PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE 10.1002/2014JC010311 Key Points: Nontidal flow structure is different in each channel of the ebb-tidal delta Nontidal flow attributed to tidal residual circulation and densitydriven flows Wind forcing modifies the basic flow structure Supporting Information: Readme Derivations
Correspondence to: P. Salles,
[email protected] Citation: Salles, P., A. Valle-Levinson, A. Sottolichio, and N. Senechal (2015), Wind-driven modifications to the residual circulation in an ebb-tidal delta: Arcachon Lagoon, Southwestern France, J. Geophys. Res. Oceans, 120, 728–740, doi:10.1002/2014JC010311. Received 16 JUL 2014 Accepted 5 JAN 2015 Accepted article online 8 JAN 2015 Published online 10 FEB 2015
Wind-driven modifications to the residual circulation in an ebb-tidal delta: Arcachon Lagoon, Southwestern France Paulo Salles1, Arnoldo Valle-Levinson2, Aldo Sottolichio3, and Nadia Senechal3 1 Laboratorio de Ingenierıa y Procesos Costeros, Instituto de Ingenierıa, Universidad Nacional Aut onoma de M exico, Sisal, Yucatan, Mexico, 2Civil and Coastal Engineering Department, University of Florida, Gainesville, Florida, USA, 3EPOC Laboratory, Universit e de Bordeaux, UMR CNRS 5805—OASU, Pessac, France
Abstract A combination of observations and analytical solutions was used to determine the modifications caused by wind forcing on the residual or nontidal circulation in an ebb-tidal delta. Observations were obtained in the lower Arcachon Lagoon, southwestern France. The basic nontidal 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: North Pass (>9 m) and South Pass (>20 m). The basic nontidal 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 nontidal 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.
1. Introduction 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., Davis and Hayes, 1984; Liria et al., 2009; de Swart and Zimmermann, 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 nontidal flow structures across channels formed by an ebb-tidal delta, and to elucidate the dynamics responsible for such flow structures. This objective shall help elucidate the processes responsible for the transport and growth of toxic algae in Arcachon Lagoon. Algal blooms have caused closures of the economically important oyster industry in the region [Batifoulier at al., 2013]. The objective mentioned above 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. 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.
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Figure 1. Study area, Arcachon Lagoon, in southwestern France showing (a) its general morphology and location; and (b) bathymetry, the site for moored instrument deployments at South Pass and North Pass (white dots), and the ADCP transect (white line) of underway sampling across both passes.
2. Study Area Arcachon Lagoon is located in the southwestern coast of France, on the southeastern portion of the Bay of Biscay in the Aquitaine region (Figure 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, with a mean depth of around 4.5 m, exhibits intertidal flats incised by a complex network of secondary channels in the northern region, and a well-defined and irregularly shaped ebb-tidal delta south of Cap Ferret (Figure 1). In particular, the ebb delta (12 km long; 8 km wide; 17 km2 surface area) extends approximately 1.5 km offshore and is cut by two channels that are dominated by tidal forcing and separated by an elongated (5 km long) supratidal bank, Arguin Bank. Surrounding shoals, a sandbar 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 [Cayocca, 2001]. Tidal currents reach different speeds throughout the lagoon but the strongest currents can be observed at the North Pass, which surpass 2 m s21 in spring tides. At South Pass, the tidal currents are typically 70–80% those at the North Pass. Winds exhibit a seasonal signal, the strongest being frequently >10 m s21 from the West and North in winter. Spring and autumn winds can be close to 10 m s21 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 m3 s21 in February and the minimum is 8.4 m3 s21 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 m3 s21, 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 welldeveloped 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].
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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 channel and periods with two channels (current configuration) at periodicities of decades. Gassiat [1989] proposed first a cyclicity of roughly 80 years, which has been used as a reference but not yet confirmed by more recent studies. Connection with the ocean seems to be maintained by an energetic wave regime and by the volume of the tidal prism of (3.5 3 108) m3, which keep the delta flushed [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.
3. Approach 3.1. Data Collection 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 provided 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 (Figure 1) during mooring deployments from 9 January to 2 March 2007. The meteorological station is maintained by Meteo 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 s21 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 Regionales de l’Environnement, de l’Amenagement 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 (Figure 1). A Teledyne RD Instruments 614.4 kHz Workhorse acoustic Doppler current profiler (ADCP) was mounted on the bottom at 44 33.30 N, 1 14.90 W, in the South Pass, over a mean depth of approximately 20 m. The instrument recorded averages of 150 pings every 10 min 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 GMT11 15 January 2007 to 09:00 on 1 March 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 44 34.40 N, 1 16.20 W over a mean depth of 9 m. It recorded averages over 2 min every 10 min with a vertical resolution of 0.5 m. Usable data were collected from 13:00 GMT11 11 January 2007 to 14:00 on 15 February 2007, when the instrument’s batteries died. 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 17 and 18 June 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 (Figure 1) was repeated 32 times over a full semidiurnal tidal cycle, on consecutive days, in order to separate tidal from nontidal 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 s with a vertical resolution of 0.5 m, while steaming at typical speeds of 1.5 m s21. 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 conductivitytemperature-depth (CTD) probe, model MPx manufactured by NKE Instrumentation in France. The probe has an accuracy of 0.05 C for temperature,
