Cross-shore sand transport under non-breaking waves in sheet flow conditions

Jolanthe JLM Schretlen1, Jebbe J van der Werf1, Jan S Ribberink1, Rob E Uittenbogaard2, Tom O’Donoghue3

1Water Engineering & Management, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands 2WL|Delft Hydraulics, P.O.Box 177, 2600 MH, Delft, The Netherlands

3University of Aberdeen, Dept. Engineering, King’s College, Aberdeen AB24 3UE, UK

1. Introduction In wave-induced sheet flow conditions the majority of the

sand transport takes place in a layer of a few centimetres thick, close to the bed. Due to their presence inside the sheet flow layer, relative small wave-induced net currents are potentially of large importance for the net sand transport rate.

Sand transport models for the sheet flow regime are largely based on flow tunnel experiments, where the above mentioned currents are not fully reproduced. This present study is focused on these wave-induced net currents and their importance for the sheet flow sand transport.

2. Background

Figure 1 shows transport rates measured in 4 different experiments with asymmetric waves in a large wave flume (Ribberink et al., 2000), being a factor 2 larger than measurements under similar flow and sediment conditions in a large oscillating wave tunnel (Ribberink & Al-Salem, 1994).

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Acknowledgements This work is carried out within the project SANTOSS, a collaboration between Twente University and Aberdeen University, with input from both Liverpool and Wales-Bangor University and WL|Delft Hydraulics. It is funded by the Dutch Technology Foundation STW, applied science division of NWO and the technology program of the Ministry of Economic Affairs (TCB 6586) and the UK’s Engineering and Physical Sciences Research Council (EPSRC) (GR/T28089/01).

Figure 1: Measured net transport rates <qs> against flow velocity moment <U3> in a large wave flume and in a large flow tunnel.

3. New experiments under surface waves To be able to understand and quantify the effects of surface

waves on sheet-flow processes and transport, new experiments are conducted in a large wave flume. Figure 2 shows the measurement set-up of these new experiments.

Figure 3 shows preliminary, phase-averaged results of both

the vertical and horizontal velocities as the surface level elevation of the water. The measured concentration shows two peaks, of which the first one results from the maximum on-shore velocity, the second one occurs around the moment of flow reversal. This seems to coincide with the findings of e.g. Hassan (2005).

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Figure 3: Flow velocities and concentrations of regular 2nd order Stokes waves, with a height of 1,0 m and period of 6,5 s. Water depth is 2,25 m, velocities measured at 1 m above the sand bed and sediment concentration measured at 3 cm above the bed.

4. Process - Based Modelling

First experiences with the

wave-current interaction Point Sand Model of Uittenbogaard (2000) indicate that the predicted sand transport rate under real waves is 40% higher than in oscillatory flows (Bosboom & Klopman, 2000).

Figure 4: Comparison by PSM of mean velocity profile for both water tunnel (solid line) and surface wave (dashed line) situations. The dots represent tunnel data (Taken from: Bosboom & Klopman, 2000).

5. Future research

New experiments in a large wave channel (GWK, Hannover) and simulations with the 1DV transport model will provide further insight in the near bed flow and sand transport processes. The influence of adequate modelling of vertical orbital flows and wave-induced net currents will be clarified and recommendations will be made how to improve practical sand transport models for these ‘real wave effects’.

As part of the SANTOSS-project this research will contribute to the development of a new practical sand transport formula for sheet flow conditions.

References Bosboom, J. & G. Klopman, Intra-wave sediment transport modelling.

Proc. 27th Int. Conf. Coast. Eng., Sydney, Australia, 2453-2466, 2000.

Hassan, W.N. & J.S. Ribberink, Transport processes of uniform and mixed sands in oscillatory sheet flow. J. Coast. Eng., 52: 745–770, 2005.

Figure 2: Measurement set-up. Left: Regular waves in Delta Flume, with horizontal velocities up to 1.5 m/s. Right: Instrument set-up which can be positioned with sub-mm accuracy in relation to the sand bed.

Ribberink, J.S. & A.A. Al-Salem, Sediment transport in oscillatory boundary layers in case of rippled beds and sheet flow. J. Geophys. Res., 99 (C6): 12707-12727, 1994.

Ribberink, J.S., C.M. Dohmen-Janssen, D.M. Hanes, S.R. McLean & C.E. Vincent, Near-bed sand transport mechanisms under waves, a large wave flume experiment (Sistex99). Proc. 27th Int. Conf. Coast. Eng., Sydney, Australia, 3263-3275, 2000.

Uittenbogaard, R.E., 1DV simulation of wave current interaction. Proc. 27th Int. Conf. Coast. Eng., Sydney, Australia, 255-268, 2000.

Email: J.L.M. Schretlen@ctw.utwente.nl

Cross-shore sand transport under non-breaking waves in sheet flow conditions

Jolanthe JLM Schretlen1, Jebbe J van der Werf1, Jan S Ribberink1, Rob E Uittenbogaard2, Tom O’Donoghue3

1Water Engineering & Management, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands 2WL|Delft Hydraulics, P.O.Box 177, 2600 MH, Delft, The Netherlands

3University of Aberdeen, Dept. Engineering, King’s College, Aberdeen AB24 3UE, UK