1. Rip currents or undertow in the surf zone? Niels Vinther and Troels Aagaard Institute of Geography, University of Copenhagen, Denmark Annual CoastView-meeting, Bologna, March 2004

2. From the web. Rip currents or undertow Feeder channels Rip head Longshore bar

3. This project Based on data from four existing field experiments: Skallingen 1995, 2000, 2002 and Egmond 2002 Based on data from four existing field experiments: Skallingen 1995, 2000, 2002 and Egmond 2002 Field experiments include measurements of current velocities/directions, sediment concentrations, wave heights/periods and morphological surveys. Field experiments include measurements of current velocities/directions, sediment concentrations, wave heights/periods and morphological surveys. The final aim of this study is to set up a general model based on hydrodynamic and morphological parameters to distinct between 2D and 3D circulation patterns in the surf zone. The final aim of this study is to set up a general model based on hydrodynamic and morphological parameters to distinct between 2D and 3D circulation patterns in the surf zone.

4. Skallingen TR=1.5 m (1.3 and 1.7) H mean =0.5 m D 50 =160-200 β: 0.007 (nearshore zone), 0.02-0.03 (intertidal zone). After Aagaard et al., 1995

5. Egmond After Kroon et al., 2003 TR=1.65 (1.4 and 2.1) H mean = 1.3 m D 50 250-300 µm β: 0.02-0.03 (nearshore zone) and 0.1-0.02 (intertidal zone)

6. Bore-model to predict mass transport across longshore bar C=(gh) 0.5 C=(gh) 0.5 B 0 ~0.1 B 0 ~0.1 A=0.9H 2 A=0.9H 2 H rms =wave height H rms =wave height h=depth h=depth T=waveperiod T=waveperiod Svendsen, 1984

7. Skallingen 1995

8. Skallingen 2002

9. Egmond 2002

10. Mass transport versus trough area/dist to rip – Skallingen 1995

11. Mass transport versus trough area/dist to rip – Skallingen 2002

12. Mass transport versus trough area/dist to rip – Egmond 2002 ?

13. Mass transport versus trough area/dist to rip for all data sets Mistake? Or is it the damning point of using the model globally?

14. Zoom in on mass transport versus trough area/dist to rip Slope of separation line: 1 (m/s)

15. Some questions Is it a globally applicable model? Is it a globally applicable model? How can we improve the model? How can we improve the model? How can we get the required data How can we get the required data Can the model be integrated in the Argus routines? Can the model be integrated in the Argus routines?

16. Some answers We need more data - preferably from other field sites. We need more data - preferably from other field sites. More 3D data (cell circulation). More 3D data (cell circulation). From Argus-images we can get trough area, level of bar crest and distances to rips From Argus-images we can get trough area, level of bar crest and distances to rips From off-shore buoys we can get wave heights/period and water levels From off-shore buoys we can get wave heights/period and water levels

17. Conclusions Surf zone circulation is depended not only on hydrodynamics but also on surf zone bar morphology Surf zone circulation is depended not only on hydrodynamics but also on surf zone bar morphology When currents in the trough exceed 1 m/s, undertow is initiated – and vice versa When currents in the trough exceed 1 m/s, undertow is initiated – and vice versa Deviations at Egmond, may turn down the model as a global model Deviations at Egmond, may turn down the model as a global model The model may be integrated in the Argus- routines The model may be integrated in the Argus- routines

18. Thank you!

20. Extra slides

21. Skallingen field site

22. Egmond field site

23. Instrumentation Current velocity/direction Wave heights Wave periods Morphology

24. Calculation of trough areas SK1995

25. Skallingen 2000

26. Mass transport versus trough area/dist to rip – Skallingen 2000 No positive data