under large random waves plus current has been examined by a classical sediment diffusion model in conjunction with a dynamic turbulent boundary layer model based on the linearized boundary layer equations with horizontally uniform forcing. The turbulence closure is provided by a high Reynolds number k – e model. Under such conditions wave or current-induced sea bed ripples are washed out, such that a sheet flow and a suspension layer of high sediment concentration exist. This sediment diffusion model has been verified against a range of existing measurements, for regular waves alone, as well as regular waves plus current. Overall, these comparisons show that the present sediment diffusion model does a fair job except in the outer suspension layer, where the existence of a `convective' peak near flow reversal becomes important. These peaks cannot be captured by standard sediment diffusion models. Correlations between suspended sediment concentration and boundary layer quantities due to the grouping of the largest waves in a realistic sea state have been examined. Sediment pump-up under a wave group is visualized. Estimates of probability density functions for individual sediment concentration maxima are given. Superposition of a mean current on the waves at the outer boundary induces a mean sediment flux as well as an enhancement of the suspended sediment concentration; these effects have been demonstrated and quantified. A non-zero angle between the waves and the current leads to a decrease in both the mean sediment concentration and the mean sediment flux. The resulting mean sediment concentration and mean sediment flux for random waves plus current has been shown to agree reasonably well with those obtained by an equivalent sinusoidal wave plus current. Overall, the results obtained in this paper support the application of an equivalent wave to predict mean sediment concentrations, mean sediment fluxes, as well as the total sediment transport under sheet flow conditions.

Rivers provide the dominant pathway of terrigenous sediment to the ocean. The density difference between riverine and salt water as well as the density anomaly contributed by sediment have important consequences on the delivery, transport and ultimate fate of the sediment issued from the land. Most fresh water outflows produce positively buoyant plumes; however, the aggregation and settling of sediment cause the route of sediment transport to diverge from the route of fresh water flow. Sediment is trapped in frontal zones, both in estuaries and on the inner shelf, often resulting in large increases in the concentration of sediment relative to the riverine source. At high concentrations, the density anomaly due to the sediment itself contributes to the vertical stability of the flow, often increasing the trapping efficiency of the frontal zone. Sediment trapping may also occur within the wave boundary layer on the continental shelf, leading to high concentrations within a layer only on the order of 10 cm thick, but which may represent an important cross-shelf conduit of sediment. The high concentration layers formed both by frontal trapping and by wave boundary layer trapping can attain excess densities large enough to initiate down-slope motion on the continental shelf, resulting in an important cross-shelf transport process. Because of these trapping processes, the formation of high-concentration layers, and the occurrence of hyperpycnal plumes, the transport of sediment in river-influenced environments is often dominated by near-bottom fluxes rather than fluxes in the surface plume.

A three-dimensional baroclinic model, including suspended sediment transport, is used in cross-sectional form to examine processes (tidal, along-shelf current, wind-waves, and wind) influencing suspended sediment transport off the west coast of Scotland. Sub-grid scale processes are parameterized using a turbulence energy model. Calculations using appropriate sediment types and tidal forcing for the region, show that the barotropic tide alone essentially cannot suspend bottom sediment in the area. However, when an along-shelf flow is added some sediment is suspended. An off-shelf net transport of sediment occurs in the bottom layer, with a weaker on-shelf transport in the surface region. This is consistent with recent measurements in the area which show the lack of tidal resuspension, and the importance of an along-shelf flow in sediment suspension. Observations reveal a weak off-shelf flow in the bottom boundary layer. The magnitude of this transport is increased by the presence of wind-waves, or by a reduction in the sediment settling velocity. Observations during a storm clearly show the importance of wind-waves in causing sediment resuspension particularly close to the shore. When density stratification was included, internal tides were generated with associated regions of enhanced bottom friction velocity u* and hence increased sediment resuspension. Although the dominant feature of the flow is similar to that found with the barotropic tide, this is modified by smaller scale currents due to the presence of the internal tide. Summer observations indicate that strong internal waves can enhance sediment suspension. However from the limited number of observations and the high spatial variability of the internal tide found in the model, the importance of the internal tide is difficult to assess from the measurements. When an upwelling favourable wind is added, the vertical density gradient in the near-bed region increases, which changes the internal tide and hence its effect upon sediment suspension and transport. The presence of an along-shelf flow induced by the wind significantly increases the bed stress leading to enhanced sediment resuspension. Sediment transport is dominated by the wind-driven flow, with coastal upwelling moving sediment from the bottom boundary layer to the surface layer where it is advected off-shelf. Observations made during upwelling favourable winds, confirm this off-shelf advection of sediment in the upper part of the water column. Conversely with a downwelling favourable wind, the thickness of the bottom boundary layer is increased, and in the shelf edge region sediment is suspended to a greater height in the water column. In the near-coastal area, under downwelling conditions, the sediment is confined to the bottom boundary layer and is advected away from the shore in the near-bed region.

