Surface wave effects on sheet-flow sand transport Jolanthe JLM Schretlen 1 , Jebbe J van der Werf 1 , Jan S Ribberink 1 , Rob E Uittenbogaard 2 , Tom O’Donoghue 3 1 Water Engineering & Management, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands 2 WL|Delft Hydraulics, P.O.Box 177, 2600 MH, Delft, The Netherlands 3 University of Aberdeen, Dept. Engineering, King’s College, Aberdeen AB24 3UE, UK 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). The National Institute for Coastal and Marine Management (RWS RIKZ) is acknowledged for the funding of the Delta flume experiments. In addition, the authors highly appreciate the collaboration with Utrecht University. 1. Introduction Under storm conditions, the majority of sand on the upper shore-face is transported in the sheet-flow layer, only a few cm’s thick. Existing sand transport models, used for these conditions, are primarily based on data from oscillatory flow tunnel experiments. This poster addresses the importance of surface wave effects on sheet-flow sand transport, which are not fully reproduced in tunnels. 2. Experimental set-up The Figure above shows the waves simulated in the new Delta Flume experiments (left); regular, 2 nd order Stokes waves with H = 1 m and T = 6.5 s. The water depth is 1.25 m and the D 50 of the bed’s sediment is 250 μm. The right picture shows the measurement frame, specifically designed for these measurements, with the used instruments. 3. Experimental results The above Figure shows the results of detailed velocity measurements near the bed (measured with UVP). (a): Time- dependent horizontal velocities, measured at z = -3.5, -1, 0, 3, 6, 9, 12, 16, 26, 36 and 56 mm in reference to the still bed- level, (b): Velocity profiles of the same measurement, showing 40 phases during one wave. 4. Comparison of the new Delta Flume experiments and oscillatory flow tunnel experiments In this comparison, the left Graph shows both the time-averaged velocities measured in the Delta Flume (solid line) and in an oscillatory flow tunnel (dashed line) (Wright and O’Donoghue, 2002). Differences between the two are caused by the fact that in tunnel conditions vertical orbital motions are absent and wave-induced net-currents (e.g. Longuet-Higgins streaming) are not fully reproduced. The centre box shows the model of O’Donoghue and Wright (2004a) to calculate the time-dependent concentration profile in the sheet-flow layer and the expression of Ribberink et al. (2007) to determine erosion depth. Applying these to the experimental data and multiplying this with the measured net velocities, results in the time-averaged flux profiles shown in the right Graph. Again the solid line represents results from the new Delta flume experiments and the dashed line oscillatory flow tunnel experiments. 5. Results from process-based modelling Simulations with a 1DV RANS transport model, the Point Sand Model (PSM), (Uittenbogaard, 2000) also give a net velocity profile with an onshore maximum in ‘real wave mode’ (solid line) and an offshore maximum for ‘wave tunnel mode’ (dashed line) (see adjacent Figure). When combined with a mean concentration profile, the PSM gives current-related sediment fluxes with the same variation in direction as shown in the experimental results. Apart from this, the PSM also takes the wave-related transport component into account. 6. Conclusions and recommendations The detailed time-dependent velocities under full-scale surface waves are measured here for the first time. The differences between these results and previous oscillatory flow tunnel experiments occur mainly in the first few mm’s to cm’s above the bed. Under sheet- flow conditions, the far majority of the sand is transported in this layer directly above the bed. Therefore, it can be stated that these differences are of importance for the net (current-related) sand transport under sheet-flow conditions. This is supported by the process-based 1DV Point Sand Model. The contribution of this current-related transport relative to wave-related transport is expected to vary for different hydrodynamic conditions and will be investigated in a later stage of this research project. References O’Donoghue, T. & Wright, S., 2004a. Concentrations in oscillatory sheet flow for well sorted and graded sands. Coastal Engineering, 50: 117 - 138. Ribberink, J.S., Van der Werf, J.J. & O’Donoghue, T., 2007. Sand motion induced by oscillatory flows: sheet flow and vortex ripples. Submitted to Journal of Turbulence, special issue about ‘Particle-laden flow, from geophysical to Kolmogorov scales’ (Euromech colloquim 477, Enschede, The Netherlands, 2006). Wright, S. and O'Donoghue, T., 2002. Total sediment transport rate predictions in wave current sheet flow with graded sand. Oscillatory flow tunnel experiments at Aberdeen University. Experimental report EPSRC "LUBA" Project., University of Aberdeen, Aberdeen, UK Uittenbogaard, R.E., 1DV simulation of wave current interaction. Proc. 27 th Int. Conf. Coast. Eng., Sydney, Australia, 255-268, 2000. Email: J.L.M. [email protected] () 50 1 1 1 1 1 1 0 D e piv z e z piv C C z C e βθ δ α δ δ = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + + ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − + = X
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Surface wave effects on sheet-flow sand transport 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
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). The National Institute for Coastal and Marine Management (RWS RIKZ) is acknowledged for the funding of the Delta flume experiments. In addition, the authors highly appreciate the collaboration with Utrecht University.
