Tidal modulation of wave-setup and wave-induced

currents on the Aboré coral reef, New Caledonia

Philippe BONNETON, Jean-Pierre LEFEBVRE, Patrice BRETEL,

Sylvain OUILLON, Pascal DOUILLET

Department of Geology and Oceanography, UMR EPOC, CNRS,

University of Bordeaux 1, France

Institut de Recherche pour le Développement, UR Camelia,

Nouméa, New Caledonia, France

18th April 2007

New Caledonia

Nickel : Opencast mining, 30 % of global reserves (3rd global producer)

Total lagoon area : 23 400 km2

Nouméa (main city) : 120 000 people

Context : urbanism, mining Focus on the Nouméa lagoon (red box : 2100 km2)

166°10' 166°20' 166°30'

-22°20'

-22°30'

Pass ofDumbéa

30

5

N

Coulée

1020

20

30

20

10

20

20

30

20

D54

B30

P03

20° S

21° S

22° S

23° S

164° E 165° E 166° E 167° E 168° E

Currentmetermoorings

Meteorologicalstations

Doppler profiling

CTD profiling

Wavemeter

Tiderecorders

History : Physical measurement strategy South-West lagoon of New Caledonia

Ouillon, Douillet, Fichez, Panché, 2005, C R Geoscience, 337 (16), 1509-0517

Bathymetry Tide Meso-scale circulation Wind

Lagoon circulation

Suspended particulateTransport

Mud & Sand 2001-…

WavesWavewatch III

2002-2007

Biological ModelECO3M 2000

WRF 2005/081980-90’s 1998

Mars3D 1999

ROMS 2006/08

History : Modeling strategy South-West lagoon of New Caledonia

166°10'E 166°20'E 166°30'E 166°40'E

22°20'S

22°30'S

0.20 m/s

Wind 8 m/s

SurfaceBottom

Ex : Simulation under mean trade wind

History : hydrodynamics modeling South-West lagoon of New Caledonia

Douillet, Ouillon, Cordier, 2001, Coral Reefs, 20 (4), 361-372

Local flushing time (days)

166°20'E 166°30'E 166°40'E

22°20'S

(days)

> 60

30 - 35

10 - 15

< 0.2

1 - 2

5 - 6

Method : tracer concentration – Case : 8 m/s trade wind + Tide

History : residence time South-West lagoon of New Caledonia

Jouon, Douillet, Ouillon & Fraunié, 2006, Continental Shelf Research, 26, 1395-1415

• Estimation of the wave setup• Estimation of the cross reef-wave induced current

Caracterisation of energy transfert across the coral reefField campaign : oct-nov 2005

• Input and output fluxes across the barrier reef

• Transformation processes of waves propagating across the reef

What about the reef influence on the lagoon ?

Aboré reef

Study area

Study area

Living corals

outer rim

rubbles and boulders

reef flat

boulders

reef slope

Reef profile

DGPS Trimble 5700(vertical accuracy 5 cm)

Hei

ght A

bove

Elli

psoi

d IA

G G

RS8

0 (m

)

East Coordinates (m) (RGNC91, UTM58S)

Open Sea

Lagoon

Highest Measured Sea Level

Lowest Measured Sea Level

estimated morphologymeasured morphology

1.4 mhigh tide level

low tide level 3 m

120 m

External sl

ope (β= 1/10)

Electromagnetic current meter S4 (2Hz)

Synchronized pressure sensors (8Hz)

Wave and Tide recorder Aanderaa

Acoustic Doppler velocimeters (8Hz)

Instruments deployment

Synchronized pressure sensors (8Hz)

0-2 bar absolute pressure sensor

(26W Keller)

Acoustic Doppler velocimeters (8Hz)

acoustic Doppler velocimeter(ADV Vector Nortek)

Electromagnetic current meter S4 (2Hz)

electromagnetic current meter

(S4 InterOcean)

Wave and Tide recorder Aanderaa

non-directional wave and tide recorder(WTR9 Aanderaa)

150 m

4 to 65 to 10T02 (s)

0.25 to 0.650.3 to 1.8Hs (m)

