Damien Sous

Jean-Luc Devenon

Jean Blanchot

Marc Pagano

Circulation patterns in a channel reef-lagoon system, Ouano lagoon, New Caledonia

This paper reports on two three-months field experiments carried out in the Ouano lagoon, New Caledonia. This channel-type lagoon, exposed to meso-tides, south pacific swells and trade winds, has been monitored thanks to a network of currents profilers to understand the dynamics of the lagoon waters. Four typical circulation patterns have been identified, covering all together more than 90% of the survey period.

These patterns are mainly driven by the waves and wind features. In particular, obliquely incident waves or strong winds blowing over a sufficient period are able to reverse the typical circulation pattern. The analysis of the vertical structure of the currents through passages shows the regular presence of a nearly linear vertical shear within the water column.

Introduction

Coral reefs are both invaluable and endangered living systems in the nearshore areas of tropical regions.

They provide a unique habitat for countless species as well as a natural and efficient protection against erosion process and submersion events induced by storms or tsunamis [START_REF] Fernando | Coral poaching worsens tsunami destruction in sri lanka[END_REF]. Unfortunately, the reef colonies and all their benefits for biological and human populations are threatened by the combined effects of the increasing anthropic pressure and the climate change (sea level rise, acidification, warming, etc).

The hydrodynamical functioning of reef-lagoon systems remains a challenging task for coastal oceanographers. It is, from a physical point of view, a striking example of interacting processes over a wide spatio-temporal range but also a fundamental step for the characterization of the biogeochemical processes which finally govern the health and resilience of the ecosystema [START_REF] Carassou | Spatial and temporal distribution of zooplankton related to the environmental conditions in the coral reef lagoon of new caledonia, southwest pacific[END_REF][START_REF] Szmant | Nutrient enrichment on coral reefs : is it a major cause of coral reef decline ?[END_REF].

Reef-lagoon systems are potentially exposed to a wide set of physical forcings such as tides, waves, wind, coastal currents, rainfalls, river discharges and evaporation which affect the dynamics and the quality of lagoon waters. Density-driven currents have been observed between lagoon and ocean [START_REF] Atkinson | Circulation in enewetak atoll lagoon[END_REF], but the overwhelming trend is that tides, waves and wind are, by far, the main drivers of lagoon circulation and water renewal in most configurations [START_REF] Wolanski | Modeling the fate of pollutants in the tiahura lagoon, moorea, french polynesia[END_REF][START_REF] Kraines | Wind-wave driven circulation on the coral reef at bora bay, miyako island[END_REF][START_REF] Kraines | Rapid water exchange between the lagoon and the open ocean at majuro atoll due to wind, waves, and tide[END_REF][START_REF] Tartinville | Wave-induced flow over mururoa atoll reef[END_REF][START_REF] Andréfouët | Water renewal time for classification of atoll lagoons in the tuamotu archipelago (french polynesia)[END_REF][START_REF] Kench | Hydrodynamics and sediment flux of hoa in an indian ocean atoll[END_REF][START_REF] Angwenyi | Wave-driven circulation across the coral reef at bamburi lagoon, kenya[END_REF][START_REF] Hench | Episodic circulation and exchange in a wave-driven coral reef and lagoon system[END_REF][START_REF] Lowe | Wave-driven circulation of a coastal reef-lagoon system[END_REF][START_REF] Taebi | Nearshore circulation in a tropical fringing reef system[END_REF][START_REF] Hoeke | Drivers of circulation in a fringing coral reef embayment : a wave-flow coupled numerical modeling study of hanalei bay, hawaii[END_REF][START_REF] Chevalier | Hydrodynamics of the toliara reef lagoon (madagascar) : Example of a lagoon influenced by both waves and tide[END_REF][START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF].

The tidal cycles have a direct effect on the lagoon water : the lagoon fills during the flow and empties during the ebb, inducing the so called tidal ellipses representing periodically rotating currents. This basic scheme can be significantly complicated by the presence of complex lagoon bathymetry with multiple openings and Preprint submitted to Elsevier passages toward the open ocean and the neighbouring lagoons. The dynamics of the lagoon waters, the mixing, exchanges and renewal processes as well as the residence time [START_REF] Delhez | Toward a general theory of the age in ocean modelling[END_REF][START_REF] Monsen | A comment on the use of flushing time, residence time, and age as transport time scales[END_REF][START_REF] Delhez | Residence time vs influence time[END_REF] are thus dramatically dependent on the lagoon geometry (atoll, barrier reef or fringing reef) and volume and the passage cross-sections. The fluctuations of the still water level, mainly due to tides, induces modifications of both lagoon volume and channels/passages sections. Furthermore, they strongly affect the wave breaking process over the reef which determines in a large part, i.e. as soon as waves break on the reef top, the cross-reef fluxes. This explains why lagoon circulation studies must take into account the wave effect, and also justifies the great amount of research efforts spent to understand wave transformation over reefs and related currents [START_REF] Lowe | Spectral wave dissipation over a barrier reef[END_REF][START_REF] Monismith | Hydrodynamics of coral reefs[END_REF][START_REF] Lugo-Fernandez | Tide effects on wave attenuation and wave set-up on a caribbean coral reef[END_REF][START_REF] Hearn | Wave-breaking hydrodynamics within coral reef systems and the effect of changing relative sea level[END_REF][START_REF] Gourlay | Wave-generated flow on coral reefs-an analysis for two-dimensional horizontal reef-tops with steep faces[END_REF][START_REF] Bonneton | Tidal modulation of wave-setup and wave-induced currents on the aboré coral reef, new caledonia[END_REF]. The main physical processes during the wave propagation, which are now fairly well understood, include refraction, reflection and shoaling on the outside reef slope [START_REF] Kraines | Wind-wave driven circulation on the coral reef at bora bay, miyako island[END_REF][START_REF] Symonds | Wave-driven flow over shallow reefs[END_REF]Gourlay, 1996a,b;[START_REF] Massel | On the modelling of wave breaking and set-up on coral reefs[END_REF], bathymetric breaking occuring generally before the reef top [START_REF] Hardy | Field study of wave attenuation on an offshore coral reef[END_REF][START_REF] Hearn | Hydrodynamic processes on the ningaloo coral reef, western australia[END_REF][START_REF] Kraines | Wind-wave driven circulation on the coral reef at bora bay, miyako island[END_REF], harmonic transfers toward infragravity (IG) waves [START_REF] Pomeroy | The dynamics of infragravity wave transformation over a fringing reef[END_REF][START_REF] Van Dongeren | Numerical modeling of low-frequency wave dynamics over a fringing coral reef[END_REF] but also possibly to higher frequency (superharmonics) waves [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF][START_REF] Masselink | Field investigation of wave propagation over a bar and the consequent generation of secondary waves[END_REF], dissipation by friction and interaction with co-or counter-current [START_REF] Roberts | Wave-current interactions on a shallow reef (nicaragua, central america)[END_REF]. The relative importance of each process during the wave propagation toward the shore is controlled by the offshore wave features, the bathymetry, the mean water level and slope and the reef roughness.

This rich literature shows that a great research effort has been engaged during the last two decades to understand the bulk dynamics of reef-lagoon systems exposed to a set of time-varying, and often interacting, forcings. In this context, the present study aims to present and analyse a long-term field survey of the Ouano lagoon, New Caledonia. More than six months of current measurements have been performed in strategic points of the system to better understand the lagoon interaction with the open ocean and the neighbouring lagoons. The first section of the paper is dedicated to the presentation of the studied site and the experimental setup. The second section summarizes the results in order to identify the main drivers of the lagoon dynamics and the most typical circulation patterns, with a subsection devoted to the analysis of the vertical structure of the currents. The third section discusses the observed mechanisms in a more general context, including biogeochemical issues.

Field site and experiments

Field site

The New Caledonia archipelago hosts one of the largest reef structures worldwide, partly inscribed to the UNESCO World Heritage List in 2008[START_REF] Grenz | New caledonia lagoon : a threatened paradise under anthropogenic pressure ? Lagoons : Habitat and Species[END_REF]. The study site is the Ouano lagoon (Fig. 1), located on the south-west coast. It is an approximately 30 km long, 10 km wide and 10 m deep channeltype lagoon [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF] mostly exposed to south pacific swell waves, trade winds and meso-tidal fluctuations.

The lagoon is directly opened to ocean through two reef-openings in the north-west section of the reef barrier. The southern opening is about 1 km wide and 10-20m deep (the Isié reef opening) while the northern (the Ouaraï reef opening) is the deepest, down to -60m and 1.5 km wide. The lagoon is connected to northern In the present study, the instrumentation is focused to a well-defined lagoon system extending from the Tenia passage to the N'Digoro passage where the lagoon topography is rather simple with a limited number of openings (see Fig. 1). This simple geometry allows that the physics of the problem can be more easily understood before extending to more complex systems. In this part of the Ouano lagoon, the reef barrier is 25 km long and only opened at the Isié reef opening. The total volume of the considered portion of the lagoon is then about 1.3 10 9 m 3 .

Field experiments and methods

Two field campaigns have been carried out in the Ouano lagoon : the first in 2013 from August 28 to December 4 and the second in 2015, from January 10 to April 15. A first analysis of the 2013 experiment has focused on the parameterization of cross-reef fluxes in a coastal circulation numerical model [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF]. The 2013 data is here further analysed and combined with the 2015 experiment to provide a general view of the circulations patterns in the Ouano lagoon, over two different seasons including a wide range of wave, wind and tide conditions.

