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The processes of sediment entrainment, transport, and deposition over bedforms are highly dynamic and temporally and spatially variable. Obtaining measurements to understand these processes has led to ongoing developments in instrumentation for studying near-bed sediment dynamics, with the outputs applied to the development and assessment of sediment transport modelling. In the present study results are reported from three acoustic systems deployed to make observations of bedforms, bedload, suspended concentration and horizontal and vertical velocity components. To evaluate the instruments a series of near-bed boundary layer measurements were collected in a large scale flume facility over a rippled bed of medium sand under regular waves. The observations were conducted as part of Joint Research Activities within the EU funded Hydralab project. The suite of acoustic instruments consisted of a Bedform And Suspended Sediment Imager, BASSI, a three dimensional acoustic ripple profiler, 3D-ARP, and three Acoustic Concentration and Velocity Profilers, ACVP's. Here results are reported from the deployment of the instruments, to illustrate the ongoing developments in acoustics and the expanding capability of the application of acoustics to the study of near-bed flow and sediment dynamics. [ABSTRACT FROM AUTHOR]

Abstract: Sediment transport models require appropriate representation of near-bed processes. We aim here to explore the parameterizations of bed shear stress, bed load transport rate and near-bed sediment erosion rate under the sheet flow regime. To that end, we employ a one-dimensional two-phase sheet flow model which is able to resolve the intrawave boundary layer and sediment dynamics at a length scale on the order of the sediment grain. We have conducted 79 numerical simulations to cover a range of collinear wave and current conditions and sediment diameters in the range 210–460 . The numerical results confirm that the intrawave bed shear stress leads the free stream velocity, and we assess an explicit expression relating the phase lead to the maximum velocity, wave period and bed roughness. The numerical sheet flow model is also used to provide estimates for the bed load transport rate and to inspect the near-bed sediment erosion. A common bed load transport rate formulation and two typical reference concentration approaches are assessed. A dependence of the bed load transport rate on the sediment grain diameter is observed and parameterized. Finally, the intrawave near-bed vertical sediment flux is further investigated and related to the time derivative of the bed shear stress. [Copyright &y& Elsevier]

Abstract: Laboratory data of free surface elevation and fluid velocities (cross-shore and vertical velocities) were obtained using a laser-Doppler velocimeter (LDV) for the case of a periodic wave plunging over an impermeable, steep (1:10) slope with a fixed bottom roughness (nominal diameter=2.2mm). The measurements were conducted over 14 cross-shore locations from the outer surf zone to the swash zone with approximately 10 vertical points at each location. The vertical resolution included measurements in the bottom boundary layer and some points above the trough level. Various hydrodynamic quantities, including ensemble-averaged turbulent kinetic energy (TKE) (〈k〉) and turbulence intensity (〈u′2〉 and 〈w′2〉), were estimated to better understand inner surf and swash zone hydrodynamics induced by strongly plunging waves. In the outer region of the bottom boundary layer, the ratio of the time-averaged horizontal turbulence intensity to the time-averaged vertical turbulence intensity agreed well with the case of plane wake turbulence . The average ratio near the bottom was larger which is closer to flows typified by the inner region of a boundary layer . In the surf zone, the ensemble-averaged horizontal fluid velocity (〈u〉) near the bottom leads in phase compared to 〈u〉 in the upper layer, and 〈u〉 at the bore front exceeds the theoretical wave celerity. At the impinge point, a strong return flow occurred which was greater in magnitude than the onshore directed velocity . In the surf zone, 〈k〉 was largest just below trough level with a forward shift in phase of the peak intensity. The turbulence energy was mostly dissipated after the passage of the crest at this location. 〈u〉 near the bottom was leading in phase with a sharp vertical gradient in 〈u〉 indicating that boundary layer processes may have been important. In the swash zone, the vertical gradient of 〈u〉 was relatively small compared to the vertical gradient of 〈u〉 in the surf zone and may be due to the effect that the strong downrush had on turbulent mixing. Finally, the time-averaged estimate of TKE was vertically uniform over the inner surf and swash zone. [Copyright &y& Elsevier]

A process-based sediment transport model is presented that combines the knowledge of sediment transport in rivers and estuaries with the understanding of physical processes specific for the shelf seas. Due to formation of bottom Ekman spiral, the near-bottom current and hence the sediment transport deviate from the direction of surface current. The model takes into account velocity veering induced by Ekman spiral and estimates both direction and rate of transport of suspended particulate matter (SPM) generated by a steady or slowly varying current, through suspension, relocation and deposition of sediment in a shallow sea. The model uses a combination of analytical and numerical methods: analytical integration, in the vertical, and numerical integration, in the horizontal. The model predicts also vertical erosion/deposition fluxes at the seabed. The deviation angles and other parameters of SPM transport are computed for a range of water depths 5–50 m, particle settling velocities 0.1–6 cm/s, and current speeds 0.4–1.2 m/s. The model converges automatically to traditional engineering-style formulations, in extreme case of strong current in very shallow water, where velocity veering is of minor importance. [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]