SENWWind direction

105Wind speed (m.s-1)

October, 19th to November,2nd(days 8 to 15)

October, 18th to 25th(days 0 to 7)

(a) incident significant wave height in a 6.5 m mean water depth(b) incident wave period

Wave conditions

• Estimation of the wave setup• Estimation of the cross reef-wave induced current• Estimation of the wave setup• Estimation of the cross reef-wave induced current

Caracterisation of energy transfert across the coral reefField campaign : oct-nov 2005

Reef-top wave setup - Measurements

01 Apr ζζζ −=

ζ : 30 minutes averaged elevation mesurementsat the reef-top (p1) and offshore (A0)

ζr : setup

dr : mean water level over the reef (with no setup)

hb : depth at the breaking point

A0 p1

MWL

Reef-top wave setup – Estimation (Symonds et al., 1995)

Wave setup on the reef-top, ζr, as a function of hb-drfrom data acquired between day 2 and day 7.

ζr=0.081(hb-dr)(Least squares best fit : R² = 0.83)

( )rbr dh −=αζζr : wave setup at the reef-tophb : water height at the breaking pointxL : reef flat widthβ : reef slopeH0 : incident wave heightHsb : Hs at the breaking pointγb : constant breaking coefficient = 0.7

(Symonds et al., 1995)

With the conditions :β xL / dr >> 1hb≡ Hsb/γb

rAr zd −= 0ζ

: 30 minutes averaged elevation measurementsoffshore (A0)0Aζ

zr : reef-top elevation

dr : mean water level over the reef-top (with no setup)

hb (m)

Reef-top wave setup - Results

from ζr = α (hb – dr)

ζrmax is reached for (hb – dr)max

Comparison between observed (solid line) and calculated (dashed line) wave setup on the reef-top

• Estimation of the wave setup• Estimation of the cross reef-wave induced current• Estimation of the wave setup• Estimation of the cross reef-wave induced current

Caracterisation of energy transfert across the coral reefField campaign : oct-nov 2005

1.4 mhigh tide level

low tide level 3 m

S4adv1

Cross reef current - Measurements

Thirty minute averaged cross reef currents measured on the reef-top at site adv1 (black line) and in the lagoon at site S4 (blue line)

Cross-reef current – Estimation (Hearn, 1999)

Square of the reef-top current ur as a function of dr(hb-dr)from data acquired between day 2 and day 7

ur²=0.22 dr(hb-dr)(Least squares best fit : R² = 0.83)

ur : cross-reef currenthb : water height at the breakingdr : mean water level over the reef-topKH : Hearn’s coefficientγb : constant breaking coefficient = 0.7

)(² rbrHr dhdKu −=

With the condition : ζr/dr << 1

Comparison between observed (solid line) and calculated (dashed line) reef cross-current ur on the reef-tophb / 2 (m)

Cross-reef current – Results

from ur² = KH dr(hb – dr)

• if drmin > hb/2

urmax is reached for dr = hb/2

Comparison between observed (solid line) and calculated (dashed line) reef cross-current ur on the reef-tophb / 2 (m)

Cross-reef current – Results

• if drmin > hb/2

urmax is reached for drmin

u r(m

/s)

Bathymetry TideMeso-scalecirculation Wind

Lagoon circulation

Suspended particulateTransport

Mud & Sand 2001-…

WavesWavewatch III

2002-2007

Biological ModelECO3M 2000

WRF 2005/081980-90’s 1998

Mars3D 1999

ROMS 2006/08

Next step (1)

Input across the

reef

Harmonic generation (triads)

Next step (2)

« Boussinesq type » approach to model harmonic generation

Transformation processes of waves propagating across the reef

Strong correlation between wave induced setup and tidal level oscillations:• ζr is maximum when dr is minimum • when hb > 2dr : ur oscillated with a maximum twice a tide, out of phase with dr

• when hb < 2dr : ur oscillated with a maximum at low tide, 180° out of phase with dr

CONCLUSION

Further details in the proceedings :

Bonneton, Lefebvre, Bretel, Ouillon & Douillet, 2007, J Coastal Research, in press