Sediment dynamics on the outer s helf of the GoL during an onshore storm: an approach based on acoustic glider an d nu mer ical modelling 

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Hydrodynamical conditions

The hydrodynamics of the shelf involves numerous physical processes, with heterogeneous spatiotemporal scales. The Mediterranean Sea is a micro-tidal environment, where tides have small amplitudes, in order of 30 cm in the GoL (Fanget et al., 2013), and associated currents are weak by a few mm s-1 (Carrère et al., 2012). In the GoL, most hydrodynamical processes are generated directly in response to wind forcing. Wind induces stress at the air-sea interface, shearing the water layer and setting it in motion. Depending on the direction, intensity, and duration of the wind, different hydrodynamic processes will be generated, such as barotropic circulation, mesoscale structures, upwellings, and inertial currents. The dimension of the Mediterranean system is presented below before focusing on the shelf circulation scenarios, according to the main winds.

Circulation induced by the NW Mediterranean system

The North Current or Liguro-Provençal Current (LPC) is the northern part of the Mediterranean surface water gyre (cyclonic circulation) of the western basin. This vein of warm water, less salty compared to the open sea waters (Millot, 1990), reaches speeds of several tens of cm s-1 (Millot and Taupier-Letage, 2005) in winter, and spreads in the GoL shelf over a width of several tens of km. In the GoL, this current follows the upper part of the continental slope and is therefore constrained by the bathymetry (Petrenko, 2003).

Circulation induced by offshore winds

Barotropic circulation—Estournel et al. (2001) showed from numerical simulations that the Mistral alone induces an anticyclonic circulation at the shelf scale, which tends to isolate the shelf waters from the slope waters. While Tramontane alone induces a cyclonic circulation only on the western part of the shelf. These winds generate waves of small amplitude and period (<2 m and <6 s), due to the short fetch.
Upwellings—Estournel et al. (2001) showed that Mistral and Tramontane induce coastal currents that export shelf water and prompt, in turn, compensating the inflow of the slope water onto the shelf (upwellings). These offshore winds also lead to upwellings on the coasts of Provence and Hérault, as observed in summer by Millot (1990) (Fig. 2.2), where the surface water is drained to the southwest and the shelf waters are upwelled.
Dense shelf waters—In winter, wintry heat losses as well as evaporation—caused by cold and dry northerly winds—induce cooling and mixing of the shelf waters which may cascade along the slope (Durrieu de Madron et al., 2005; and the references therein) (Fig. 2.3). As reported by Durrieu de Madron et al. (2008), these events are unusual but very important in the export of suspended particulates matter (SPM) from the shelf to the deep ocean.
Mesoscale eddies—Several studies (Forget et al., 2008; Rubio et al., 2009, 2005) have revealed the presence of isolated but recurrent eddies over the GoL shelf. For instance (Rubio et al., 2009) showed an eddy of 25 to 40 km of diameter with a vertical extension of 80–100 m depth, and
Figure 2.3: Schematic diagram depicting the dense shelf water cascading and open sea convection processes across the margin from Puig et al. (2013).
surface velocities around 0.2-0.3 m s-1 over the Catalan shelf (West of the GoL). It would be generated by the detachment to the south of Cap Creus of a coastal current formed during a strong Tramontane (Rubio et al., 2009).

Circulation induced by onshore winds

In fall and winter, intense onshore winds (higher than 10 m s-1) may generate storms. As reported by Ulses et al. (2008a), these intense winds are generally brief (3 days) and rare (6% of the time). However, storms can produce east to southeast significant wave heights of up to 10 m with a period of 12 s (Guizien, 2009), associated with strong near-bottom currents. At the shelf scale, storms induce a cyclonic circulation (Fig. 2.4) and can generate coastal downwelling (surface water sinking due to its accumulation near the coast). This coastal water can then be exported deep into the Cap de Creus Canyon due to the narrowing of the shelf, which is only a few kilometers wide at the cap (Ulses et al., 2008a). In winter, these exports can be enhanced by the presence of dense cold
Figure 2.4: Simulated depth-averaged current field (m s-1) on the storm of 4 December 2003, showing the cyclonic circulation in the Gulf of Lions from Ulses et al. (2008a).
waters on the shelf and the weak stratification of the water column on the slope (Palanques et al., 2006). In the GoL, where tide forcing can be neglected, various observational (Bonnin et al., 2008 ; Bourrin et al., 2015, 2008b, 2008a ; Ferré et al., 2005 ; Guillén et al., 2006 ; Martín et al., 2013 ; Ogston et al., 2008 ; Palanques et al., 2008, 2006) and modeling (Dufois et al., 2008 ; Ferré et al., 2008 ; Ulses et al., 2008b) studies emphasized the role of east, southeast storms induced by onshore winds in the resuspension and redistribution of the shelf sediments. These marine storms may also be combined with river floods due to the transport of humidity over coastal relief (Pyrenees, Massif Central, Alps) which induces high precipitation.