Four current profilers (ADCP) were deployed during the experiment to provide data on temporal variability of current velocity and direction along vertical profiles. Three Acoustic Doppler Current Profilers (ADCP) were deployed (Fig. 1) in lagoon passages (N'Digoro and Tenia) and reef-opening (Isié). An additional profiler was deployed in shallower area at the onshore end of the reef flat (the Platier site) to measure the cross-reef exchanges between lagoon and ocean. Mooring depths and profilers parameters are summarized in table 1.

Wave dynamics on the outside reef slope was measured thanks to autonomous pressure sensors OSSI Wave Gauge (5Hz sampling frequency) and RBR Duo (1Hz sampling frequency) for the first and second campaigns.

Pressure sensors are fixed on the bottom, at immersion depths 14.4 and 12m for the first and second experiments, respectively. Linear theory is used to estimate free surface oscillations and related significant wave height H s from pressure measurements at the bottom over 30-min time window. Section 3.5 presents an analysis of the vertical structure of currents. A quantitative estimation of the vertical shearing over the water column is provided by the calculation of the horizontal component of the vorticity vector ∂U/∂z. The vorticity is first calculated for each bin and then depth-averaged over the water column. The vertical shear of current is generally quite linear for main velocity components at Isié, Tenia and N'Digoro sites, so that the depth-averaged vorticity can be generally considered as a relevant indicator.

Along the reef (Platier site), the vertical structure of the current is usually much more complex which can not be simply analysed in terms of depth-averaged quantities and will not be discussed here.

Field conditions

In most reef-lagoon systems, water circulation is mainly controlled by fluxes through passages, reef opening and above the immersed part of the coral barrier in response to external forcings, such as tides, waves and wind [START_REF] Bonneton | Tidal modulation of wave-setup and wave-induced currents on the aboré coral reef, new caledonia[END_REF][START_REF] Gourlay | Wave-generated flow on coral reefs-an analysis for two-dimensional horizontal reef-tops with steep faces[END_REF][START_REF] Roberts | Physical processes in fringing reef system[END_REF][START_REF] Roberts | Wave-current interactions on a shallow reef (nicaragua, central america)[END_REF]). In the considered lagoon, the main driver is the tide, as demonstrated for instance by the and nearly offshore (north-west to north). They increase during the day while clockwise rotating and blowing from the east, then south and finally west before slowing down in late afternoon. This typical trend shows a slight seasonal variation is observed when comparing Figs. 4 and5, the wind tends to be stronger during the spring. The mean and maximal measured values are 2.5 and 12.4 m/s for the 2013 experiment and 2.9 and 12.2 for the 2015 experiments.

The wave distribution shown in Fig. 2, right plot, shows that waves are coming from a quite narrow sector between 140 and 215 o . The main peak period is 11.7s which indicates the dominance of long swell waves.

Mean and maximal wave heights are about 0.96 and 2.74m for the 2013 experiment, and 1.22 and 3.79m for the 2015 experiment. The strong wave events (H s > 1.5m) are generally associated to direction about 200 o , with a noticeable exception around March 14, 2015 with a more south-eastern swell event.

Results

Overview

Let us first have a look on an overall directional and spectral analysis of the measured currents which allows to define the variables used hereinafter. Figure 3 depicts direction probability and energy spectrum for depth-averaged current at each site. As expected in the presence of strong bathymetric constraints, the current are well-channelized in the reef passages and opening during the filling-emptying cycles of the lagoon induced by the tide. At the Platier (reef) site, a larger directional spread is observed, but a main tendency toward north is clearly observed. A more detailed analysis of current dynamics is presented hereinafter, but this result allows to define, for each site, a projection axis along the main flow direction to obtain the main and transverse components called U and V respectively. The sign convention is that an inward lagoonentering current is related to a positive value of the main component. In most cases, the analysis will be performed on the main component U while the tranverse component can be neglected. The only exception is the Platier site for which the transverse component V platier can be important.

The spectral analysis of depth-averaged currents presented in Fig. 3 (right plot) demonstrates the strong influence of tidal components in the velocity signal : by order of decreasing importance M2 (12h25) combined with S2 (12h), K1 (23h56), M4 (6h12) and M6 (4h08). As described by [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF], the amplitude of water level variation shows the prevalence of the semi-diurnal and diurnal tides M2, S2 and K1 which determine about 97% of the signal. To remove the influence of tides on the velocity measurements, currents can be either day-averaged or detided. The former will be denoted with the superscript da in the following.

The latter, identified with the superscript dt , are obtained by applying band-stops filters to the current data around the three dominant harmonics frequencies : f = 2.28.10 -5 Hz (M2/S2), f = 1.16.10 -5 Hz (K1) and f = 4.47.10 -5 Hz (M4). As exposed in the following, day-averaged currents are the most relevant variable to identify the circulation pattern at the lagoon scale while detided values will mainly be used to highlight the timelag between forcings evolution and currents response.

One notes that the M4 component is more significant for current than for free surface spectra (dashed line in Fig. 3). However, rather than a real M4 signature, this observation reveals the modulation of cross-reef currents at twice the tidal frequency [START_REF] Symonds | Wave-driven flow over shallow reefs[END_REF][START_REF] Kraines | Wind-wave driven circulation on the coral reef at bora bay, miyako island[END_REF] which drives associated fluctuations at other measurement sites.

An ensemble view of measured depth-averaged currents and main external parameters during the 2013 and 2015 experiments is shown in Figs. 4 and5, respectively. The influence of tide on the depth-averaged currents is strong but appears to be site-dependent, as observed on the energy spectra in Fig. 3. The tide effect is clearly dominant for Isié and N'Digoro, less strong but still significant at Tenia and much smaller on the reef (Platier site). In this latter location, the overwhelming trend for lagoon-entering current is related to the nearly permanent wave breaking over the reef, as described by [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF]. At the other sites, the tide effect is much more significant, inducing intense currents alternatively inward and outward during the emptying-filling cycles of the lagoon in response to tidal fluctuations. As expected, this tidal effect is more pronounced at the Isié reef opening which is the only direct connection to the open ocean in the considered zone. Day-averaged currents reveal that the mean general tendency is a positive (lagoon-entering) current at the Platier site and a negative (lagoon-leaving) current for the three other sites. A striking feature is that large wave events are associated with entering flux on the reef (Platier) and outward currents at each other site as shown by [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF] but this latter trend is not systematic as denoted for instance during the Feb. 8 or Mar. 14, 2015 wave events.

Further understanding on the lagoon dynamics is provided by the statistical inter-sites relationships depicted in Tab. 2 and in Fig. 6. Note that, for the sake of clarity, all relationships between sites are not plotted in Fig. 6 but a focus is made on the most significant ones in terms of lagoon circulation. In order to withdraw the effect of tide, the analysis of the dominant trends is performed on the day-averaged currents.

The effect of external parameters, waves and wind, on the measured currents is discussed later on in sections 3.2 and 3.3. Further analysis will be carried throughout the text to finally characterize the circulation patterns in section 3.4.

The following trends on links between sites can be deduced from Tab. 2 and Fig. 6.

-Strong connection is observed between the two northern sites N'Digoro and Isié. The day-averaged currents are generally negative (outward) and nearly linearly related (black circles in Fig. 6 A).

-The correlation is weaker between northern and southern passages (see U da tenia and U da ndigoro in Fig. 6 A). One notes that strong currents at Tenia can be either in-or outward while weak northern flow conditions are associated to outflow in Tenia.

-The Platier main component is fairly anti-correlated to the N'Digoro site. A similar link, not shown here, is observed with Isié.

-A strong connection is observed between U da tenia and V da platier . However, slightly different slopes in the U da tenia /V da platier relationships are observed for positive and negative currents (see dash-dotted and dashed lines in Fig. 6,B). As discussed later on in section 3.4, this latter trend tends to indicate two distincts functionings of the southern part of the lagoon : south-eastward currents should be associated to converging water fluxes in the Tenia passage leading to a magnitude increase of U da tenia with respect to V da platier while, on the opposite, periods of north-westward velocities correspond to more defined (channelized) current entering the lagoon through Tenia and flowing along the reef barrier allowing a better flux conservation between both sites.

-The linear correlation between day-averaged U da tenia and V da platier is very weak. However, Fig. Correlation coefficients for complete velocity signals presented in Tab. 2 are generally much lower than their day-averaged counterparts. This highlights in particular the role of phase shift during the tide propagation inside the lagoon : the tidal filling-emptying cycles are not in phase at each site, as described by [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF].

U isie U ndigoro U platier U tenia V platier U da isie U da ndigoro U da platier U da tenia V da platier U isie - 73.4 -31.3 12.1 -40.6 - - - - - U ndigoro - - -38.3 -14.6 -41.4 - - - - - U platier - - - 6.3 9.7 - - - - - U tenia - - - - 54.5 - - - - - U da isie - - - - - - 81.5 -81.5 -44.2 -37.7 U da ndigoro - - - - - - - -71.3 -56.1 -49.1 U da platier - - - - - - - - -1.3 -0.3 U da tenia - - - - - - - - -

Wave effect

Table 2 shows that, as expected, the influence of waves and wind is greater on the day-averaged currents as the tidal-related components have been removed from the signal. The significant wave height H s is, apart from the tide, the dominant external parameter affecting the main current component at Isié, N'Digoro and Platier sites. The sign of correlation coefficients indicates that an increase of significant wave height promotes (negative) outward directed currents at Isié and N'Digoro sites and inward (positive) current over the reef at Platier. This is confirmed in Fig. 7A which shows a monotonic increase of the magnitude of U da isie and U da platier when increasing the incoming significant wave height. The overall tendency is that the increase of offshore wave energy induces stronger inward cross-reef fluxes generated by wave-breaking above the barrier. This water input all along the reef barrier is compensated by outward currents in each passage and reef opening, see [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF] for a parameterization of wave-induced cross-reef fluxes. The response of the

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 U NDigoro (m/s) Velocity (m/s) A U Isie U Platier U Tenia -0.3 -0.2 -0.1 0 0.1 0.2 0.3 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 U Tenia (m/s) Velocity (m/s) B V Platier U Platier
Figure 6: Inter-connections between day-averaged detided currents at selected sites. A : U da isie (black circles), U da platier (red stars) and U da tenia (blue dots) vs U da tenia . B : V da platier (black circles) and U da platier (red stars) vs U da tenia . Dash-dotted and dashed lines are linear regression for V da platier vs U da tenia for negative and positive values of U da tenia , respectively.

