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Abstract: 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–ε 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. [Copyright &y& Elsevier]

For moderate Shields number flows, the characteristic minimum sediment transport prediction error is estimated to be of the order of a factor of 2, even in stationary and horizontally homogeneous flows over a flat bottom. For oscillatory flows over ripples the same error is judged to be comparable to a factor of 4.In actual flows when the flow, sediments and bottom shape have to be predicted, moderate estimation errors for the near bottom turbulent stress results in characteristic minimum sediment transport error that is probably at least of the order of a factor of 10.The equations for the large-scale average flow require nonstandard parameterizations of the smaller scale ripple flow and boundary conditions. Only weakly founded closure relations are available for this. The error of the large-scale sediment transport can therefore not be quantified without extensive data comparisons. As extrapolated from the simpler estimates, it appears plausible that it could be significantly larger than a factor of 10.When the sediment transport variability is large, both in reality and in models, many models can probably be adjusted to fit the few data available. This may not be relevant for judging the prediction accuracy. [Copyright &y& Elsevier]

Field observations of shoreface benthic processes made in the Middle Atlantic Bight off Duck, N.C. and on the Eel River shelf off northern California have yielded several types of evidence that gravity-induced downslope transport of suspended sediment may constitute an important, but episodic, mode of across-shelf transport during storms. Observed manifestations of the inferred gravity flows include side scan images showing abrupt interfaces between fine sediment “blankets” and coarser sands, rapid bed accretion during the waning phases of storms and short-lived periods of accelerated seaward sediment flux following groups of higher waves. To explore the possibility that gravity-driven transport may be episodically significant in energetic shoreface and shelf environments, we applied a relatively simple analytical model that considers the effects of wave and current turbulent friction in modulating the downslope response of turbidity-induced gravity currents as well as in suspending sediment to produce negative buoyancy. The theory was applied to data on benthic flows from three phases of a storm event off Duck in October 1994. Results indicate that the gradient Richardson number, Ri, remained appreciably below the critical value of 0.25 during average conditions and during high wave groups. During lulls, however, Ri briefly became supercritical within a few tens of centimeters above the bed implying short-lived turbulence suppression. On the Eel River shelf, direct observations show that, within the thin ((O)10 cm) muddy layer, Ri values also fluctuated between subcritical (<1/4) during groups and supercritical (>1/4) during lulls again implying that stratification suppressed production of turbulence during lulls. Downslope transport was observed during both high and low wave packets but accelerated during lulls as in the Duck case. We conclude that gravity-induced downslope transport of sediment suspended by waves is likely to be an important mode of across-shelf transport on the inner shelf of energetic sandy environments and the mid-shelf of energetic muddy environments. Such transports are most likely pulsational, with high turbidities and buoyancy anomalies being created by the high stresses associated with groups of large waves and moderately rapid but brief downslope movement occurring when bed stresses are temporarily relaxed during the lulls between high groups. This mechanism operates out of phase with group-forced long waves. [Copyright &y& Elsevier]