Rivers influence

In the GoL, riverine inputs can be separated into two main categories: (i) the seasonal inflow from the Rhone River, and (ii) the inflow from the coastal rivers with torrential character.
The Rhone River—is one of the largest rivers in terms of liquid and solid contributions to the Mediterranean Sea, along with the Po in Italy and, to a lesser extent, the Ebro in Spain, as reported by Bourrin (2007) in Table 2.1, and confirmed by the recent update of Sadaoui et al. (2018). The Rhone River (Fig. 2.5b) presents a highly seasonal variability (mean annual discharge of 1,700 m3 s-1) with maximum discharges observed in autumn and winter, during which the largest floods generally occur, or during the melting snow period in spring. The river supplies 80% of the sedimentary input to the GoL (Bourrin and Durrieu de Madron, 2006 ; Courp and Monaco, 1990), with an average annual particle flux of 8 +/- 4.5 × 106 t y-1 (1977–2013 period) (Sadaoui et al., 2016). The contribution of sediments during a flood event is estimated at 70% (Pont, 1996), highlighting the role of these episodic events in the SPM delivery to the shelf.
The Coastal Rivers—of the GoL (i.e., from east to west: Vidourle, Lez, Hérault, Orb, Aude, Agly, Têt, and Tech [Fig. 2.5a]), have a high seasonal variability and their functioning is close to the North African Wadis. These small mountainous rivers present low discharges during summer and large flood events in winter, fall, and also spring season (Serrat et al., 2001). Most of the sedimentary material of these coastal rivers is therefore brought during episodic events such as floods. For instance, Bourrin et al. (2008b) estimated, during a 5-year flood, to ~25 and ~75% the fraction of sands and clays/silts in the total year SPM delivered by small rivers to the GoL shelf.

Morpho-bathymetry and sedimentary properties

The GoL is composed of different sedimentary units that we will name by their location: inner-, mid- and outer-shelf. The main part of the sediments brought by the rivers is stored, sometimes temporarily, on the GoL sediment units.
The inner shelf (~20 m) is mainly composed of coastal sands (Fig. 2.6). This material is mobilized during storm events and moved along the longshore drift. There are several cells with different drift directions along the GoL coastline which depend on the angle of incidence of the main waves with the coast. The main cells and directions of these longshore drifts are well known and have been established by bathymetric methods (Certain, 2002) or radioactive tracing (Courtois and Monaco, 1969).
The mid shelf is mainly composed of a mud belt from silt to clay (Fig. 2.6). This sedimentary unit is located between 30 and 90 m depth and corresponds to the average wave action limit (Jago and Barusseau, 1981). This median mudflat is directly connected to the Rhône pro-delta but detached from the prodeltas of other coastal rivers (Fig. 2.6).
Figure 2.6: Morphological and bathymetric map of the Gulf of Lions’ shelf, from Bourrin, (2007). This map compiled several sedimentary studies carried out during past decades (Aloısi, 1986; Got, 1973; Monaco, 1971).
The outer shelf (>90 m) is mainly composed of muddy sands, which are homogeneous and bioturbated (Bassetti et al., 2006). The sandy fraction corresponds to relict “offshore sands”, which cover many continental shelves around the world, at water depths generally between 80 and 120 m (Emery, 1968). These sediments represent littoral relict formations from the last eustatic low stage that were reworked during the first phase of the eustatic sea-level rise (Berne et al., 1998; Perez Belmonte, 2003). The muddy fraction has a more recent origin and is mainly sourced from the Rhone River. In the GoL, sand ripples blanketed with mud may be remobilized from the outer-shelf and supply sediments to canyon heads (Gaudin et al., 2006).