N'Digoro currents (not depicted here) is very close to the Isié one. However, this general circulation trend, already depicted in [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF], is not the sole circulation system in the lagoon. In particular, at Tenia (see blue dots in Fig. 7A), the day-averaged currents show no clear correlation with wave height and most of wave conditions are possibly associated to inward or outward currents.

The question arises now on the external conditions controlling the flushing or filling dynamics through the Tenia passage. While wave period does not show any noticeable statistical effect on depth-averaged currents, wave direction tends to anti-correlated with day-averaged U da tenia and V da platier with moderate correlation coefficients being -40.9 and -38.5 % respectively. Fig. 7B depicts the corresponding relactionships. In spite of the measurement spread, one notes the overall trend that eastern wave direction tend to promote positive current in Tenia and along the reef at Platier while western swells induce outflow at Tenia.

Wind effect

The correlation coefficients on both complete and day-averaged currents are presented in Fig. 8. The statistical effect of wind on the day-averaged lagoon dynamics is noticeable. It is negatively correlated with the northern sites and positively correlated anywhere else : strong south-east winds promote a north-west bulk motion within the lagoon (V platier > 0) and throughout the reef passages (U tenia > 0 and U ndigoro < 0)

and openings (U isie < 0). In addition, additional computations have shown that the wind effect is quite weaker when using detided currents : correlation coefficients are -30.3, -23.8, 17.5, 28.2 and 25.3% for U dt isie ,

U dt ndigoro , U dt platier , U dt tenia and V da platier , respectively. This trend is confirmed in Fig. 8 in wind intensity around 3m/s from which V da platier increases with wind intensity. For wind speed greater than 4m/s, V da platier is systematically positive. As observed in the wind rose in Fig. 2, such strong winds are coming from a narrow directional band between 125 and 140 o (in nautical convention). Similar tendencies with same wind intensity thresholds are observed at Tenia, Isié and N'Digoro sites.

Circulation patterns

The careful consideration of the presented data leads us to identify four dominant circulation patterns in the Ouano lagoon : the Classic Pattern (CP), the Tenia Reversal Pattern (TRP), the Isié Reversal Pattern (IRP) and the Southward Pattern (SP).

The hydrodynamical features (Fig. 9), occurence probabilities (Tab. 3) and onset conditions (Fig. 10) for each pattern are described below. The two dominant ones, i.e. CP and TRP, are described in more details through two four days selected events in Fig. 11. Furthermore, in order to understand their role in sites is not able to compensate the outward flow. Currents are maximal aruond the swell apex, at high tide at Platier and during the ebb for other sites.

The onset conditions of CP for day-averaged currents are synthesized in Fig. 10. It is generally associated to large south-western swells and rather moderate winds. It appears that for the largest swells, CP is systematically observed, i.e. the lagoon-entering cross-reef flux is strong enough to dominate any other process. The breaking process over the reef is dependent of the ratio between wave height and water level above the reef [START_REF] Hearn | Wave-breaking hydrodynamics within coral reef systems and the effect of changing relative sea level[END_REF][START_REF] Bonneton | Tidal modulation of wave-setup and wave-induced currents on the aboré coral reef, new caledonia[END_REF][START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF]. When strong waves combinates with low tidal amplitude, the cross-reef flux induced by wave breaking can be so important that it pushes out water through each passages and reef opening (i.e. even at Isié) all along the tidal cycles, as depicted in Fig. 4 around Sept. 29 or Oct. 31, 2013. However, more data in large wave conditions (H s > 3m) should be gathered to substantiate this observation.

Day-averaged currents and estimated fluxes are given in Tab. 4 for the October, 6. One notes that, if depth-averaged velocities are of the same order of magnitude for each site, the input flux over the reef barrier is much more important than through passages and reef openings. Furthermore, the outward flux in Tenia is 2.5 times the along-reef flux in Platier, indicating the convergence process in the southern part of the lagoon.

The Tenia reversal pattern.

The Tenia reversal pattern (TRP) is characterized by : -An inward flow above the reef barrier (U platier > 0) -An inward flow through Tenia passage (U tenia < 0) generally associated to north-west current along the reef (V platier > 0).

-An outward flow in the northern sites (U isie < 0, U ndigoro < 0)

The occurence probabilities of TRP for day-averaged currents are 28.6 %, respectively. A typical TRP is depicted in Fig. 11 (right plots) between Feb. 6 to 9, 2015. The main difference with CP is the observation of an inward flow at Tenia which magnitude oscillates with the tide but can remain in the same direction during several tidal cycles. The cross-reef flux measured at Platier site is still positive but a bit less intense during the swell peak than in CP presented above for similar type of wave height. This is attributed to the combined effects of, (i), a more southern wave direction which induces more westward (indeed WNW) flow above and along the reef and, (ii), a stronger south-east wind which drives lagoon waters toward north-west.

However, in the considered case, the wind peak occurs after the establishment of the pattern. This latter appears thus to be mainly initiated by the swell arrival. TRP is related to a bulk north-west motion along the barrier (see the relationship between U tenia and V platier in Fig. 6,B), with water entering through Tenia from the neighbouring south-east lagoon and above the reef-barrier pushed by wave-breaking of south/south-east swell and leaving the lagoon through the northern openings.

Figure 10 shows that TRP is promoted by south to south-east swells and strong winds which are generally from south-east (Fig. 2). The comparison of estimated fluxes in Tab. 4 shows that, for similar range of incoming wave height, the exchanged fluxes through Isié and N'Digoro are much stronger in the TRP case. This emphasizes the major role played by the circulation patterns, driven by wave direction and wind magnitude, on the lagoon flushing dynamics and water renewal [START_REF] Chevalier | Hydrodynamics of the toliara reef lagoon (madagascar) : Example of a lagoon influenced by both waves and tide[END_REF].

The Isié reversal and Southward patterns.

The Isié reversal and Southward patterns are discussed conjointly because they occur in quite similar conditions, i.e. small to moderate waves (typically H s < 1.2m) and weak winds (typically W < 4m/s) as depicted in Figs. 4 and 10. These patterns are not very stable and small waves periods are often characterized by alternating periods of IRP and SP.

Both Isié Reversal Pattern (IRP) and Southward Pattern (SP) are characterized by : -An inward flow above the reef barrier (U platier > 0) -An inward flow through Isié reef opening (U isie > 0) -An outward flow in Tenia (U tenia < 0)

The only difference is that N'Digoro current is inward (U ndigoro < 0) for SP and outward (U ndigoro > 0)for IRP. The occurence probabilities for IRP and SP are respectively 12.5 and 3.6 % in terms of day-averaged currents. The estimation of exchanged fluxes in Tab. 4 for selected 24h period of IRP and SP indicates that these patterns, associated to low wave energy, are much less effective in the advection of water mass.

Vertical structure

The vertical structure of lagoon currents and its effect on the lagoon hydrodynamics, water renewal and biogeochemical processes have been little studied to date. Strong vertical variability has been highlighted in the Majuro Atoll numerical model [START_REF] Kraines | Rapid water exchange between the lagoon and the open ocean at majuro atoll due to wind, waves, and tide[END_REF], with fast wind-driven surface layer (down to few meters) and much weaker or even return flows deeper in the water column possibly affected by baroclinic effects. The present data is purely hydrodynamical, i.e. the water properties such as salinity or temperature, which play a dominant role in stratification processes, are not documented by measurements. However, limiting our discussion to a purely currentological analysis, interesting observation can be performed on the vertical structure of flows through passages and reef openings.

As described in Section 2.1, the depth-averaged horizontal component of the vorticity ∂U ∂z is computed to provide a quantitative estimation of the vertical shear. Fig. 12 shows the relationship between depth-averaged vorticity and main component of currents for each site. Color levels in left and right columns indicate wind magnitude and wave height, respectively. Only one third of the dataset are depicted for the sake of clarity.

These plots should be interpreted as follows :

-Zero vorticity : no vertical shear in the water column.

-Both positive vorticity and velocity : the current over the whole water column is entering into the lagoon, the surface layer moves faster than the bottom water.

-Both negative vorticity and velocity : the current over the whole water column is leaving the lagoon, the surface layer moves faster than the bottom water.

-Positive vorticity and negative velocity : the depth-averaged current leaves the lagoon, the bottom layer moves faster than the surface layer. In some cases, this latter can even flows in an opposite direction (i.e. being positive lagoon-entering) than the bottom layer.

-Negative vorticity and positive velocity : the depth-averaged current enters the lagoon, the bottom layer moves faster than the surface layer. In some cases, this latter can even flows in an opposite direction (i.e. being negative lagoon-leaving) than the bottom layer.

One notes first, for a substantial portion of the measured data, the importance of the vertical shear. For illustration, a vertical vorticity of 0.05 s -1 over a 10m deep measurement water colum leads to a vertical variation of the velocity of 0.5m/s. The presence of flow reversal within the water column, i.e. opposite directions for surface and bottom layers, is observed in 16, 28, 29 % of the considered data for the Isié, Tenia and N'Digoro sites, respectively. It occurs generally during the tide reversal, when depth-averaged velocities are small.

