Wind-driven modifications to the residual circulation in an ebb tidal delta:
Arcachon Lagoon, Southwestern France
Paulo Salles Afonso de Almeida1*, Arnoldo Valle-Levinson2, Aldo Sottolichio3, Nadia Senechal3
1Instituto de Ingeniería, Universidad Nacional Autónoma de México, Sisal, México
2Civil and Coastal Engineering Department, University of Florida, Gainesville, Florida
3 Université de Bordeaux, UMR CNRS 5805 EPOC – OASU, Pessac Cedex, France
A combination of observations and analytical solutions were used to determine the modifications caused by wind forcing on the residual circulation in an ebb tidal delta. Observations were obtained in the lower Arcachon Lagoon, southwestern France. The basic residual or non-tidal circulation was established with acoustic Doppler current profilers (ADCPs) that were i) moored in the delta’s two deepest channels, and ii) towed along a cross-lagoon transect. The bathymetry of the lower lagoon, or ebb tidal delta, featured two channels that exceeded 15 m: North Pass and South Pass. The basic residual circulation consisted of mostly inflow with weak surface outflow in the South Pass, and laterally sheared bidirectional flow, dominated by outflow, in the North Pass. Analytical solutions and comparison of observed dynamical terms suggested that, in addition to the conventionally accepted influence of tidal nonlinearities, density gradients contributed to the basic residual circulation in the lagoon. Observations also indicated that wind forcing altered the basic circulation by driving simultaneous upwind flows in both passes. This response was supported by an analytical solution to wind-driven flows over the bathymetry of the transect sampled. The response to seaward winds was to enhance inflow in South Pass and reduce outflow in North Pass. Landward winds caused diminished inflow in South Pass and increased outflow in North Pass.
Ebb tidal deltas are shaped by tidal currents in the lower part of lagoons or estuaries dominated by tides and are found throughout the world (e.g. Donda et al., 2008; Fitzgerald et al., 2004; Morales et al., 2001). Ebb tidal deltas tend to have reduced morphological influence from river discharge and wave action (e.g. Finley, 1978). However, there are instances in which both waves and tides influence an ebb tidal delta, causing a mixed wave-tidal energy regime (e.g. Davies and Hayes, 1984; Liria et al., 2009). Other combinations in which river input is relevant can also develop. Most of the body of research on these systems has been focused in their morphology and stratigraphy (e.g. Hicks and Hume, 1997; Sha and Van den Berg, 1993; Imperato and Sexton, 1988), and even in their morphodynamic evolution (e.g. Elias and van der Spek, 2006; van Leeuwen et al., 2003; Fontolan et al., 2007). Fewer efforts have concentrated on the spatial structure of residual, or nontidal, flows and the hydrodynamics associated with such ebb-tidal delta bays and lagoons at particular times of the morphodynamic evolution. The objective of this investigation was to determine the non-tidal flow structures across channels formed by an ebb tidal delta, and to elucidate the dynamics responsible for such structures. The objective was addressed with observations in Arcachon Lagoon in southwestern France and with application of theoretical concepts that helped explain the flow structures observed.
In the case of Arcachon Lagoon, present-day ebb-tidal delta was formed by reshaping of sand by tidal currents and waves (Cayocca, 2001). However, the delta has changed its shape from having one channel to having two channels with periodicities on the order of decades (~80 years). To understand such relatively fast changes in delta morphology, it is essential first to elucidate the flow structures associated with the delta. This represents the main motivation for the present investigation.
Arcachon Lagoon is located in the southwestern coast of France, on the southeastern portion of the Bay of Biscay in the Aquitaine region (Fig. 1). It has a high-tide surface area between 160 (Cayocca, 2001) and 174 (Plus et al., 2009) km2 and a low-tide area between 40 and 50 km2. The lagoon exhibits a well-defined ebb tidal delta south of Cap Ferret (Fig. 1). The mean depth of the lagoon is around 4.5 m. In particular, the ebb delta is cut by two channels (>15 m deep) that are dominated by tidal forcing and that are separated by an elongated (~5 km long) bank, Arguin Bank. Surrounding shoals, to the west of the delta channels, are influenced by waves that can reach >5 m height in winter storms. The ebb delta tidal channels are called North Pass and South Pass because of their distribution at the connection between the lagoon and the ocean. Inside the lagoon, these channels change their orientation in such a way that they are located to the ‘west’ and to the ‘east.’