An analytical model of down-slope sediment transport and deposition by wave-supported gravity-driven flows is applied to examine the formation of the mid-shelf flood deposit on the continental margin off of the Eel River in northern California. The model reproduces observed time series of near-bed velocity and deposition following flood events when sufficient fine sediment is available to critically stratify the wave boundary layer. Comparative estimates of down-slope gravity-driven flux and along-shelf sediment delivery suggest that critical stratification will dominate the near-bed dynamics when greater amounts of sediment are delivered to the inner shelf by river floods than can be removed by gravity-driven processes. If insufficient sediment is delivered to cause critical stratification in the wave boundary layer, energetic waves can enhance drag, retard down-slope transport, and limit deposition. Analytic predictions of deposition suggest that the magnitude of wave energy is more important than the magnitude of the flood event in controlling the local thickness of mid-shelf gravity-driven deposition following floods of the Eel River. Higher wave energy increases the capacity for critically stratified gravity flows to transport sediment to the mid-shelf and results in greater gradients in flux and hence deposition. Flood magnitude determines how close to shore the flood deposit begins and how far along-shelf it extends. The bathymetry of the Eel margin plays a critical role controlling wave-supported gravity flows. Analytic predictions indicate that gravity-driven deposition on the mid-shelf begins roughly 7–8 km north of the river mouth. Closer to the river mouth, the seaward increasing mid-shelf slope associated with the concave downward subaqueous delta causes gravity-driven flux divergence, preventing significant mid-shelf deposition by wave-supported gravity flows and favoring sediment bypassing. A seaward decrease in shelf slope in the vicinity of the observed flood depo-center leads to greater flux convergence by gravity-driven flows, and hence greater deposition. Farther north, the supply of sediment diminishes sufficiently to prevent significant gravity-driven deposition. The analytic predictions of mid-shelf mud deposition are spatially and temporally consistent with field observations and provide strong evidence that wave-supported gravity flows control the emplacement and location of the Eel margin flood deposit.

The ability of internal tides to resuspend and advect sediment over continental shelves and slope regions is investigated theoretically using an internal wave and sediment transport model. Numerical experiments are carried out, firstly, with a linearly sloping continental slope and the ratio of bathymetry and internal waves characteristics creating critical, subcritical, and supercritical conditions, and secondly, for an observed section of the Australian North West Shelf. In the former cases, the model is forced with an internal tide propagating through the model domain. The latter application involves forcing by a barotropic tide which in turn generates internal waves at the shelf slope. Internal wave-generated bottom layer shear stresses are large enough to resuspend sediment. The application of a turbulence closure scheme results in the creation and maintenance of a thin nepheloid layer. The thickness of the suspended sediment layer is controlled by vertical diffusion which is large within the bottom boundary layer, but very small outside. The residual velocity and the asymmetry associated with the velocity field, result in both down- and upslope net suspended sediment fluxes, and deposition of resuspended material onto the shelf. These suspended sediment fluxes are largest for critical bottom slopes. The parting point between down- and upslope net sediment flux is found to be sensitive to the formulation of vertical mixing with the parting point moving downslope for increased mixing. At the Australian North West Shelf, near the shelf break and upper slope, the net flux of resuspended material is influenced by both the barotropic and internal tide. The phase relationship in bottom layer shear stresses generated from those two tides causes regions of enhancement and regions of reduction in the resuspension rates and net suspended sediment fluxes.