The data distribution observed in Fig. 12 shows two overall tendencies : vertical shear generally increases with current velocity and vorticity is preferentially of the same sign as velocity, i.e. surface water moves faster than bottom one. The main exception to this trend is observed at Isié where inward current can show significant vorticity either positive or negative. In addition to this base functioning, the effects of wind and wave on the vertical shear are displayed by color labels in left and right columns, respectively. Strong winds (blowing from south-east) are generally associated to strong (negative) vertical shear associated with the outward currents at Isié and N'Digoro. This describes the expected effect of wind which enhances the north-west surface transport over the whole lagoon. At Tenia, the wind does not show a direct effect on the vertical shear and strong wind events are mainly associated to strong inward weakly sheared currents. One notes also that, at each site, large value of vorticity can be observed during calm wind periods. The wave effect is quite similar to that of the wind, although less marked. Strong wave events are generally related to outward sheared currents at Isié and N'Digoro whereas at Tenia no clear dependency is observed.

From the data presented in Fig. 12 and an overall analysis of the measured profiles, the vertical structure of the currents in passage and reef opening can be summarized as follows :

-During well-established currents and circulation patterns, the surface layers are generally moving faster than the bottom ones.

- The top plot provides a scheme to help the data interpretation of the results in terms of vertical profiles, the xand y-axis being the depth-averaged current and the vorticity, respectively. Current vorticity vs intensity for Isié, N'Digoro and Tenia. Color labels provide information of the wind magnitude (in m/s) and wave height (in m) for left and right plots, respectively.

-In most cases, the vorticity is roughly constant in the water column (linear velocity profile)

-The wind, generally eastern, affects as expected the vertical shear by accelerating the surface layer when depth-averaged current is inward (e.g. during TRP events) or slowing down the surface layer of outward flows.

-Opposite signs of depth-averaged velocity and vorticity can be observed, i.e. bottom layers moving faster than surface ones, in particular (but not exclusively) during switchovers from TRP to CP.

Note that due to the time scales of the external forcings, in particular tide and thermal breeze, and the lagoon depth (typically about 10m) and topographical constraints, the Ekman's effect on the current vertical structure is expected to be weak, in particular in comparison to the direct action of wind stress at the surface. Additional data processing will be carried out to further explore this issue.

Discussion

A generic functionning for channel lagoons ?

The hydrodynamical field measurements presented here provide a valuable dataset for the understanding of lagon circulation and a relevant benchmark for numerical modeling. It shows the importance of the combined effects of tides, waves and wind which must all be taken into account in numerical models to provide a proper characterization of water renewal dynamics. The main processes observed in the Ouano lagoon should be representative of similar types of reef-lagoon systems that we defined as channel lagoons.

The distinctive features of such lagoons are :

-a well-defined barrier reef -a typical lagoon depth (5-50m) much greater than the depth over the reef top (typically outcropping during low spring tides), -a reef-parallel dimension (length) of the lagoon much greater than the reef-normal dimension (width), i.e. with an aspect ratio of the order of 2-10, -longitudinal bathymetric gradients smaller than their transverse counterparts, -one or several passages toward open ocean or adjacent lagoons.

Typical channel lagoons can be found for instance in the Pacific Ocean :

-New Caledonia, west and east coasts ;

-French Polynesia, east coast of Moorea, Huahiné, Raiatea-Tahaa, Tahiti Iti and Nui ;

-Japan, south-west Okinawa (Bibi beach) ; -Fiji, Viti Levu, Nairai and Kandavu ;

-Samoa, Naunonga (Vanikoro), Utupoa ;

or in the Indian Ocean in Madagascar (Tulear) or in Maurice (north Mahebourg). Various tides, waves and wind conditions can be encountered at each site but the renewal time of lagoon waters typically ranges from few days to few weeks. Such time scales are, on one hand, sufficient to allow the setting up of a wide range of bio-geochemical processes (the lagoon can produce its "own" waters) and, on the other hand, short enough to be permanently affected by the fluctuations of ocean and atmospheric forcings : swell events, storms, spring/neap tide cycles, winds, etc. The present experiments demonstrates that, in addition to the tidal cycles, the wave and wind plays an important role in the lagoon circulation. The wave-breaking cross-reef fluxes are generally able, indepently to the tide, to renew the lagoon water in few days to few weeks. This wave effect, which is enhanced as the length to width ratio of the lagoon increases, can drive different current patterns depending on the swell direction and magnitude, possibly blocking or reversing the tidal fluxes through passages and reef openings. The wind stress affects the vertical flow structure and also participate to the reverse of the whole lagoon circulation when strongly blowing over a sufficient period of time (typically few days). Depending on the lagoon geometry, the difference between patterns, which can alternatively drive water from/into the open ocean or from/into the neighbouring lagoons in variable proportions, can lead to important consequences in terms of water properties and biogeochemical processes.

Seasonal variability

The All of these open questions should drive further ambitious research efforts. In particular, field campaigns must now be designed to measure simultaneously both hydrodynamical and biogeochemical properties of the reef-lagoon systems in order to gain a comprehensive understanding of these coupled processes.

Conclusion

From two field campaigns carried out in the Ouano lagoon, west New Caledonia, we have identified the main drivers of a typical channel lagoon. The selected lagoon -barrier reef system is exposed to south pacific swells, meso-tides and trade winds modulated by the thermal breeze. A network of current profilers has been deployed during two successive three-months field campaign in order to monitor the current within reef passages and openings in a wide range of hydrodynamical and meteorological conditions. Pressure sensors are used to monitor the incoming wave features on the outside reef slope.

The first driver of the lagoon hydrodynamics is the tide which induces periodic filling/flushing cycles of the lagoon well identified on both free surface and currents measurements. In addition, a modulation of currents at twice the tidal frequency has been observed, in response to the modulation of cross-reef flow [START_REF] Symonds | Wave-driven flow over shallow reefs[END_REF][START_REF] Kraines | Wind-wave driven circulation on the coral reef at bora bay, miyako island[END_REF]. The analysis of day-averaged depth-averaged currents allows to identify both the inter-connections between measurement sites and their dependence to the external forcings.

The effect of wave is straightforward : as soon as waves break on the reef top, an entering flow is observed above the reef barrier whatever the tide, increasing with wave energy. For the strongest swells, the cross-reef water input is such as depth-averaged currents are permanently outward at each other site, even during rising tide. For all other conditions, the current dynamics in reef openings and passages is mainly controlled by wave direction and wind magnitude. The comparison between detided and day-averaged currents highlights the slow day-scale adjustement of the lagoon circulation to the wind stress. Four typical circulations have been characterized, with the two first controlling the lagoon dynamics more than 70% of time :

-The Classic Pattern corresponds to a day-averaged outflow at each passage and opening, occuring during moderate wind and south-west wave conditions.

-The Tenia Reversal Pattern is defined by an input day-averaged flow at the southern opening driving an overall bulk water motion toward north-west. This pattern is forced by strong winds and/or swells coming from south-east -The Isié and Southwards Patterns are observed in calm conditions. They are characterized by a reversal of the day-averaged current in the northern opening of the lagoon. They are much less stable than the two previous patterns which last for several days as long as the wave or wind forcings maintain.

Following the fluctuations of the meteorological forcings, the lagoon is expected to show a seasonal functioning, with dominant inputs from open ocean waters and from neighbouring eastern lagoons during austral summer and winter, respectively. Moreover, the sensitivity of the lagoon circulation to forcings direction is expected to increase during severe and extreme wave and wind conditions encountered during tropical storms or cyclones.

The analysis of the vertical structure of the current in lagoon passages shows the regular presence of a significant nearly linear vertical shear in the water column. This shear often appears during strong wind events but are also observed in calm conditions. The main tendency is that surface layer are faster than bottom ones, either for in-or out-flows. Period of reversal between patterns are generally associated to complex vertical structure of the current with opposite flows in the upper and lower parts of the water column.

Figure 1 :

 1 Figure 1: Top view of the Ouano lagoon system. Black dots and stars represent pressure sensors and velocity profilers locations, respectively.

Figure 2 :

 2 Figure 2: Wind rose (left) and wave rose (right) for cumulated data of both 2013 and 2015 experiments.

  spectral analysis shown in Fig 3 or the periodic oscillations of velocities observed in Figs. 4 or 5. In this paper, a particular attention is paid on the role played by the other main forcings of the lagoon system, i.e. waves and wind, through the study of five parameters : the incoming wave height H s , peak period T p and direction θ ww3 and the wind magnitude W and direction θ w . Rainfalls have been sparse on the studied periods and are neglected. Statistical features of waves and wind over the cumulated data (2013 and 2015 experiments) are summarized in the wind and wave roses shown in Fig 2. Wind measurements performed at the nearby Tontouta airport station revealed the typical wind pattern observed during the experiments. Trade winds are modulated by thermal breeze and guided by the mountainous topography of New Caledonia. The dominant pattern is clear : strong winds always blow from the south east and almost no winds are coming from the north-west sector. Daily variations are observed in Figs. 4 and 5. Winds are minimal during nights (lower than 1 m/s)

Figure 3 :

 3 Figure 3: Depth-averaged current direction and normalized Spectral Density of Energy (dashed line is the SDE for the water level over the reef flat)

Figure 7 :Figure 8 :

 78 Figure 7: Influence of incoming wave features on the depth-averaged currents at selected sites. A : U da isie (black circles), U da platier

Figure 9 :

 9 Figure 9: Schematic view of the circulation patterns in the Ouano lagoon.

  the lagoon flushing dynamics, one representative 24h-period has been selected for each pattern among the dataset : Sept. 21 2013 for SP, Sept. 22 2013 for IRP, Oct. 6 2013 for CP and Feb. 7 2015 for TRP. The measured depth-averaged currents are thus time-averaged over the corresponding 24h period and displayed in Tab. 4. Estimation of related exchanged daily fluxes can be computed by using the cross-section areas of each site provided by the numerical bathymetry used by[START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics : a simple parameterization for coral lagoon circulation model, with application to the ouano lagoon, new caledonia[END_REF]. For the N'Digoro, Isié and

Figure 10 :

 10 Figure 10: Classic and Tenia patterns

Figure 11 :

 11 Figure 11: Examples of Classic Pattern and Tenia Reversal Pattern. From top to bottom : depth-averaged currents, depthaveraged detided currents, wave height and peak period, wave direction, wind intensity and direction.