1. Introduction Under storm conditions, the majority of sand on the upper shore-face is transported in the sheet-flow layer, only a few cm’s thick.
Existing sand transport models, used for these conditions, are primarily based on data from oscillatory flow tunnel experiments. This poster addresses the importance of surface wave effects on sheet-flow sand transport, which are not fully reproduced in tunnels.
2. Experimental set-up
The Figure above shows the waves simulated in the new
Delta Flume experiments (left); regular, 2nd order Stokes waves with H = 1 m and T = 6.5 s. The water depth is 1.25 m and the D50 of the bed’s sediment is 250 μm. The right picture shows the measurement frame, specifically designed for these measurements, with the used instruments.
3. Experimental results
The above Figure shows the results of detailed velocity measurements near the bed (measured with UVP). (a): Time-dependent horizontal velocities, measured at z = -3.5, -1, 0, 3, 6, 9, 12, 16, 26, 36 and 56 mm in reference to the still bed-level, (b): Velocity profiles of the same measurement, showing 40 phases during one wave.
4. Comparison of the new Delta Flume experiments and oscillatory flow tunnel experiments
In this comparison, the left Graph shows both the time-averaged velocities measured in the Delta Flume (solid line) and in an
oscillatory flow tunnel (dashed line) (Wright and O’Donoghue, 2002). Differences between the two are caused by the fact that in tunnel conditions vertical orbital motions are absent and wave-induced net-currents (e.g. Longuet-Higgins streaming) are not fully reproduced. The centre box shows the model of O’Donoghue and Wright (2004a) to calculate the time-dependent concentration profile in the sheet-flow layer and the expression of Ribberink et al. (2007) to determine erosion depth. Applying these to the experimental data and multiplying this with the measured net velocities, results in the time-averaged flux profiles shown in the right Graph. Again the solid line represents results from the new Delta flume experiments and the dashed line oscillatory flow tunnel experiments.
5. Results from process-based modelling
Simulations with a 1DV RANS transport model, the Point Sand Model (PSM), (Uittenbogaard, 2000) also give a net velocity profile with an onshore maximum in ‘real wave mode’ (solid line) and an offshore maximum for ‘wave tunnel mode’ (dashed line) (see adjacent Figure). When combined with a mean concentration profile, the PSM gives current-related sediment fluxes with the same variation in direction as shown in the experimental results. Apart from this, the PSM also takes the wave-related transport component into account.
6. Conclusions and recommendations
The detailed time-dependent velocities under full-scale surface waves are measured here for the first time. The differences between these results and previous oscillatory flow tunnel experiments occur mainly in the first few mm’s to cm’s above the bed. Under sheet-flow conditions, the far majority of the sand is transported in this layer directly above the bed. Therefore, it can be stated that these differences are of importance for the net (current-related) sand transport under sheet-flow conditions. This is supported by the process-based 1DV Point Sand Model. The contribution of this current-related transport relative to wave-related transport is expected to vary for different hydrodynamic conditions and will be investigated in a later stage of this research project.
References O’Donoghue, T. & Wright, S., 2004a. Concentrations in oscillatory sheet flow for well sorted and graded sands. Coastal Engineering, 50: 117 - 138. Ribberink, J.S., Van der Werf, J.J. & O’Donoghue, T., 2007. Sand motion induced by oscillatory flows: sheet flow and vortex ripples. Submitted to Journal of Turbulence, special
issue about ‘Particle-laden flow, from geophysical to Kolmogorov scales’ (Euromech colloquim 477, Enschede, The Netherlands, 2006). Wright, S. and O'Donoghue, T., 2002. Total sediment transport rate predictions in wave current sheet flow with graded sand. Oscillatory flow tunnel experiments at Aberdeen
University. Experimental report EPSRC "LUBA" Project., University of Aberdeen, Aberdeen, UK Uittenbogaard, R.E., 1DV simulation of wave current interaction. Proc. 27th Int. Conf. Coast. Eng., Sydney, Australia, 255-268, 2000. Email: J.L.M. [email protected]
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