Large wave ripples and upper-plane bed sheet flow were observed under combined waves and currents during storms at 39 m water depth on Scotian Shelf. These data show that the critical wave mobility number and Shields parameter for sheet flow are not constant, but rather their values decrease with an increase in grain size. The critical sheet-flow Shields parameter under combined flows is found to be about 50% smaller than that for pure waves. Though the bed shear stress based on bedload roughness or wave parameterization using the one-tenth largest waves both offer adequate resolution to this difference, a definitive explanation does not exist at present. The large wave ripples (LWR) observed under combined flows are generally symmetrical, rounded and 3-dimensional, with small current ripples superimposed. The wavelengths of these LWR are approximately half of the wave orbital diameter and their average steepness is about 0.1. Analyses of storm processes on continental shelves show that as a storm builds up, ripples generally change directly into upper-plane beds. LWR generally form following the peaks of storms due to sediment fall out from suspension and moulding of the seabed by the long wave oscillation. If the spin up of a storm is gradual and strongly wave dominant, however, LWR can also develop from small ripples before sheet-flow conditions are reached. We interpret the three-dimensional, mound-like, low-relief LWR as combined-flow hummocky megaripples. This suggests that hummocky megaripples and hummocky cross-stratification (HCS) on continental shelves are formed under wave-dominant combined flows, and that they can form in medium sand as well as in silt and fine sand. The application of the combined-flow boundary layer model of Grant and Madsen (1986) and a modified Rouse suspension equation shows that the incorrect prediction of the presence of ripples in place of upper-plane bed causes over-estimation of shear velocities and suspension concentration and a 300% over-prediction of the suspended sediment transport rate. By contrast, failing to predict correctly the formation of LWR causes an under-estimation of shear velocities and sand resuspension, and will result in under-prediction of the suspended sediment transport rate by nearly one order of magnitude.

Modeling shelf sediment transport rates and bed reworking depths is problematic when the wave and current forcing conditions are not precisely known, as is usually the case when long-term sedimentation patterns are of interest. Two approaches to modeling sediment transport under such circumstances are considered. The first relies on measured or simulated time series of flow conditions to drive model calculations. The second approach uses as model input probability distribution functions of bottom boundary layer flow conditions developed from wave and current measurements. Sediment transport rates, frequency of bed resuspension by waves and currents, and bed reworking calculated using the two methods are compared at the mid-shelf STRESS (Sediment TRansport on Shelves and Slopes) site on the northern California continental shelf. Current, wave and resuspension measurements at the site are used to generate model inputs and test model results. An 11-year record of bottom wave orbital velocity, calculated from surface wave spectra measured by the National Data Buoy Center (NDBC) Buoy 46013 and verified against bottom tripod measurements, is used to characterize the frequency and duration of wave-driven transport events and to estimate the joint probability distribution of wave orbital velocity and period. A 109-day record of hourly current measurements 10 m above bottom is used to estimate the probability distribution of bottom boundary layer current velocity at this site and to develop an auto-regressive model to simulate current velocities for times when direct measurements of currents are not available. Frequency of transport, the maximum volume of suspended sediment, and average flux calculated using measured wave and simulated current time series agree well with values calculated using measured time series. A probabilistic approach is more amenable to calculations over time scales longer than existing wave records, but it tends to underestimate net transport because it does not capture the episodic nature of transport events. Both methods enable estimates to be made of the uncertainty in transport quantities that arise from an incomplete knowledge of the specific timing of wave and current conditions. © 1997 Elsevier Science Ltd

