Presentation by Camille Grimaldi, University of Western Australia, Australia, at the Delft3D - User Days (Day 2: Hydrodynamics), during Delft Software Days - Edition 2018. Tuesday, 13 November 2018, Delft.

1. D R I V E R S O F C O R A L R E E F
H Y D R O D Y N A M I C S A T T H E
R O W L E Y S H O A L S , W A U S I N G
D E L F T 3 D F M
Camille Grimaldi, Ryan Lowe, Rebecca Green and Jessica Benthuysen
Photo- Sharyn Hickey

2. Coral reef hydrodynamics
NOAA
• Retention/circulation of water and
material in the reef (e.g. heat, nutrient,
organisms)
• Flushing time and residence time
• Affect most of the biological and chemical
processes on the reef
INTRODUCTION

3. 3rd Global-scale coral bleaching
Benthuysen et al., 2018
Temperature anomaly
INTRODUCTION

4. 2015/2016 - 3rd global-scale coral bleaching
NOAA
Northwestern Australia
Coral cover and species diversity dramatically reduced (Gilmour et al., 2018)
INTRODUCTION

5. Only minor (<10%) coral bleaching observed (Gilmour et al., 2018)
INTRODUCTION
2015/2016 - 3rd global-scale coral bleaching
Northwestern Australia

6. Atmospheric and oceanographic conditions influence the reef variability.
INTRODUCTION
2015/2016 - 3rd global-scale coral bleaching
Northwestern Australia

7. What drivers of hydrodynamics on reef systems?
INTRODUCTION
2015/2016 - 3rd global-scale coral bleaching
Northwestern Australia

8. Rowley Shoals
- Pristine environment
- 3 atolls (=ring-shaped coral reef
including a lagoon)
- Channel on the NE side
INTRODUCTION
Imperieuse
Clerke
Mermaid

9. Photo : Lady Cruises
Rowley Shoals
INTRODUCTION
Imperieuse
Clerke
Mermaid

10. Mermaid Reef
INTRODUCTION
Mermaid reef
Imperieuse
Clerke
Mermaid

11. Coral reef systems’ hydrodynamic drivers
Lowe and Falter 2015
CONCEPTS
• Waves
~ 2/3 of the world’s reefs are wave-dominated
Significant wave height (Hs)
>1.5 m

12. Coral reef systems’ hydrodynamic drivers
INTRODUCTION
OutflowInflow
Top view
Reef channel
inflow
Wave
setup
1
inflow
2
outlfow
3
Forereef
Reef
flat
Lagoon
Cross section of an atoll
• Waves
CONCEPTS

13. ~ 1/3 of the world’s reefs are tide-dominated
Coral reef systems’ hydrodynamic drivers
• Tides
Mean tidal range (MTR)
Significant wave height (Hs)
> 1
CONCEPTS
Lowe and Falter 2015

14. Coral reef systems’ hydrodynamic drivers
• Tides OutflowInflow
Top view
Reef channel
inflow
1
outflow
2
Forereef
Reef
flat
Lagoon
Risingtide
inflow
Fallingtide
Cross section of an
atoll
CONCEPTS

15. Global models estimates:
- Mean Tidal Range = 2.4 m
- Signif. Wave Height = 1.4 m
(Egbert & Erofeeva, 2002; Lowe and Falter 2015)
NTRODUCTION RESULTS & DISC. CONCLUSIONCONCEPTS METHODS
Rowley Shoals - hydrodynamic
MTR / Hs > 1
Hs ~ 1.5m
CONCEPTS
Influenced by both waves and tides?

16. NTRODUCTION RESULTS & DISC. CONCLUSIONCONCEPTS METHODS
Waves and tides dominated atolls
CONCEPTS
Costa et al., 2018

17. Numerical simulations vs In situ observations
METHODS

18. RESULTS & DISC. CONCLUSIONCONCEPTS METHODS
Numerical simulations - Delft3D Flexible Mesh
METHODS
• Spatially-complex bathymetry
• Influence of range of hydrodyanmic processes
• Interest in temperature and water quality
applications
Challenges Delft3D Flexible Mesh
• Flexible Mesh Grid
• Coupling D-Flow FM and D-Waves
• Residence time, heat exchanges, water quality, etc

19. RESULTS & DISC. CONCLUSIONCONCEPTS METHODS
D-Flow FM – Preliminary simulations
Input parameters
METHODS

20. RESULTS & DISC. CONCLUSIONCONCEPTS METHODS
Input parameters
• Flexible Mesh Grid
Inside reef: 50x50m
Outside reef 100x100m
D-Flow FM – Preliminary simulations
METHODS

21. RESULTS & DISC. CONCLUSIONCONCEPTS METHODS
Input parameters
• LiDAR Bathymetry
30m resolution
Triangular interpolation
D-Flow FM – Preliminary simulations
METHODS

22. RESULTS & DISC. CONCLUSIONCONCEPTS METHODS
Input parameters
• Boundary conditions
Tidal constituents
TPXO 8.0
D-Flow FM – Preliminary simulations
METHODS

23. RESULTS & DISC. CONCLUSIONCONCEPTS METHODS
D-Flow FM – Preliminary simulations
METHODS

24. Nov. 2017 - October 2018 : FULL year of in situ observations
In situ observations
AWAC retrieval
METHODS

25. Open Ocean
Channel
Lagoon
Reef flat
Current sensors
In situ observations
Water levels and current speed
and directions at 4 key locations
METHODS
Pressure sensors
Water levels and significant wave
height across the reef

26. PRELIMINARY RESULTS
Water levels – Numerical simulations

27. Water levels – Numerical simulations
PRELIMINARY RESULTS

28. Effect of atoll morphology
Water levels – in situ observations
PRELIMINARY RESULTS

29. INTRODUCTION
Water levels – Numerical simulations vs observations
PRELIMINARY RESULTS

30. Currents - numerical modeling
Channel
Lagoon
Reef flat
INTRODUCTION PRELIMINARY ESULTS

31. Currents - numerical modeling
PRELIMINARY RESULTS

32. CONCLUSIONCONCEPTS METHODS RESULTS
Currents - In situ observations vs Numerical simulations
10 days
4 days
Completely tidally driven
PRELIMINARY RESULTS

33. DISCUSSION CONCLUSIONCONCEPTS METHODS
Potential wave forcing
DISCUSSION

34. Mermaid reef
Potential wave forcing
DISCUSSION
Wave influence on the Western Side

35. Hs from the western side
Potential wave forcing
DISCUSSION

36. Conclusions and next steps
- Good agreement for water level between model
and observations
- Good agreement for current between model and
observations in the channel
- Tides are not enough to explain the entire
hydrodynamics
- Mermaid reef influenced by both waves and tides
Mermaid reef
Tides
Waves
CONCLUSION

37. Conclusions and next steps
Next steps :
- Drivers of reef hydrodynamics :
- Incorporate D-Waves
- Couple D-Flow FM and D-Waves
- Determine residence time
- Test different grids and sensitivity analysis
- Regionnal hydrodynamics
- Temperature variability simulations
- Water quality (carbonate chemistry) simulations
CONCLUSION

38. Aknowledgements
- Ryan Lowe, Rebecca Green, Jessica Benthuysen
and Carlin Bowyer
- University of Western Australia, Center of
Excellence for Coral reef studies and Australian
Institute of Marine Science
- Deltares
CONCLUSION

39. Questions?
CONCLUSION