  Fig. 13. The whole wave partitions are explored to compute the occurence frequency of swells components (H s > 1m) coming from eastern (θ < 200 o , black circles in Fig 13) and western (θ > 200 o , red circles in Fig 13) sectors. A clear seasonal variation is observed : western swells are clearly dominant between may and november while eastern swells are more present during the austral summer. In addition Meteo France wind data at Tontouta airport from 2006 to 2015 are processed to depict the occurence probability for strong eastern winds (W > 5 m/s) in green circles in Fig. 13. A statistical trend is observed, with more frequent strong wind events between September and March than during the rest of the year. According to the previous analysis, it is likely that the difference in lagoon circulation observed between 2013 and 2015 (and 2011) experiments are indeed representative of a cyclic seasonal variation. Even if the available hydrodynamical data does not cover the full annual variations, one can expect a dominance of CP from may to september when swells are predominantly from the west and winds are rather calm and, conversely, a dominant TRP during the austral summer between December to April when strong winds and eastern swells are more frequent. The renewal of lagoon waters should then be mainly controlled by inflow of open ocean waters during the austral winter and inflow from south-eastern neighbouring lagoon through Tenia passage the rest of the year.

Table 1 :

 1 Current profilers characteristics during the field experiments (asap : as soon as possible -ping per ensemble, nc : no theoretical value calculated by Sontek).

		Site		Profiler Parameters
	Location	Depth	Sensor	Samp.-Aver. Vert. res. 1st Bin Pings/ens Time/Ping	SD
		(m)	(min -min)	(m)	(m)			(s)	(cm/s)
				2013
	N'Digoro 14.84 ±0.05 Rdi 300KHz	10-1	1.5	3.73	100	0.55	1
	Isié	16.80 ±0.05 Rdi 300KHz	10-1	1.5	3.73	100	0.55	1
	Platier	3.53 ±0.05 Sontek 3MHz	10-1.5	0.25	0.45	asap	nc	nc
	Tenia	13.89 ±0.05 Rdi 300KHz	10-1	1.5	3.73	100	0.55	1
				2015
	N'Digoro 15.71 ±0.05 Rdi 300KHz	10-1	1.5	3.73	100	0.55	1
	Isié	16.19 ±0.05 Rdi 300KHz	10-1	1.5	3.73	100	0.55	1
	Platier	3.31 ±0.05 Sontek 3MHz	10-1.5	0.25	0.45	asap	nc	nc
	Tenia	14.20 ±0.05 Rdi 300KHz	10-1	1.5	3.73	100	0.55	1

Table 3 :

 3 Tenia sites, these cross-sections are about 2.09.10 4 , 1.15.10 4 and 2.85.10 4 m 2 , respectively. For Platier, as the CP da TRP da IRP da SP da Occurence probabilities (in %) for the four selected patterns for day-averaged (da) currents.

			Sept. -Nov. 2013 54.4	29.8	10.5	3.5
			Jan. -Apr. 2015	27.9	40.7	19.8	5.8
				Total			38.5	36.4	16.1	4.9
				Classic Pattern
				Tenia Reversal Pattern
				Isie Reversal Pattern
	Occurrence probability (%)			Southward Pattern
	5
	0
	140	160	180	200	220	240	260
				θ ww3 ( o )

Table 4 :

 4 Day-averaged currents (U in cm/s) and corresponding estimated exchanged fluxes (F in 10 3 m 3 .s -1 ) for four selected days corresponding to the four circulation patterns. Day-averaged values of wave and wind features are indicated for comparison.

						Platier						Platier
						Tenia						Tenia
			0.6			NDigoro Isie			0.6		NDigoro Isie
	Velocity (m/s)	-0.2 0 0.2 0.4						Velocity (m/s)	-0.2 0 0.2 0.4
		-0.4							-0.4
		-0.6							-0.6
	Detided velocity (m/s)	05/10 -0.2 0 0.2 0.4	06/10	07/10	Platier Tenia NDigoro Isie	08/10	Detided velocity (m/s)	06/02 0.4 -0.2 0 0.2	07/02	08/02	09/02 Platier Tenia NDigoro Isie
		05/10 -0.4	06/10	07/10			08/10		06/02 -0.4	07/02	08/02	09/02
			2					20			2		20
	05/10 0 1 s H (m)	06/10	07/10	H s T p	(m) (s)	08/10 0 10	T p (s) (m) H s 06/02 0 1	07/02	08/02	H s (m) T p (s)	09/02 15 10 5	(s) p T
		240							200
	θ ww3 ( o )	200 220						θ ww3 ( o )	190 195
		05/10 180	06/10	07/10			08/10		06/02 185	07/02	08/02	09/02
	W (m/s)	5 10 15						W (m/s)	5 10 15
			05/10 0	06/10	07/10			08/10			06/02 0	07/02	08/02	09/02
		400							200
	θ W ) ( o	200						θ W	100
			05/10 0	06/10	07/10			08/10			06/02 0	07/02	08/02	09/02

Acknowledgements

This study was sponsored by the Action Sud MIO/IRD (A.S. OLZO and A.S. CROSS-REEF) and the ANR MORHOC'H (Grant No. ANR-13-ASTR-0007). The GLADYS group (www.gladys-littoral.org) supported the experimentation. We are grateful to all the contributors involved in this experiment. The authors are particularly indebted to David Varillon, Eric Folcher and Betrand Bourgeois whose efforts were essential to the deployment. A special thanks is extended to Jerôme Aucan for giving immediate and unconditional access to his data.

and outflow through the northern sites over more than 4 days which totally overcome the tidal dynamics (see Fig. 5). The clear regime shift from TRP to CP on March 14/15, while wave height and wind intensity remain strong, should probably be attributed to the slight shift in wind and waves directions. The proper validation of such an hypothesis would have require direct measurements of wave direction at the system entry, which are not available with the present dataset. However, this observation tends to highlight the increased sensitivity of the circulation system to the forcings direction during severe and extreme conditions.

Biogeochemical issues

The existence of well defined circulation patterns in the reef-lagoon system is of primary importance to analyse and predict the biological productivity and its evolution [START_REF] Cuif | Wind-induced variability in larval retention in a coral reef system : A biophysical modelling study in the south-west lagoon of new caledonia[END_REF]. During CP conditions, the lagoon is mainly fueled by across-barrier water inputs. During the reef barrier crossing, coral organisms and plankton-eating fishes tend, through ingestion and metabolism, to modify the nutrient composition (uptake and regeneration), to deplete food content of the incoming waters by grazing and predation (bacterio-, phyto-and zooplankton content) [START_REF] Houlbreque | Picoplankton removal by the coral reef community of la prévoyante, mayotte island[END_REF][START_REF] Hamner | Behavior of antarctic krill (euphausia superba) : schooling, foraging, and antipredatory behavior[END_REF][START_REF] Cuet | Cnp budgets of a coral-dominated fringing reef at la réunion, france : coupling of oceanic phosphate and groundwater nitrate[END_REF] and to increase the release of non living particles (mucus) by corals [START_REF] Cuet | Cnp budgets of a coral-dominated fringing reef at la réunion, france : coupling of oceanic phosphate and groundwater nitrate[END_REF] . Furthermore, from the exchanged volumes estimated hereinbefore, one can expect that CP would be less efficient in terms of water renewal than TRP for which water are entering the lagon both through the Tenia passage and above the reef barrier. Longer residence times for CP may be associated with an increase of the phytoplanktonic productivity of the lagoon [START_REF] Delesalle | Residence time of water and phytoplankton biomass in coral reef lagoons[END_REF][START_REF] Andréfouët | Water renewal time for classification of atoll lagoons in the tuamotu archipelago (french polynesia)[END_REF], which can compete with cross-reef transformation processes. TRP is expected to foster the inflow from neighbouring south-eastern lagoons through Tenia passage rather than open ocean waters. Visual observations notably revealed the arrival of water loaded with appendicularians and jellyfishes during TRP.

Current shearings generated by thermal and salinity fronts have been observed to induce strong plankton aggregation in the lagoon context [START_REF] Gomez-Gutierrez | Influence of thermo-haline fronts forced by tides on near-surface zooplankton aggregation and community structure in bahía magdalena, mexico[END_REF]. The present study demonstrates the presence of significant vertical shear induced by the wind forcing alone. The effect of such vertical shearing of the currents may significantly affect the plankton migration. For instance, in the passages and reef openings, a strongly sheared current with inflow at the surface and outflow near the bed (as revealed by the present


Fabien Locatelli

Damien Sous

Vincent Rey

Cristele Chevalier

Frederic Bouchette

Julien Touboul

Jean-Luc Devenon

Frederic Bouchette

Frédéric Bouchette

WAVE TRANSFORMATION OVER THE OUANO REEF BARRIER, NEW CALEDONIA

Keywords: coral reef barrier, wave transformation, infragravity waves, very low frequency

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

A large part of tropical shores are protected by coral reef barriers. These living systems are both priceless and threatened. They offer a nourishing shelter for a large amount of marine species. Coral reef barriers have been also shown to provide a natural and efficient protection against erosion and submersion events induced by storms or tsunamis. Unfortunately, the reef colonies and all their benefits for biological and human populations are endangered by the combined effects of the increasing anthropic pressure and the climate change (sea level rise, ocean acidification and warming).

