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

2. Outline 1.Background 2.Laboratory experiments 3.Flow over ripples 4.Sand dynamics over ripples 5.Practical sand transport modelling 6.Conclusions & further research backgroundexperimentsflowsand dynamicstransport modellingconclusions

3. Surface waves and oscillatory flow backgroundexperimentsflowsand dynamicstransport modellingconclusions shoreface surf zone wave boundary layer

4. 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 λ η backgroundexperimentsflowsand dynamicstransport modellingconclusions

5. Sand transport processes over vortex ripples Vortex ripples strongly influence wave boundary layer structure, near-bed turbulence intensity and sand transport mechanisms z ≈ 2 η η Lower layer: organised convective processes dominant Upper layer: turbulent processes dominant backgroundexperimentsflowsand dynamicstransport modellingconclusions

6. Ph.D. research 1.New full-scale laboratory experiments 2.Improvement ripple predictors 3.Improvement practical models to predict time-averaged concentration profile 4.Development new practical sand transport model 5.Improvement 1DV-RANS sand transport model backgroundexperimentsflowsand dynamicstransport modellingconclusions

7. Experimental facilities Oscillatory flow tunnels Flow equivalent to near-bed horizontal flow generated by full-scale surface waves backgroundexperimentsflowsand dynamicstransport modellingconclusions

8. 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 backgroundexperimentsflowsand dynamicstransport modellingconclusions

9. Suspended sand concentration measurement Transverse suction system backgroundexperimentsflowsand dynamicstransport modellingconclusions

10. Suspended sand concentration measurement Transverse suction system Optical concentration meter backgroundexperimentsflowsand dynamicstransport modellingconclusions

11. Suspended sand concentration measurement Transverse suction system Optical concentration meter Acoustic backscatter system backgroundexperimentsflowsand dynamicstransport modellingconclusions

12. 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 D 50 = 0.22-0.44 mm time onshore offshore u backgroundexperimentsflowsand dynamicstransport modellingconclusions

13. Instantaneous flow field backgroundexperimentsflowsand dynamicstransport modellingconclusions D 50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m

14. Instantaneous flow field backgroundexperimentsflowsand dynamicstransport modellingconclusions D 50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m

15. Time-averaged flow field backgroundexperimentsflowsand dynamicstransport modellingconclusions

16. Time- and ripple-averaged flow backgroundexperimentsflowsand dynamicstransport modellingconclusions

17. Instantaneous suspended concentration field D 50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m backgroundexperimentsflowsand dynamicstransport modellingconclusions

18. Instantaneous suspended concentration field D 50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m backgroundexperimentsflowsand dynamicstransport modellingconclusions

19. Horizontal suspended sand fluxes backgroundexperimentsflowsand dynamicstransport modellingconclusions

20. Horizontal suspended sand fluxes backgroundexperimentsflowsand dynamicstransport modellingconclusions

21. Horizontal suspended sand fluxes backgroundexperimentsflowsand dynamicstransport modellingconclusions

22. Horizontal suspended sand fluxes backgroundexperimentsflowsand dynamicstransport modellingconclusions

23. Horizontal suspended sand fluxes backgroundexperimentsflowsand dynamicstransport modellingconclusions

24. Horizontal suspended sand fluxes current-relatedwave-related backgroundexperimentsflowsand dynamicstransport modellingconclusions

25. Net horizontal suspended sand fluxes backgroundexperimentsflowsand dynamicstransport modellingconclusions D 50 = 0.44 mm T = 5.0 s η = 0.08 m λ = 0.41 m

26. 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 (?) backgroundexperimentsflowsand dynamicstransport modellingconclusions

27. Net sand transport rate bedload transport dominant suspended load transport dominant backgroundexperimentsflowsand dynamicstransport modellingconclusions

28. Net sand transport rate backgroundexperimentsflowsand dynamicstransport modellingconclusions bedload transport dominant suspended load transport dominant

29. Practical sand transport modelling Implemented in larger morphological modelling systems Current practical sand transport models –Quasi-steadiness: q s (t) = m |u| n-1 u – onshore (> 0) for asymmetric oscillatory flows with larger onshore velocities –Not valid in vortex ripple regime where net transport is generally offshore (< 0) backgroundexperimentsflowsand dynamicstransport modellingconclusions

30. Phase-lag effects schematically included Four transport contributions F(θ’ c,θ’ t,P) New practical sand transport model onshore flowoffshore flow backgroundexperimentsflowsand dynamicstransport modellingconclusions

31. New practical sand transport model backgroundexperimentsflowsand dynamicstransport modellingconclusions

32. Conclusions 1.Flow and suspended sand dynamics controlled by vortex generation and ejection 2.Net sand transport controlled by offshore-directed suspended fluxes and onshore-directed near-bed transport 3.New practical sand transport model backgroundexperimentsflowsand dynamicstransport modellingconclusions

33. 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) backgroundexperimentsflowsand dynamicstransport modellingconclusions