As part of the 1990–1991 Sediment TRansport Events on Shelves and Slopes (STRESS) experiment, a 5 MHz Acoustic BackScatter System (ABSS) was deployed in 90 m of water to measure vertical profiles of near-bottom suspended sediment concentration. By looking at the vertical profile of concentration from 0 to 50 cm above bottom (cmab) with 1 cm vertical resolution, the ABSS was able to examine the detailed structure of the bottom boundary layer created by combined wave and current stresses. The acoustic profiles clearly showed the wave-current boundary layer, which extends to (order) 10 cmab. The profiles also showed evidence of an “intermediate” boundary layer, also influenced by combined wave and current stresses, just above the wave-current boundary layer. This paper examines the boundary-layer structure by comparing acoustic data obtained by the authors to a 1-D eddy viscosity model formulation. Specifically, these data are compared to a simple extension of the Grant-Glenn-Madsen model formulation. Also of interest is the appearance of apparently 3-D “advective plume” structures in these data. This is an interesting feature in a site which was initially chosen to be a good example of (temporally averaged) 1-D bottom boundary-layer dynamics. Computer modeling and sector-scanning sonar images are presented to justify the plausibility of observing 3-D structure at the STRESS site. © 1997 Elsevier Science Ltd

During the 1988–1989 Sediment Transport Events on Shelves and Slopes (STRESS) experiment, a 1-MHz acoustic backscatter system (ABSS), deployed in 90 m of water off the California coast measured vertical profiles of suspended sediment concentration from 1.5 to (nominally) 26 meters above bottom (m.a.b.). An 8-week-long time series was obtained, showing major sediment transport events (storms) in late December and early January. Comparison of the acoustics measurements from 1.5 m.a.b. are made with optical backscatter system (OBS) concentration estimates lower in the boundary layer (0.25 m.a.b.). Correlations between ABSS and OBS concentration measurements and the boundary layer forcing functions (waves, currents, and their non-linear interaction) provided a variety of insights into the nature of the sediment transport of the STRESS site. Transport rates and integrated transport are seen to be dominated by the largest storm events.

Vertical profiles of near-bottom currents and suspended sediment concentrations were measured on the inner shelf at 20 m depth off the US Army Corps of Engineers Field Research Facility at Duck, NC, U.S.A. over the month of October 1994. The observations embraced an initial period of calm conditions and a prolonged and evolving high-energy event. The data were analyzed to examine the effects of both synoptic and local-scale meteorological events on bed stress and resulting suspended-sediment transport. Four storm phases were identified and related to distinct sets of bottom boundary layer responses: (I) during the initial calm period of low bottom stress, non-diffusive suspended-sediment transport was confined within the thin wave boundary layer; (II) during the onset of the storm, increasingly diffusive suspended-sediment transport and active bedform changes prevailed in association with rapidly increasing wave boundary layer thickness and bed stress; (III) at the peak of the storm, wave boundary layer thickness and bed stress reached their maxima and caused high suspended-sediment transport and sheetflow conditions; and (IV) during the swell-dominated post-storm stage, bedforms developed and suspended-sediment transport was confined within the thick wave boundary layer because of the small current shear. Synoptic-scale winds with local wind perturbation controlled the near-bottom flow during Phase II, whereas local winds dominated during Phase III. Wave-current interactions were most intense during Phase III and were responsible for the high bottom stress.

Time variations in horizontal velocity and suspended concentration can significantly influence the net sediment transport rate in an unsteady boundary layer flow. Here the “current-related” and “wave-related” contributions to the sediment flux are examined for co-linear wave-current flow using a one-equation turbulence-closure model. Two aspects of unsteadiness are investigated by means of a case study. Initially, the phase relationship between the bottom reference concentration and the bed shear stress is shown to influence both the mean (time- and depth-averaged) suspended sediment concentration and also the sediment flux. These quantities are then shown to be influenced also by variations in the value of the ratio (β) of the sediment diffusivity to the eddy viscosity (even at a fixed value of Ws/β, Ws = sediment settling velocity. It is demonstrated that the wave-related contribution may dominate the net sediment flux for grains confined to the oscillatory boundary layer, whereas, if grains are well diffused into the outer current boundary layer region, the wave-related contribution becomes unimportant. More generally, the effect of wave size in relation to the current is investigated, and estimates of the net flux for differing reference concentration phase lags are discussed. The results presented are applicable primarily to sediment transport above plane beds (i.e. sheet flow layers) above which turbulent diffusion is the dominant mechanism of entrainment. However some tentative conclusions are also reached concerning sediment transport above rippled beds.