The transformation of incoming waves over the reef barrier is of primary interest to understand the mass, energy and nutrient transfers from the open ocean to the inner lagoon which in turn affect the wealth and the renewal time of lagoon waters [START_REF] Monismith | Hydrodynamics of coral reefs[END_REF]. A range of processes involved in wave transformation over reef bottom has been documented by in-situ and laboratory measurements [START_REF] Bonneton | Tidal modulation of wave-setup and waveinduced currents on the Aboré coral reef, New Caledonia[END_REF][START_REF] Lowe | Spectral wave dissipation over a barrier reef[END_REF], Pomeroy et al. 2012) as well as theoretical and numerical approaches [START_REF] Massel | On the modelling of wave breaking and set-up on coral reefs[END_REF]Gourlay 2000, Van Dongeren et al. 2013), including reflexion and refraction over the outside reef slope, breaking, dissipation and energy transfers toward lower and higher frequency bands.

An essential issue is to understand how the incoming swell energy is transferred to the shore or to the lagoon for fringing and barrier reef systems, respectively. Field and numerical studies demonstrated the importance of infragravity (IG) waves in reef environments (Pomeroy et al. 2012[START_REF] Van Dongeren | Numerical modeling of low-frequency wave dynamics over a fringing coral reef[END_REF]. In nearshore areas, these long waves are usually forced by the groupiness of incoming gravity short waves (SW), i.e. the envelope modulation of swell waves. The threshold between short and IG waves bands is generally about 0.04-0.06Hz (see [START_REF] Sénéchal | Observation of irregular wave transformations in the surf over a gently sloping sandy beach on the French Atlantic coastline[END_REF] for a discussion). Under certain circumstances, a distinction can be made within the IG band itself between IG and very low frequency (VLF) bands. The usual motivation for such distinction is that in the VLF band, free surface fluctuations are more controlled by topo-bathymetric features inducing standing and/or resonating oscillations. The IG/VLF frequency boundary varies depending on the authors and the studied sites (beaches or reefs) but is typically between 0.004 [START_REF] Ruessink | Bound and free infragravity waves in the nearshore zone under breaking and nonbreaking conditions[END_REF], Pomeroy et al. 2012[START_REF] Van Dongeren | Numerical modeling of low-frequency wave dynamics over a fringing coral reef[END_REF]) and 0.005Hz [START_REF] Sénéchal | Observation of irregular wave transformations in the surf over a gently sloping sandy beach on the French Atlantic coastline[END_REF][START_REF] Gawehn | Identification and classification of very low frequency waves on a coral reef flat[END_REF]. The lower limit for VLF (or for total IG when the IG/VLF distinction is not made) is usually taken at 0.001Hz [START_REF] Aagaard | Infragravity wave contribution to the surf zone sediment transport -the role of advection[END_REF]Greenwood 2008, Gawehn et al. 2016).

In reef environments, IG waves are generated around or just before the reef top. As the outer reef slope is strong (1:20 to 1:10), the generation mechanism is generally related to the moving of the breaking point 1Université de Toulon, CNRS/INSU, IRD, MIO,UM110, 83051 Toulon Cedex 9, France. sous@univ-tln.fr 2 Aix-Marseille Université, Université de Toulon, CNRS/INSU, IRD, MIO, UM 110, 13288, Marseille, Cedex 09, France 3 GEOSCIENCES-Montpellier, Université de Montpellier II, Montpellier, France Paper No. 17 (Pomeroy et al. 2012[START_REF] Baldock | Dissipation of incident forced long waves in the surf zone -Implications for the concept of "bound"wave release at short wave breaking[END_REF]. At the inner end of fringing reef, the shore can induce reflection of IG waves and development of standing wave motions. When the forcing period of incoming wave groups matches one of the natural seiching periods (eigenmode) of the reef flat, resonance can appear [START_REF] Péquignet | Forcing of resonant modes on a fringing reef during tropical storm Man-Yi[END_REF], Pomeroy et al. 2012b). [START_REF] Gawehn | Identification and classification of very low frequency waves on a coral reef flat[END_REF], which proposed a detailed classification of such processes in the case of fringing reef, indicates that resonance over reef flat is mostly observed for the longest fundamental mode around high tide.

Based on field measurement in the Ouano lagoon, New Caledonia, the present study aims to provide a new insight of wave transformation processes over a coral barrier reef. A particular attention will be paid on long waves, in order to understand how they propagate across the reef flat and are transmitted into the lagoon.

Experiments

Field site

The studied site is the Ouano lagoon, south-west New Caledonia. The field site is a coastal weakly anthropized narrow lagoon, nearly 30 km long, 10 km wide and 10 m deep (see Fig. 1). This reef-lagoon system is a typical "channel" lagoon with strong aspect ratio of horizontal dimensions: the lagoon is much longer than it is wide. Such configuration implies a strong effect of cross-reef fluxes induced by wave breaking on the lagoon dynamics and water renewal time. The lagoon is directly open to ocean through two reef-openings in the north-west section of the reef barrier. The southern opening is about 1 km wide and 10-20m deep while the northern is the deepest, down to -60m and 1.5 km wide. The lagoon is connected to northern and southern lagoons by two passages, one toward north and one toward south. The northern passage is about 5m deep while the south passage is 10-15 m deep. Note that this latter is close to a further south reef-opening. At high tide, the coral reef barrier is fully submerged, whereas at low tide it can be partly emerged depending on tide and wave conditions as well as large-scale sea level fluctuations. For further description of the Ouano reef-lagoon system, the reader should refer to [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics: a simple parameterization for coral lagoon circulation model, with application to the Ouano Lagoon, New Caledonia[END_REF].

Instrumentation and methods

The field measurements presented here are part of a large hydro-biogeochemical field campaign (OLZO and CROSS-REEF projects) conducted from April to August, 2016. The instrumentation used for the present analysis is dedicated to the study of cross-reef wave transformation and propagation within the lagoon. Incoming wave conditions are provided by a S4 electro-current meter (8 m deep) on the reef foreshore on 20 min bursts recorded each 3h. Due to the failure of the embedded pressure sensor, free surface fluctuations at S4 are deduced from the velocity measurements using linear wave theory. Seven autonomous pressure sensors have been deployed from the reef top to the inner lagoon. The reef flat sensors CP2 to CP5 are OSSI Wave Gauge® sensors recording bottom pressure at 5 Hz. The lagoon pressure sensors are RBR Duo® at 2Hz. Positioning of the sensors array is given in Fig. 2. Energy spectra presented in Fig. 3 are computed continuously over 40min burst for each pressure sensor (see Sec. 3.1 for discussion). Figure 3 displays the average spectra over the April 25 to July 31 time period. The average spectrum of the wave field envelope measured by S4 is computed each 3h over 20min bursts using Hilbert transform.

Bathymetric data results from in-situ surveys performed during the campaign and combined with the ZONECO atlas. It should be noted the acquisition of reliable bathymetric data in reef area remains quite challenging, in particular in the zones exposed to waves. The bathymetry of the inner reef coral colony has not been properly documented and is given as indicative information in Fig. 2 from sparse data survey and visual observations.

Results

Theoretical seiching modes

Fringing reefs have been observed to develop a specific dynamics for long waves, partly controlled by strong reflection process at the shore inducing standing or resonant VLF modes attached to the reef flat. Barrier reef systems are not that constrained due to the presence of a fluid rather than a rigid boundary at the inner limit of the reef flat. They however show strong topo-bathymetric gradients able to potentially induce partial reflection of surface waves. It is well-know that wave reflection in closed or semi-open basins leads to the development of standing waves, the so-called seiches. In order to estimate the seiching modes for our reef-lagoon system, we use the analytical work of [START_REF] Wilson | Seiches[END_REF], restated by [START_REF] Rabinovich | Seiches and harbour oscillations[END_REF].

Two types of standing oscillations can a priori be expected. First, the lagoon can be considered as a basin either closed or semi-open at the reef boundary. In the former case, the expected fundamental seiching wave has a node near the basin center and two anti-nodes at the lagoon boundaries. The oscillation period depend on the water depth and the exact bathymetric profile, but assuming a 12m deep 6000m long flat basin, the fundamental frequency is f0=9.4.10 -4 Hz. Higher nth harmonics are at n.f0. If the basin is considered semi-open, seiching frequencies are ((2n+1)/2).f0. The fundamental mode (n=0) shows a node at the reef barrier and an anti-node at the inner reef.

Second, the reef flat itself shows a strong bathymetric gradients at its boundaries, in particular a nearly vertical step at its inner end. This induces discontinuities in the wave celerity which can possibly induce partial reflection of surface wave. The reef standing wave pattern would thus consists in a node/anti-node at the barrier outer/inner boundary. Possible seiching oscillations attached to the reef barrier would naturally be more dependent on depth, because relative depth fluctuations are much more important on the reef than in the lagoon. The fundamental mode for the reef barrier seiche is at f0=1.4.10 -3 Hz.

