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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