Presentation by Damien Sous (Université De Toulon) at the XBeach X (10th Year Anniversary) Conference, during Delft Software Days - Edition 2017. Thursday, 2 November 2017, Delft.

1. Wave-induced barrier reef currents:
XBeach vs Field
Damien Sous
Mediterranean Institute of Oceanography
XBX - Nov. 2017 - Delft

2. Introduction
Increasing interest on reef environments
Major role in tropical ecological systems (nursery, nearshore
protection)
Threatened by climate change and anthropic pressure
Most studies on fringing reefs and wave dynamics (Taebi et al. 2011,
Pomeroy et al. 2012, Van Dongeren et al. 2013, Gawehn et al. 2016, ...)
Present study: Focus on barrier reefs
Strong hydrodynamical interactions with lagoon
Wave-induced currents over the reef: Control of the exchanges
between lagoon and open ocean (residence time)
Increased effect in channel lagoons

3. Field site: Ouano lagoon

4. Field site: Ouano lagoon

5. Field site: Ouano lagoon

6. Field site: Ouano lagoon

7. Circulation patterns

8. CROSS-REEF project
Importance of the Cross-Reef fluxes on the lagoon dynamics
C.R. fluxes controlled by wave forcing and tide level
Affect the lagoon circulation and water renewal time

9. CROSS-REEF project
Importance of the Cross-Reef fluxes on the lagoon dynamics
C.R. fluxes controlled by wave forcing and tide level
Affect the lagoon circulation and water renewal time
How to implement C.R. fluxes in coastal circulation model ?
Empirical parameterization (Chevalier et al. 2015)

10. CROCO modelling
Chevalier et al., Ocean Dynamics 2017

11. CROSS-REEF project
Importance of the Cross-Reef fluxes on the lagoon dynamics
C.R. fluxes controlled by wave forcing and tide level
Affects the lagon circulation and water renewal time
How to implement C.R. fluxes in coastal circulation model ?
Empirical parameterization (Chevalier et al. 2015)
Analytical models (Symonds et al. (1995), Hearn (1999), Gourlay and Colleter
(2005), Bonneton et al. (2007), Lowe et al. (2009))

12. CROCO modelling
Lowe et al., JPO 2009

13. CROSS-REEF project
Importance of the Cross-Reef fluxes on the lagoon dynamics
C.R. fluxes controlled by wave forcing and tide level
Affects the lagon circulation and water renewal time
How to implement C.R. fluxes in coastal circulation model ?
Empirical parameterization (Chevalier et al. 2015)
Analytical 1D models (Symonds et al. (1995), Hearn (1999), Gourlay and Colleter
(2005), Bonneton et al. (2007))
X-Beach ?

14. Is XBeach able to simulate the wave-induced cross-reef currents ?

15. CROSS-REEF project
New field campaign 2016

16. CROSS-REEF project
New field campaign 2016
1 Complete reef barrier bathymetric survey

17. CROSS-REEF project
New field campaign 2016
1 Complete reef barrier bathymetric survey
2 High resolution velocity profiles => friction processes

18. CROSS-REEF project
New field campaign 2016
1 Complete reef barrier bathymetric survey
2 High resolution velocity profiles => friction processes
3 Long term current measurements => dataset for Xbeach
benchmark

19. CROSS-REEF project

20. Friction over corals reefs ?

21. Friction over corals reefs ?
Depends on coral colony and depth
Depth ≈ coral height, flow through canopy (Rosman and Hench 2011)
Depth > coral height, canonical turbulent log-layer (Nepf and Vivoni,
2000, Pomeroy et al. 2012)
Cf = (
κ
(1 + z0
h
)ln( h
z0
+ 1) − 1)
)1/2
(1)

22. Friction over corals reefs ?
Depends on coral colony and depth
Depth ≈ coral height, flow through canopy (Rosman and Hench 2011)
Depth > coral height, canonical turbulent log-layer (Nepf and Vivoni,
2000, Pomeroy et al. 2012)
Cf = (
κ
(1 + z0
h
)ln( h
z0
+ 1) − 1)
)1/2
(1)
Friction coefficients
Red sea measurements Cf=0.009-0.015 (Reidenbach et al. 2006)
Analytical model Cf=0.03-0.1 (Hearn et al. 1999)
IG wave field study Cf=0.06 (Pomeroy et al. 2012)
IG-tides field study CF=0.06-0.2 (Lugo-Fernandez et al. 1998)

23. Extracting friction from 3 weeks field
data

24. Extracting friction from 3 weeks field
data
Comparison of 4 methods
Log-layer method
U
u
=
1
κ
ln(
z
z0
) (2)
Correlation method
u2
= − < u w > (3)
Direct dissipation method
u = ( κz)1/3
(4)
Sww = 0.34 2/3
k−5/3
(5)
Balance dissipation method
= − < u w >
∂U
∂z
(6)

25. Friction coefficient

26. The X-Beach model
Model settings
761 2m cells
Hydrostatic, stationary
48 cases: Hs =0.2, 0.7, 1.2, 1.8, 2.5 and 4m, Depth=0.1-1.5m above
reef top
Wave friction fw = 0.6 (Van Dongeren et al. 2013)

27. XBeach simulations
Fixed drag coefficient

28. XBeach simulations
Variable drag coefficient

29. Preliminary conclusions
Consistent friction measurements over the reef flat
Confident in the XB capacity to simulate cross-reef currents in most
conditions
Prospects
Understand the turbulence dissipation/production balance
Take a closer look at extreme conditions
Study obliquely incident waves
2D modelling of the whole reef barrier
Waves...

30. Locatelli et al. 2017, In Coastal Dynamics 2017

31. Thanks !