Each of these fundamental modes are in the usual VLF range. The selected 40 min time window for spectral analysis is long enough to resolve the expected VLF scales [START_REF] Gawehn | Identification and classification of very low frequency waves on a coral reef flat[END_REF]. It is emphasized that the theoretical values, which are used as a guide for the data analysis, must be considered keeping in mind the limitations of the idealized case. First, the predictions are provided for flat basin which do not account for bathymetry variations which can strongly affect long waves structure [START_REF] Michallet | Long waves and beach profile evolutions[END_REF]) neither for the effect of bed friction. Second, this purely cross-reef longitudinal analysis discard any wave pattern developing in the long axis of the lagoon. Finally, one notes the possible overlapping between seiche harmonics in the lagoon or over the reef which increases the complexity of the analysis.

Wave classification

The first step of the data processing is the characterization of the free surface waves observed along the instrumented cross-shore transect. Figure 3 shows that in the short-wave (SW) or so-called "gravity" band (0.04<f<0.3 Hz) which combines swell and wind waves, the average spectrum depicts a swell-dominated environment. Peak periods between 14 and 22s correspond to long south pacific swells reaching the studied reef with a nearly normal incidence. Shorter waves with peak periods around 7s generally correspond to more south-easterly waves. At higher frequencies, the free surface oscillations are mostly wind waves locally generated by trade winds modulated by thermal winds but, further over the reef flat, they can also result from the energy transfer to higher harmonics already observed here [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics: a simple parameterization for coral lagoon circulation model, with application to the Ouano Lagoon, New Caledonia[END_REF] or in other sites [START_REF] Masselink | Field investigation of wave propagation over a bar and the consequent generation of secondary waves[END_REF]. The averaged spectrum of SW envelope over the reef foreshore (thin line in Fig. 3, left) shows that 3-months averaged SW modulations increase with decreasing frequency (as denoted for instance by [START_REF] Van Dongeren | Numerical modeling of low-frequency wave dynamics over a fringing coral reef[END_REF] and reach a peak around 0.0055Hz, i.e. just above the selected IG-VLF limit. Envelope energy remains quite high in the upper VLF band and falls quickly below 0.0015Hz (Fig. 3, right plot). Fig. 3 (right plot) shows a zoom on low frequency for the 3-months averaged spectra. The SW-IG limit is here taken at 0.04Hz as suggested by [START_REF] Van Dongeren | Numerical modeling of low-frequency wave dynamics over a fringing coral reef[END_REF] and well adapted to our long swell dominated environment. For the IG-VLF limit is less straightforward, the 0.005Hz value proposed by Gawehn et al. for fringing reef is depicted in Fig. 3 as a guide. For the reef flat sensors (CP2-CP6), a peak is observed is the VLF band around 0.0015Hz. Below this value, VLF energy on the reef barrier quickly decreases, which can be directly related to the drop of SW envelope energy. However, the 0.0015Hz value also appears in good agreement with the theoretical fundamental reef flat seiching oscillation but further analysis is required to better identify the possible presence of standing wave motions. The energy decreases with increasing frequency and increases again in the IG band after a trough around 0.01Hz. The overall energy decreases significantly when moving into the lagoon and this shoreward attenuation increases with frequency. The inner lagoon is mainly controlled by VLF motions. Specific spectral modulations are observed for CP7 and CP8 sensors. They both show a common peak around 0.0035Hz but CP7 (mid lagoon) also shows the 0.0015Hz peak than the reef sensors whereas CP8 (inner reef) is characterized by a clear energy minimum around 0.0023Hz and is maximal for the lowest part of the VLF band around 0.0009 Hz. This latter may fairly correspond to the anti-node of the fundamental seiching mode of the lagoon considered as a closed basin.

Wave climate

The analysis of surface waves climate in the reef-lagoon system is based here on the data recovered during the month of May, 2016, during which various conditions of waves, tides and sea level have been encountered. Time series for significant wave height for VLF, IG and SW are plotted in Fig. 4 together with 1h and 24h averaged mean sea level over the reef flat (CP5 sensor, top plot).

Let us first focus on the SW transformation depicted in Fig. 4. The results confirm the well-know strong filtering role played by the reef on the incoming swell energy [START_REF] Hardy | Field study of wave attenuation on an offshore coral reef[END_REF]Young 1996, Hearn 1999). The SW amplitude decreases as propagating over the reef barrier due to the combined effects of breaking, frictional dissipation and harmonic transfers. Each of these processes are governed by the water depth over the reef. The reef barrier acts thus as a tide-controlled filter of SW energy: the lower the depth, the stronger the attenuation. Wave breaking occurs in most cases between S4 and CP2, leading to a strong decrease of SW energy. It is only during very calm wave and high tide conditions that wave breaking occurs higher on the reef flat, as already observed by [START_REF] Chevalier | Impact of cross-reef water fluxes on lagoon dynamics: a simple parameterization for coral lagoon circulation model, with application to the Ouano Lagoon, New Caledonia[END_REF].

A series of observations on IG and VLF surface waves dynamics can be performed from Fig. 4. One notes first that overall VLF, IG and SW significant heights are well correlated: strong IG/VLF periods correspond to strong incoming waves. During the very strong swell event on May, 22 (Hs=6m, Tp=16s), the IG and VLF amplitudes reach 1.2 and 0.7m, respectively. An example of free surface elevation recording at CP5 on May, 22, is depicted in Fig. 5. One notes the combination of VLF, IG and SW fluctuations with periods of the order of 10 min, 1-2min and 15s, respectively. The wave pattern is here quite different than the VLF waves measured by [START_REF] Gawehn | Identification and classification of very low frequency waves on a coral reef flat[END_REF] on the Roi-Namur Island fringing reef. VLF waves are here more regular and do not show the bore waveform observed by Gawehn et al. These long oscillations rather appear to act as carrier waves for the higher frequency wave field resulting from the superposition of IG and SW. Figure 4 shows that, when the incoming wave energy is significant (typically Hs>2m), the IG waves energy generally decreases along the reef transect toward the inner lagoon. This process is modulated by the tidal elevation with more energy propagating along the barrier as the tide level rises. This trend is less straightforward when the swell is small and/or the tidal range is large. For the higher tidal ranges (see e.g. the low tides of May 8-11), the reef top is at least partly emerged at low tide. A very strong dissipation of IG waves is observed on the offshore part of the reef flat and nearly no waves are observed after CP3. At high tide, CP2-CP6 sensors show very close IG amplitude indicating a small IG waves dissipation when water depth is high and incoming swell rather moderate. The VLF dynamics is also strongly dependent on the incoming swell amplitude but the effect of tide is quite lower than for IG. The general trend is a general decay along the instrumented transect. However, for the stronger swells (May 14-15 or 22) the VLF energy is nearly steady across the reef barrier (CP2-CP6 sensors). The dependence of IG waves produced on the reef top on the incoming wave field is best predicted by the parameter Hs²Tp measured at the reef foreshore (see Fig. 6,A), similarly to the data obtained by [START_REF] Inch | Observations of nearshore infragravity wave dynamics under high energy swell and wind-wave conditions[END_REF] on dissipative sandy beach. The VLF amplitude over the reef top (Fig. 6,B) show a lower dependence on incoming energy, with more dispersion. The VLF/IG ratio over the reef top is around one third. The same analysis is performed for the inner reef sensor (CP8). The general trend is similar, i.e. more incoming energy produces more IG/VLF within the lagoon, but the role of water depth over the reef top is much more important. This is particularly visible for IG waves at CP8: the higher the free surface over the reef, the stronger the IG wave motions inside the lagoon. This confirms the strong dissipation of IG waves on the reef barrier, modulated by the water depth. As water level over the reef decreases, bottom friction over the reef increases the damping of IG energy produced in the surf zone and further reduces the IG wave propagating within the lagoon. The same tendency is observed for VLF at CP8, although with a larger spread. One notes also that, in a general manner, long waves are strongly attenuated in the lagoon itself. This is particularly true for VLF, for which a minimal threshold of energy flux is necessary to induce clear fluctuations whereas VLF waves over the reef top are observed to rise as soon as the incoming SW energy increases.

An additional observation can be performed on the reef top MWL (CP3) depicted in Fig. 4: the strong wave setup on the reef top, up to 40cm during swell events, which will be involved in the development of significant pressure gradients and currents across the reef barrier [START_REF] Hearn | Wave-breaking hydrodynamics within coral reef systems and the effect of changing relative sea level[END_REF][START_REF] Bonneton | Tidal modulation of wave-setup and waveinduced currents on the Aboré coral reef, New Caledonia[END_REF].

Wave transformation

Figure 7 depicts the IG and VLF transformations along the studied transect. Let us first focus on the IG waves evolution. Between CP2 and CP3, the IG dynamics depends on both incoming SW amplitude and tide elevation. When incoming swell is strong (typically greater than 2.5m at the foreshore), the breaking zone is on the outer reef slope. Consequently most of IG waves are produced well upstream the reef top and a significant dissipation is already observed between CP2 and CP3. This process is dependent on tide, and the dissipation decreases as water level increases. However, in any case of strong swell, the ratio between significant IG heights at CP3 and CP2 remains less than 1. For moderate and small incoming SW, the ratio can reach values up to 1.9. This indicates that, for such moderate wave events, production of IG waves occurs between CP2 and CP3. The tendency is again controlled by the tide: when the still water level lowers, the breaking zone and IG production shift offshore and dissipation process can dominate. Further onshore on the reef flat (CP3 to CP4 and CP4 to CP5), the transformation, which should mainly result from the competition between production and dissipation, is now fully controlled by tidal fluctuations. At low tide, the dissipation is very strong and IG waves amplitude are observed to decrease of more than 90% in 120m between CP3 and CP5). Between mid and high tide, the tidal effect on wave transformation is less intense than between low and high tide and amplification of IG waves can be observed. This non-linear response to tide indicates that for the lower levels, IG are only produced offshore the reef top and the IG dynamics is only controlled by dissipation, while for higher mean water levels, IG production is still active over the reef flat. Between CP5 and CP6, one notes a nearly unimodal dependence of IG transformation on the tide elevation and a systematic damping. This trend confirms the previous observations of the progressive transition from production to dissipation regimes for IG waves across the reef flat. Within the lagoon, i.e. between CP6 and CP8, the tide effect is nearly invisible but a strong damping (about 70%) is observed. Such a damping can be attributed to bottom friction processes, both over the sand bottom and the inner reef of the Ouano lagoon.