Tides in the lagoon are predominantly semidiurnal with ranges that can exceed 4.5 m at spring tides and that are typically 0.8 m at neap tides. Tidal currents reach different speeds throughout the lagoon but the strongest currents can be observed at the North Pass, which surpass 2 ms-1 in spring tides. At South Pass, the tidal currents are typically 70 to 80% those at the North Pass. Winds exhibit a seasonal signal, the strongest being frequently > 10 ms-1 from the West and North in winter. Spring and Autumn winds can be close to 10 ms-1 blowing from the northwest or southwest. Summer winds are variable and appreciably weaker. The main sources of buoyancy to the lagoon come from the Leyre River and the Porges Canal. The maximum monthly mean discharge for the Leyre is 38.6 m3s-1 in February and the minimum is 8.4 m3s-1 in August (Plus et al., 2009). For the Porges, the maxima and minima occur in the same months with values of 12.9 and 1.8 m3s-1, respectively. Consistent with wind forcing, swells show maximum heights in winter and minimum in the summer with an annual mean significant height of 1.4 m and a period of 6.5 s (Butel et al., 2002). Because of the well-developed sandbar (continuation of Cap Ferret) and the ebb-tidal delta, swells do not propagate inside the lagoon (Salles et al., 2008). These morphological features cause the outer inlet to be saturated with wave breaking (Senechal et al., 2013).
The lower lagoon has undergone dramatic morphological changes in the last 300 years at the delta region (Cayocca, 2001). It has exhibited periods with one pass and two passes (current configuration) at periodicities of roughly 80 years (Gassiat, 1989). Connection with the ocean seems to be maintained by the tidal prism of 3.5108 m3 and the energetic wave regime (Cayocca, 2001). Despite the morphodynamic complexity of the lagoon, basic descriptions of circulation are missing. It is therefore essential to understand flow conditions associated with present morphology in order to be able to infer future changes. Such is the main thrust of this investigation.
The objective of elucidating the flow structure across the two channels of an ebb tidal delta was determined with a combination of moored and towed current profilers. Moored profilers provided data to describe temporal variations, and a towed profiler furnished information on the spatial structure. Wind data were used to aid in the interpretation of the moored instrument records. Hydrographic profiles (temperature and salinity) and river discharge data were used to help propose mechanisms responsible for the flow structures observed.
Wind data were obtained at the Cap Ferret station (Fig. 1) from January 9th to March 2nd, 2007. The meteorological station is maintained by Météo France, the French National Meteorological Service, with data being collected at 10 m height. Wind speed and directions values were available at intervals of 3 h. Daily river discharge values in m3/s were obtained at Leyre River station during the same period as wind measurements. This station is maintained by the Public Regional Service of Environment and Development (Directions Régionales de l’Environnement, de l’Aménagement et du Logement, DREAL).
Two acoustic Doppler current meters were deployed at each one of the two tidal channels of the ebb delta at the entrance to the lagoon (Fig. 1). A Teledyne RD Instruments 614.4 kHz Workhorse acoustic Doppler current profiler (ADCP) was mounted on the bottom at 44º 33.3’N, 1º 14.9’ W, in the South Pass, over a mean depth of approximately 20 m. The instrument recorded averages of 150 pings every 10 minutes with a vertical resolution of 0.5 m. The first bin of data was at 1.61 m from the transducers, or ~2 m from the bottom. Usable current velocity data were collected from 14:40 GMT+1 January 15th, 2007 to 09:00 on March 1st, 2007. A 1000 kHz Nortek AWAC was deployed in the North Pass to complement measurements in the South Pass. This instrument was bottom-mounted at 44º 34.4’N, 1º 16.2’W over a mean depth of 16 m. It recorded averages over 2 minutes every 10 minutes with a vertical resolution of 0.5 m. Usable data were collected from 13:00 GMT+1 January 11th, 2007 to 14:00 on February 15th, 2007.
