Jan S. Ribberink

Modelling of Sand Transport with Graded Sands under Oscillatory Sheet-Flows

The present study focuses at improving our basic understanding of size- graded sand transport mechanisms under oscillatory sheet-flow conditions using different transport models. Moreover, the study aims at improving the performance of different transport model concepts for the description of both the rate and size- composition of the transported sand. Quasi-steady and intermediate models are investigated, in combination with a large dataset of oscillatory sediment transport measurements (sheet-flow conditions) as collected from two oscillatory water tunnels. A comparison is made between the measured net transport rates and transport rates per size fraction and the results from various transport formulas (Bailard 1981; Ribberink 1998; Dibajnia & Watanabe 1996; Dohmen-Janssen et al., 2002). First, the comparison is based on a 1-fraction approach i.e. the sand is modeled using D50 of the mixture. Second, a size-fraction approach is tested and developed further using the experimental dataset. Exposure corrections are proposed for fine and coarse size fractions in the sand mixture to improve model performance.

SUSPENDED SEDIMENT DYNAMICS OVER FULL-SCALE RIPPLES IN OSCILLATORY FLOW

A series of prototype-scale laboratory experiments was performed in the Aberdeen Oscillatory Flow Tunnel to investigate the intra-wave sediment transport processes in the ripple regime. Detailed measurements of velocities and concentrations were carried out for three different regular oscillatory flow conditions. It is found that the time- and bed-averaged concentrations fit an exponential profile reasonably well, and that the concentration decay length is strongly related to the ripple height. The intra- wave concentration field is dominated by the formation and ejection of vortices on the onshore (in the direction of flow asymmetry) ripple flank. In a lower layer of about two ripple heights, the net current-related flux and net wave-related flux are in the offshore direction and the wave-related flux dominates. Higher up in the flow the net wave-related flux is still offshore- directed, but the net current-related flux is onshore-directed and is dominant, resulting in an onshore total net flux.

PIV MEASUREMENTS OF VELOCITIES OVER FULL-SCALE RIPPLES IN OSCILLATORY FLOW

A cross-correlation particle image velocimetry (PIV) method has been used to obtain detailed velocity measurements of flow over ripples in the Aberdeen Oscillatory Flow Tunnel. The new data is distinguished from previous work of this kind in that the measurements were made over full-scale, mobile ripples. A total of 14 velocity measurement experiments were carried out, covering a wide range of flow and mobility conditions and ripples with dimensions in the range 2.5 < η < 10 cm and 18.5 < λ < 55 cm. The paper presents example results from the measurements, illustrating the quality of the data and some important features of the flow behaviour. Attention is given to velocities near the ripple crest, velocities near the ripple surface, the time-averaged velocity field and the time- and horizontally-averaged velocity profiles.

Sheet flow and suspension under wave groups in a large wave flume (SISTEX99)

Detailed measurements of sediment concentrations in the sheet flow layer and in the suspension layer for flat bed conditions under prototype wave groups are presented. These measurements are part of experiments on near-bed sand transport processes, carried out in the Large Wave Flume in Hannover, Germany (SISTEX99). The results show that concentrations in the sheet flow layer are highly coherent with the instantaneous near-bed velocity. In the suspension layer, however, concentrations respond much slower to changes in near-bed velocity: at some distance from the bed concentrations increase and decrease on the time scale of the wave group, with a time delay relative to the peak wave within the wave group.