For VLF waves, the general trend is generally the same, i.e. an overall decrease from the reef top to the inner reef. The tidal effect is less marked. Between CP2 and CP3 (outer reef flat), the influence of swell previously noticed for IG waves is still valid: for strong swells, the VLF amplitude generally decreases while for smaller swells, strong amplification can be observed. This trend can again be related to the offshore shifting of breaking zone and VLF production during strong swell conditions. However, more importantly, a striking observation for each reef flat sensors is that VLF are amplified in a number of cases. This naturally raises the question of the part of standing or even resonant motions in the VLF band. The question can be also triggered for IG waves, but the first, and consistent, analysis performed herein before only assumes purely propagating waves.

Standing vs progressive wave patterns

To better understand the long wave dynamics, and in particular to discriminate the possible presence of standing wave motions in our system, we use part of the approach proposed by Pomeroy et al. (2012b) and [START_REF] Gawehn | Identification and classification of very low frequency waves on a coral reef flat[END_REF]. The analysis presented in Fig. 8 is first based on the calculation of the magnitude squared coherence between the signal recorded at two distinct sensors:

where Gxy(f) is the cross-spectral density between x and y, and Gxx(f) and Gyy(f) the autospectral density of x and y, respectively. When the coherence is close to one, the signals are in direct relationship. The coherence is computed and averaged over the month of May (Fig. 8,A). Then two specific events (May 15, 16:00 and May 22, 6:00) are selected, with 40 min bursts taken at the same depth over the reef top (0.7m) in two different wave conditions: strong swell (Fig. 8,B) and very strong swell (Fig. 8,C).

As emphasized by Pomeroy et al. (2012b), the coherence can not itself reveal the presence of a standing wave because "... any wave form that propagates across a basin and maintains a (reasonable) consistent form will have a high coherence.". A second indicator is thus used to better understand the relationship between long wave pattern at two different measurement points: the phase lag between the two signals (Fig. 8, D andE). A special attention is paid on the phase lags for the CP2-CP3, CP3-CP4, CP2-CP6 and CP6-CP8. If standing wave motions develop on the reef flat or within the lagoon following the theoretical approach (Sec. 3.1), neighboring sensors CP2-3-4 should be nearly in phase (Lag=0) while CP2-CP6 or CP6-CP8 should be out-of-phase (Lag around ) with strong coherence in each case. Note that the reef flat sensors are not strictly at the limits of the expected seiching domains, so difference to these theoretical values are expected. The month-averaged coherence for the reef flat sensors (CP2 to CP5) in Fig. 8 shows that, as expected, the coherence generally increases as the frequency decreases. For the higher part of the IG band (0.03<f<0.05 Hz) and further for gravity waves, strong wave transformations occur after the reef top as observed on the smaller coherences for CP3-CP4 and CP4-CP5 sensor pairs. In the lower part of the IG band (0.005<f<0.03Hz), the coherence remains high all across the reef flat. It still increases when entering the VLF band for CP3-CP4 and CP4-CP5 pairs but is a bit lower on the outer reef flat slope between CP2 and CP3. The averaged coherence drastically decrease further onshore. Between CP5 and CP6, a moderate coherence is observed mainly for low VLF, while between CP6 and CP8 such peak is hardly visible.

The analysis of the two selected events reveals the importance of the swell energy. For the 2.4m swell depicted in Fig. 8 B and D, strong coherence is observed for CP2-3-4 in both IG and VLF bands. The CP2-CP6 coherence strongly falls when entering the VLF band. This indicates that, even if VLF energy is transmitted toward the lagoon, the wave forms are altered. The phase lags for the same event (Fig. 8,D) show ramps for the reef flat sensor. This linear dependency on frequency indicates the presence of progressive waves in both VLF and IG bands. In the inner lagoon (CP6-CP8), no clear cross-shore wave propagation can be identified. The pattern is drastically different for the 5.2m swell event of May, 22. Coherence for sensors CP2-3-4 is still very strong (even increased). In the VLF band, significant coherence peaks are observed at 0.002-0.003Hz and 0.0005Hz for CP2-6 and CP6-8 sensors pairs. Phase lags show the similar frequency ramp patterns for the reef flat sensors but the frequencies associated to coherence peaks in the VLF reveal: (i) a π/4 phase lag between CP6 and CP8 around 0.0005Hz and, (ii), a lag around 3π/4 between CP2 and CP6 in the 0.0014-0.0019Hz range. The former fairly corresponds to the fundamental seiching mode of the lagoon considered here as a semi-open basin while the latter is in the range of the fundamental mode expected for seiching oscillations on the reef flat domain due to partial wave reflection at the reef barrier boundary. Further analysis is now required to better discriminate such bimodal standing wave pattern and, in particular, to understand their response on incoming wave conditions and tidal fluctuations.

Conclusion

The present study is a part of a large scale experiments performed on the Ouano barrier reef lagoon system, New Caledonia. A long term field campaign has been carried out to monitor the wave transformation from the reef foreshore to the inner lagoon. The measurements, based on a cross-shore network of pressure sensors, revealed the presence of strong long wave dynamics over the reef flat divided into the so-called infragravity (IG) and very low frequency (VLF) waves. The IG/VLF energy produced on the reef is directly related to the incoming swell energy flux. The filtering role played by the reef barrier, through wave breaking and frictional processes, is strongly depend on the water depth: the lower the tide, the stronger the dissipation.

Dedicated analysis has been carried out to identify the possible presence of standing long waves in our system. It appears that, at least for the most energetic event recorded (Hs=4.2m, Tp=15s), two patterns of seiching modes can be observed. The first is associated to the fundamental oscillation of the lagoon, considered as a semi-open basin, in its transverse direction. The second is attached to the reef barrier itself, possibly induced by partial reflection of the long waves by the bathymetric step at the reef/lagoon boundary. Further research work has to be engaged to confirm these observation and better understand the conditions for VLF/IG waves development and attenuation. In addition, a particular attention will be paid on the role of cross-reef currents, which are strong and vertically sheared, on the wave dynamics.

Figure 1 .

 1 Figure 1. The Ouano reef-lagoon system, New Caledonia. Light grey zones indicate most of the reef colonies. Positions of the reef barrier sensors (S4, CP2-CP6) are distorted, precise values are given in Fig. 2.

Figure 2 .

 2 Figure 2. Experimental Setup along the studied cross-shore transect (upper plot is a zoom on the reef barrier area). Inner reef bathymetry is indicative (for X>6000m) as no survey data is available. Note that CP7 sensor is not aligned with other sensors (see Fig. 1)

Figure 3 .

 3 Figure 3. Average wave spectra over the whole acquisition period. Right: SW and envelope spectra deduced from velocity measurements over the outer reef slope (S4, 20min spectra every 3h). Center and right: full spectra for pressure sensors (CP2-CP8, continuous 1h measurements). The right plot is a zoom in the low frequency band.

Figure 4 .

 4 Figure 4. Time series of VLF, IG and SW significant wave amplitude continuously computed over 1h bursts. Incoming swell significant wave height at the reef foreshore (S4) is computed over 20min bursts each 3h. The top plot depicts the Mean Water Level (1h averaged at CP5) and the 24h averaged levels measured at CP3 and CP8.

Figure 5 .

 5 Figure 5. May, 22 swell event, CP3 reef top measurements. Left: free surface elevation from pressure measurements. Right: spectral density of energy.

Figure 6 .

 6 Figure 6. IG and VLF significant wave heights vs SW incoming energy flux for reef top CP3 (A,B) and inner reef CP8 (C,D) sensors for the month of May, 2016. The color levels depict the water depth above the reef top. Note the difference in y-axis for A,B and C,D plots.

Figure 7 .

 7 Figure 7. Long waves transformation along the cross-shore transect. Top and bottom plots are IG and VLF, respectively. Each plot is a ratio of significant wave heights between two consecutive sensors vs the water depth over the reef top. Color levels depict the SW significant height at the foreshore sensor (S4).

Figure 8 .

 8 Figure 8. A: Averaged magnitude-squared coherence for neighboring sensors over the month of May. B and C: Magnitude squared coherence for 40 min bursts on May 15, 16:00 and May, 22 6:00. D and E: Phase lag for 40 min bursts on May 15, 16:00 and May, 22 6:00. For both events, the mean water depth above the reef top is 0.7m. Vertical dashed line on each plot correspond to th IG-VLF threshold.

Acknowledgements

This study was sponsored by the EC2CO OLZO program (CNRS INSU), the OLZO and CROSS-REEF Action Sud (MIO IRD) and the ANR project MORHOC'H (ANR-13-ASTR-0007). The Noumea IRD center and the GLADYS group supported the experimentation. We are grateful to all the contributors involved in this experiment. The authors are particularly indebted to David Varillon, Eric Folcher and Bertrand Bourgeois whose efforts were essential to the deployment and Pascal Douillet for providing free access to bathymetric data.