Two consecutive-day 12.5 h experiments were carried out in order to place the results of the moored instruments in the context of spatial variability. Experiments were carried out on June 17 and 18, 2014 and consisted of towing a Teledyne RD Instruments 1228.8 kHz ADCP with bottom-tracking capability. The ADCP was mounted on a pole that was attached to a 7 m boat. One cross-channel transect (Fig. 1) was repeated 32 times over a full semidiurnal tidal cycle in order to separate tidal from non-tidal signals. The ADCP transducers were at roughly 0.2 m from the surface, which allowed the first usable bin to be centered at 0.8 m from the surface. Velocity profiles were recorded every 0.45 seconds with a vertical resolution of 0.5 m, while steaming at typical speeds of 1.5 ms-1. The sampling transect covered the entirety of the North (west) Pass and South (east) Pass, which exhibited a maximum depth of ~20 m at high tide. Additionally, profiles of temperature, salinity and density were measured with a conductivity-temperature-depth (CTD) probe, model MPx manufactured by NKE Instrumentation in France. The probe has an accuracy of 0.05ºC for temperature, <0.05 mS/cm for conductivity, and < 0.3 m for depth. Profiles were recorded over each channel only at the end of flood and end of ebb for a total of 4 profiles per each of the 2 experiments.
Between 2007 and 2014, the morphology of the channels and shoals have experienced small changes, as verified by satellite imagery and bathymetric surveys (Capo et al., in revison). Therefore, data from moored and towed current profilers were comparable and representative of the same configuration of the ebb delta.
Wind velocity time series were low-pass filtered with a 34-h Lanczos window to eliminate diurnal variability. Velocity profiles and water levels from moored instruments were treated with the same filter to remove tidal variations. Filtering was effected after depurating current velocity data that were close to the surface (side-lobe contamination). In both current profiler records, the criterion was to eliminate data above 0.9 of the water column. Moreover, water velocity data were rotated to their axis of maximum variance, reducing the transverse flow component. The rotation angles were clockwise, 36º for South Pass and 21º for North Pass. These angles aligned with the ‘x’ axis and already considered the magnetic declination, which was close to 1º W in 2007.
Towed ADCP data were first averaged in ensembles of 20 profiles, yielding one profile every 9 seconds or roughly every 15 m along the track. Data thus averaged were analyzed following the procedures outlined in Valle-Levinson and Atkinson (1999). A synthesis of the procedure consists of identifying each transect repetition separately and correcting the compass performance of the ADCP with the method of Joyce (1989). After current velocity data were corrected for compass bias they were gridded onto a uniform grid. Gridding was also done for time and acoustic backscatter of each transect repetition. Columns of the uniform grid represented distance from a common origin, while rows indicated depth from the surface. A total of 32 uniform grids represented each of the transect repetitions. There were time series of 32 elements at each grid point. Time series were then fit, via least-squares, to a residual contribution plus two harmonics (semidiurnal and fourth-diurnal). The residual contribution illustrated the spatial structure of the mean flow, after a tidal cycle, throughout the transect sampled. The amplitude and phase of each harmonic indicated their spatial variability and the bathymetric influences.
1. Moored current profiles
Wind forcing during mooring deployment was dominated by northeastward winds (Fig. 2a). These winds exceeded instantaneous values of 10 m/s in three pulses on January 20-22, February 9-14 (days 39-44), and February 25-27 (days 55-57), 2007. One energetic (~10 m/s) pulse of southwestward winds interrupted the predominant northeastward winds on January 23-25, 2007. Because of the orientation of the lagoon, northeastward winds caused water level set-up (relative to mean depth) toward the lagoon’s head (Fig. 2b). The opposite was expected for southwestward winds. Wind forcing during towed ADCP experiments of June 2014 (not shown) was <5 m/s and variable, so it caused negligible effects on the flows observed during the 12.5 h experiments.