Measurements of wave heights, near-bed currents, bed shear stresses and suspended-sediment concentrations and fluxes from a severe storm are described. The data are from 25-m water depth on the British North Sea shelf. The current above the wave boundary layer was retarded by bed roughness that was dominated by wakes shed from saltating grains in the bedload layer and by wave-current interaction in the wave boundary layer. The bed roughness increased by two orders of magnitude immediately prior to overturning of the instrument installation by large waves; the cause for the increase is not clear but is consistent with the formation of large-scale bedforms at high transport stage. Backscatter data revealed wave resuspension of bed sediment, modulation of sediment concentration by wave groups, and advection of dense sediment clouds by the current. The vertical profiles of mean suspended-sediment concentration were, nevertheless, consistent with a simple turbulent-diffusion model that incorporates different diffusion scales within and above the wave boundary layer. A single measurement of the sediment reference concentration at the height of the storm yielded an estimate of 0.00025 for the entrainment parameter, γo, but at other times γo was O(10−3). The change in bed roughness might have caused the decrease in entrainment rate at the height of the storm. The erosion depth required to support the suspended-sediment load was a maximum of ∼1 cm, but was typically an order of magnitude smaller than that; suspension was the dominant transport mode. A wind-driven mean flow distorted the tidal ellipse and caused a net transport of sediment along- and off-shore; depth-integrated sediment transport during the storm was up to two orders of magnitude greater than prior to the storm under tidal currents. Wave-orbital currents directly transported a small, but not negligible, fraction of the total sediment load at 34 cm above the bed.

Measurements of velocities and light transmission in the bottom boundary layer on the continental shelf off northern California demonstrate the importance of storms in the transport of sediment along the coast and offshore in this region. Time-series estimates of bottom stress obtained from a combined wave-current bottom boundary layer model in which wave and current measurements from the Geoprobe tripod were used as input show high stress values of 10 dynes cm−2 during two distinct storm events in early February and early March, 1991. These stresses induce significant offshore sediment transport, achieving maximum values of about 0.5 g cm−1 s−1. The net transport over the entire measurement period from 30 January 1991 to 13 March 1991 was along the coast toward the north and offshore. This transport pattern explains slow migration of low amplitude, broad crescentic dunes along and across this portion of the inner continental shelf.

Turbulent boundary shear stress depends on a roughness length scale which characterizes momentum transfer to the seabed. The effective roughness length is due to physical roughness geometry such as sediment ripples, grain size and processes affecting momentum transfer, such as bedload transport and wave motions. During storms on the California shelf, wave motions dominate the turbulent boundary layer, although feedback through wave-induced bed forms and sediment transport are important. Several intense storms on the Northern California Shelf were monitored with pressure sensors, acoustic current meters and an optical backscatter sensor within 5 m of the bed at 90 m depth. Wave spectra, velocity profiles, turbulent kinetic energy and suspended sediment concentrations were obtained. At 90 m depth, waves were measured in the band of 0.05-0.08 Hz, with nearbed wave velocities of 5–30 cm s −1 . In spite of wave-induced currents of up to three times the mean speed, 30 min average velocities yielded typical logarithmic profiles. Roughness, as indicated by the zero intercept of the logarithmic profiles, z 0 , varied by a factor of 25 throughout the storms, with a maximum of 18 cm, when mean currents were above 5 cm s −1 and the wave amplitude was maximal. Such large increases in z 0 are predicted by models of wave-current interaction. Effects of sediment transport and bed forms are not easily extracted from the data during storms. But, the fairly large non-storm values of ∼0.5cm indicate the effect of bed ripples.

Eulerian current measurements made 5–7 m above bottom at six stations along the United States east coast continental margin show a net downslope flow of 1–5 cm s−1. Although the scalar current speed decreases with water depth and toward the bottom, fluctuations in the cross-isobath flow were stronger and increasingly asymmetric near the bottom. Maximum downslope flow exceeded maximum upslope flow by a factor of two to three. The strength of the low-passed downslope flow was proportional to the upslope Reynolds flux of density as well as to the amplitude of the current fluctuations that have periods shorter than 30 h. These flow characteristics may be caused by differential vertical mixing in the bottom boundary layer where a stratified fluid flows upslope (unstable) and downslope (stable). The asymmetry in current strength clearly favors net downslope transport of sediments that move as bedload.