Sand Motion over Vortex Ripples induced by Surface Waves

1. Sand Motion over Vortex Ripples induced by Surface Waves Jebbe J. van der Werf Water Engineering & Management, University of Twente, The Netherlands

2. background experiments flow sand dynamics transport modelling conclusions Outline • Background • Laboratory experiments • Flow over ripples • Sand dynamics over ripples • Practical sand transport modelling • Conclusions & further research

3. surfzone shoreface wave boundary layer background experiments flow sand dynamics transport modelling conclusions Surface waves and oscillatory flow

4. λ η background experiments flow sand dynamics transport modelling conclusions Wave-generated ripples • Cover large part shoreface bed • η = 0.01-0.1 m and λ = 0.1-1.0 m • Vortex shedding if η/λ > 0.1

5. Upper layer: turbulent processes dominant z ≈ 2 η Lower layer: organised convective processes dominant η background experiments flow sand dynamics transport modelling conclusions Sand transport processes over vortex ripples Vortex ripples strongly influence wave boundary layer structure, near-bed turbulence intensity and sand transport mechanisms

6. background experiments flow sand dynamics transport modelling conclusions Ph.D. research • New full-scale laboratory experiments • Improvement ripple predictors • Improvement practical models to predict time-averaged concentration profile • Development new practical sand transport model • Improvement 1DV-RANS sand transport model

7. background experiments flow sand dynamics transport modelling conclusions Experimental facilities • Oscillatory flow tunnels • Flow equivalent to near-bed horizontal flow generated by full-scale surface waves

8. background experiments flow sand dynamics transport modelling conclusions Measurements • Bed elevation using laser displacement sensor • Particle velocities using ultra-sonic velocity profiler and PIV • Net sand transport rates by mass conservation technique using measured masses in traps and volume changes • Suspended sand concentrations

9. Suspended sand concentration measurement • Transverse suction system background experiments flow sand dynamics transport modelling conclusions

10. Suspended sand concentration measurement • Transverse suction system • Optical concentration meter background experiments flow sand dynamics transport modelling conclusions

11. background experiments flow sand dynamics transport modelling conclusions Suspended sand concentration measurement • Transverse suction system • Optical concentration meter • Acoustic backscatter system

12. u onshore time offshore background experiments flow sand dynamics transport modelling conclusions Experimental conditions • Regular and irregular asymmetric flow with T = 5.0-10.0 s and u = 0.4-1.3 m/s • Uniform sand with D50 = 0.22-0.44 mm

13. background experiments flow sand dynamics transport modelling conclusions Instantaneous flow field D50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m

14. background experiments flow sand dynamics transport modelling conclusions Instantaneous flow field D50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m

15. background experiments flow sand dynamics transport modelling conclusions Time-averaged flow field

16. background experiments flow sand dynamics transport modelling conclusions Time- and ripple-averaged flow

17. background experiments flow sand dynamics transport modelling conclusions Instantaneous suspended concentration field D50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m

18. background experiments flow sand dynamics transport modelling conclusions Instantaneous suspended concentration field D50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m

19. background experiments flow sand dynamics transport modelling conclusions Horizontal suspended sand fluxes

20. background experiments flow sand dynamics transport modelling conclusions Horizontal suspended sand fluxes

21. background experiments flow sand dynamics transport modelling conclusions Horizontal suspended sand fluxes

22. background experiments flow sand dynamics transport modelling conclusions Horizontal suspended sand fluxes

23. background experiments flow sand dynamics transport modelling conclusions Horizontal suspended sand fluxes

24. current-related wave-related background experiments flow sand dynamics transport modelling conclusions Horizontal suspended sand fluxes

25. background experiments flow sand dynamics transport modelling conclusions Net horizontal suspended sand fluxes D50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m

26. background experiments flow sand dynamics transport modelling conclusions Bedload transport • Near-bed (mm’s) transport where grain-grain interactions are important • Net bedload in the onshore direction due to flow asymmetry • Forcing mechanism for onshore ripple migration (?)

27. bedload transport dominant suspended load transport dominant background experiments flow sand dynamics transport modelling conclusions Net sand transport rate

28. bedload transport dominant suspended load transport dominant background experiments flow sand dynamics transport modelling conclusions Net sand transport rate

29. background experiments flow sand dynamics transport modelling conclusions Practical sand transport modelling • Implemented in larger morphological modelling systems • Current practical sand transport models • Quasi-steadiness: qs(t) = m |u|n-1 u • <qs> onshore (> 0) for asymmetric oscillatory flows with larger onshore velocities • Not valid in vortex ripple regime where net transport is generally offshore (< 0)

30. onshore flow offshore flow background experiments flow sand dynamics transport modelling conclusions New practical sand transport model • Phase-lag effects schematically included • Four transport contributions F(θ’c,θ’t,P)

31. background experiments flow sand dynamics transport modelling conclusions New practical sand transport model

32. background experiments flow sand dynamics transport modelling conclusions Conclusions • Flow and suspended sand dynamics controlled by vortex generation and ejection • Net sand transport controlled by offshore-directed suspended fluxes and onshore-directed near-bed transport • New practical sand transport model

33. background experiments flow sand dynamics transport modelling conclusions Future research • Comparison detailed data with more sophisticated models, 2DV-RANS models, …? • Development of a general practical model to predict sand transport in coastal waters (Dutch/UK SANTOSS project)