Leyre river discharge increased from 8 m3s-1, at the beginning of the moored instruments sampling period, to around 40 m3s-1 at the end (Fig. 2a). Discharge remained relatively constant from the beginning through day 38 (February 7th). Two river pulses caused an increasing trend throughout the rest of the observation period. The first pulse was centered on day 46 (February 15th) and the second pulse showed the maximum at the end of the observations. This maximum exceeded 50 m3s-1 on March 5-7th, after the instruments were retrieved from their mooring locations.
Water level measured at different locations in the lagoon (only those at North and South Pass are included in Fig. 2b) showed 3 periods of spring tides and 3 of neap tides. Tidal water levels at both passes were essentially the same but subtidal (low-pass filtered) water levels were different. This difference is suspicious and could have been caused by different response from the pressure sensor of the two different current profilers. Nonetheless, subtidal water levels at both locations displayed increased subtidal levels (set-up) associated with northeastward winds. Set-down, or decreased subtidal water level, was observed with southwestward winds.
Subtidal along-channel velocity profiles in the South Pass showed a general tendency for non-tidal inflow throughout the water column (mainly positive values in Fig. 2c). There were a few periods in which the inflow reversed to outflow. These periods of negative subtidal flow, at least in part of the water column, appeared on days 19-23, 38, 40-44, 51, and 55-57. In contrast, subtidal flows in the North Pass displayed outflow throughout the water column during the entire period of observations (only negative values in Fig. 2d). Non-tidal outflows weakened in the period of days 26-31, which corresponded to increased inflow in South Pass. Comparatively, the periods of strongest outflow in North Pass (days 19-23 and 38-44, Fig. 2d) coincided with reversal of flow, from inflow to outflow, at South Pass. It was evident then that outflow in the North Pass and inflow in the South Pass could be modulated by wind. Strongest outflow in North Pass coincided with reversed flow in South Pass at the times of northward wind. Similarly, weakest outflow in North Pass concurred with strongest inflow in South Pass at times of southward wind. This response seemed paradoxical as non-tidal flows at the measurement sites were in opposite direction to wind. This apparent paradox is explored further in the Discussion section. The pattern of exchange flow suggested by moored observations is synthesized then as non-tidal inflow developing in the South Pass and non-tidal outflow in the North Pass. A question that arose from these observations was whether the profiles measured at one point in each pass were representative across the entire pass. Results of the towed ADCP experiments were then analyzed to answer this question.
2. Towed current profiles
Towed ADCP data were presented as non-tidal or residual fields and tidal fields. The residual flows showed rich spatial structure that was affected by bathymetry. In general, residual flows displayed non-tidal inflow in the South (east) Pass and non-tidal outflow in the North (west) Pass (Fig. 3). Such flow structure was remarkably similar from one day to the next and was consistent with that indicated by moored results. However, a detailed inspection of this structure revealed additional complexities. The North Pass showed non-tidal inflow of up to 0.15 m s-1 in addition to the non-tidal outflow of up to 0.45 m s-1. Outflow dominated the western portion of the channel, and non-tidal inflow appeared on the eastern part (Fig. 3). The outflow-to-inflow partition depth was close to the mean depth over the entire section. In addition, non-tidal inflow dominated South Pass with values of 0.05 to 0.07 m s-1 but near-surface net outflow, albeit weak (<0.03 m s-1), also developed toward the eastern portion of the channel. The rest of the cross-section, between the two passes, was influenced by essentially non-tidal inflow of around 0.05 m s-1 but that was mainly directed across the channel. Integrated transports into and out of the lagoon were close to 550 m3 s-1 on June 17th and around 630 m3 s-1 on June 18th. These transports were estimated after reducing the cross-channel transports that artificially increased landward transports (from direct integration of positive flows) by nearly 300 m3 s-1. The flow distributions of Figure 3 could have been driven by tidal non-linear processes (tidal rectification) or by density gradients. Tidal rectification was diagnosed with the distribution of tidal current amplitudes and generation of overtides. Density gradient effects were established through hydrographic profiles measured in the passes.