Tidal modulation of wave-setup and wave-induced currents on the Aboré coral reef, New Caledonia

Transcription

1 18th April 2007 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

2 New Caledonia Nickel : Opencast mining, 30 % of global reserves (3 rd global producer) Total lagoon area : km 2 Nouméa (main city) : people Context : urbanism, mining Focus on the Nouméa lagoon (red box : 2100 km 2 )

3 istory : Physical measurement strategy South-West lagoon of New Caledoni 20 S 21 S Meteorological stations 20 D54 B30 Coulée 22 S N 23 S 164 E 165 E 166 E 167 E 168 E Tide recorders ' P03 urrentmeter oorings Pass of Dumbéa TD rofiling ' 20 Doppler profiling 20 5 Wavemeter ' ' ' 30

4 istory : Modeling strategy South-West lagoon of New Caledonia Bathymetry Tide Meso-scale circulation Wind s 1998 ROMS 2006/08 WRF 2005/08 Lagoon circulation Mars3D 1999 Waves Wavewatch III Suspended particulate Transport Mud & Sand Biological Model ECO3M 2000

5 istory : hydrodynamics modeling South-West lagoon of New Caledonia 0.20 m/s Ex : Simulation under mean trade wind Wind 8 m/s 22 20'S 22 30'S Surface Bottom 'E 'E 'E 'E

6 istory : residence time South-West lagoon of New Caledonia Local flushing time (days) Method : tracer concentration Case : 8 m/s trade wind + Tide 22 20'S > < 0.2 (days) 'E 'E 'E Jouon, Douillet, Ouillon & Fraunié, 2006, Continental Shelf Research, 26,

7 What about the reef influence on the lagoon? Input and output fluxes across the barrier reef Transformation processes of waves propagating across the reef Caracterisation of energy transfert across the coral reef Field campaign : oct-nov 2005 Estimation of the wave setup Estimation of the cross reef-wave induced current

8 reef flat reef slope Study area outer rim Living corals Aboré reef Study area rubbles and boulders boulders

9 Reef profile Height Above Ellipsoid IAG GRS80 (m) Highest Measured Sea Level Lowest Measured Sea Level Open Sea East Coordinates (m) (RGNC91, UTM58S) Lagoon estimated morphology measured morphology DGPS Trimble 5700 (vertical accuracy 5 cm)

10 Instruments deployment non-directional wave and tide recorder (WTR9 Aanderaa) 0-2 bar absolute pressure sensor (26W Keller) acoustic Doppler velocimeter (ADV Vector Nortek) electromagnetic current meter (S4 InterOcean) high tide level 1.4 m low tide level External slope (β = 1/10) 120 m 3 m 150 m Electromagnetic current meter S4 (2Hz) Acoustic Doppler velocimeters (8Hz) Synchronized pressure sensors (8Hz) Wave and Tide recorder Aanderaa

11 Wave conditions October, 18th to 25th (days 0 to 7) October, 19th to November,2nd (days 8 to 15) Wind speed (m.s -1 ) Wind direction H s (m) T0 2 (s) 5 NW 0.3 to to SE 0.25 to to 6 (a) (b) incident significant wave height in a 6.5 m mean water depth incident wave period

12 Caracterisation of energy transfert across the coral reef Field campaign : oct-nov 2005 Estimation of the wave setup Estimation of the cross reef-wave induced current

13 Reef-top wave setup - Measurements ζ r = ζ ζ p1 A0 ζ : 30 minutes averaged elevation mesurements at the reef-top (p1) and offshore (A0) h b : depth at the breaking point d r : mean water level over the reef (with no setup) MWL ζ r : setup A0 p1

14 Reef-top wave setup Estimation (Symonds et al., 1995) d r = ζ A0 z r d r ζ A0 : mean water level over the reef-top (with no setup) : 30 minutes averaged elevation measurements offshore (A0) z r : reef-top elevation ζ r =0.081(h b -d r ) (Least squares best fit : R² = 0.83) With the conditions : β x L / d r >> 1 h b Hs b /γ b ζ r = α ( h d ) b r ζ r : wave setup at the reef-top h b : water height at the breaking point x L : reef flat width β : reef slope H 0 : incident wave height Hs b : Hs at the breaking point γ b : constant breaking coefficient = 0.7 (Symonds et al., 1995) Wave setup on the reef-top, ζ r, as a function of h b -d r from data acquired between day 2 and day 7.

15 Reef-top wave setup - Results from ζ r = α (h b d r ) ζ r max is reached for (h b d r )max Comparison between observed (solid line) and calculated (dashed line) wave setup on the reef-top h b (m)

16 Caracterisation of energy transfert across the coral reef Field campaign : oct-nov 2005 Estimation of the wave setup Estimation of the cross reef-wave induced current

17 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) high tide level low tide level 1.4 m 3 m adv1 S4

18 Cross-reef current Estimation (Hearn, 1999) With the condition : ζ r /d r << 1 u r ²=0.22 d r (h b -d r ) (Least squares best fit : R² = 0.83) u r ² = K H d r ( hb d r ) u r : cross-reef current h b : water height at the breaking d r : mean water level over the reef-top K H : Hearn s coefficient γ b : constant breaking coefficient = 0.7 Square of the reef-top current u r as a function of d r (h b -d r ) from data acquired between day 2 and day 7

19 Cross-reef current Results from u r ²= K H d r (h b d r ) if d r min > h b /2 u r max is reached for d r = h b /2 Comparison between observed (solid line) and calculated (dashed line) reef cross-current u r on the reef-top h b / 2 (m)

20 Cross-reef current Results if d r min > h b /2 u r (m/s) u r max is reached for d r min Comparison between observed (solid line) and calculated (dashed line) reef cross-current u r on the reef-top h b / 2 (m)

21 ext step (1) Bathymetry Tide s 1998 Input across the reef Meso-scale circulation ROMS 2006/08 Wind WRF 2005/08 Lagoon circulation Mars3D 1999 Waves Wavewatch III Suspended particulate Transport Mud & Sand Biological Model ECO3M 2000

22 Next step (2) Transformation processes of waves propagating across the reef Harmonic generation (triads) «Boussinesq type» approach to model harmonic generation

23 Bonneton, Lefebvre, Bretel, Ouillon & Douillet, 2007, J Coastal Research, in press CONCLUSION Strong correlation between wave induced setup and tidal level oscillations: ζ r is maximum when d r is minimum when h b > 2d r : u r oscillated with a maximum twice a tide, out of phase with d r when h b < 2d r : u r oscillated with a maximum at low tide, 180 out of phase with d r Further details in the proceedings :