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Suspended particle dynamics

In the GoL, the SPM is distributed in nepheloid layers, whose characteristics (concentration, extent, thickness) depend on the river discharge and the physical forcings (stratification, wind, currents and waves) affecting the coastal zone.
During flood events of the Rhone River, the delivery of particles to the shelf is strongly enhanced, until 70% of the total solid discharge (Pont, 1996). These particles are dispersed by the Rhone River plume in the coastal zone. The Rhone River plume responds to the wind forcing (~5-10 h according to Demarcq and Wald (1984)) and is advected through the shelf by the surface currents enhanced by continental (i.e. north-northwesterly) wind or along the coast during marine (i.e. east-southeasterly) wind (Broche et al., 1998; Estournel et al., 2001; Forget and Ouillon, 1998; Naudin et al., 1997; Ody et al., 2016).
Over the shelf, deposited surface sediments are resuspended by waves and currents and dispersed by bottom currents. Several studies highlight the presence of a bottom nepheloid layer (1-5 mg L-1) up to 15 m thick over the shelf (Aloisi et al., 1979; Durrieu de Madron et al., 1990; Durrieu de Madron and Panouse, 1996). This nepheloid layer highly participates to the particulate transport over the shelf during all year. Marine storm events have been described as a principal factor producing sediment resuspension and redistribution overt the shelf (Dufois, 2008; Ferré et al., 2008; Ulses et al., 2008a,b) (Fig. 2.7). The sediment erosion induced by waves and strong currents, which can reach several centimeters, enhances the SPM concentration (>30 mg L-1 ) in the water column (Ferré et al., 2005; Guillén et al., 2006; Bourrin al., 2008b). Also, the SPM dynamics is accentuated by the cyclonic circulation (∼ 50-70 cm s-1 close to the coast), which favors the suspended particle transport along the shelf (see red arrow on the map in Fig. 2.7).
Over the SW part of the shelf, the export of suspended particles offshelf occurs by downwelling through the Cap de Creus and Lacaze-Duthier submarine canyons as well as by bypassing the Cap de Creus to the Catalan shelf (Palanques et al., 2006, 2008; Bonnin et al., 2008; Martín et al., 2013).
Figure 2.7: Sediment budget of the GoL shelf. The morphology of the sediment is specified. The red rectangles highlight the main areas of SPM source (Rhone River ROFI) and export (Catalan shelf).

Historical observations of hydro-sedimentary processes

In section 2.2, we defined the main hydro-sedimentary characteristics of the Gulf of Lions and their dynamics. The knowledge of the GoL’s dynamics is intimately linked to our ability to accumulate physical, sedimentological, and geological measurements. Main observations carried out in the GoL are resumed in Table 2.2 and 2.3 for hydrodynamic and sediment dynamics respectively, as a function of their programmatic context. As on a larger scale (see Chapter 1.), the GoL is experiencing an increase in measurements at different spatiotemporal scales, thanks to technological developments. Indeed, until 2015, most in situ observations were gathered at few fixed locations over one or some components of the shelf (inner-, mid-, outer-, and shelf break) (Table 2.2 and 2.3). Since 2015, autonomous underwater gliders have been used in several projects (CASCADE, TUCPA, MATUGLI, CHIFRE), especially for the study of the SPM dynamics in the coastal zone during extreme events (Table 2.3). Over the last decade, the use of a holistic approach in projects has been developed, thanks to technological advances, through a multi-platform strategy (mooring, research vessel, satellite, glider, model) to better understand and quantify sediment dynamics in the Gulf of Lions (Durrieu de Madron et al., 2008 ; Many, 2016 ; Weaver et al., 2006).

Datasets of the thesis work

Section 2.3 shows that the evolution of technologies and the robotization of instruments have allowed the use of gliders to improve knowledge of hydro-sedimentary dynamics in the GoL. However, the rapid evolution of technologies with the integration of new sensors such as the ADCP onto underwater gliders needs to be investigated. The objective of this thesis is to develop the processing chain and to explore the capability of a glider newly equipped with an acoustic Doppler current profiler to study hydro-sedimentary processes, particularly during extreme events.
The glider-ADCP data acquired during two experiments conducted respectively in 2016 and 2017 at the level of the Rhone plume (Site 1; Fig. 2.8a) and in 2018 on the continental shelf (Site 2; Fig. 2.8a) of the GoL, were analyzed by combining them with other data from traditional platforms (coastal buoys, ships, satellites) or numerical simulations (Table 2.4). Figure 2.8b-d shows glider deployments carried out during these two experiments and Table 2.4 summarizes the platforms used according to the study sites. The observations acquired by these different platforms and their processing will be detailed in chapters 3, 4, and 5.

Table of contents :

Chapter1. General i ntroduction
Chapter2 Regional Settings: The Gulf of Lions
Chapter3 Glider ADCP toolbox: a MATLAB toolbox for processing active acoustic d ata onto underwater gliders
Chapter 4 Glider based active acoustic monitoring of currents and turbidity in the coastal zone
Chapter 5 Sediment dynamics on the outer s helf of the GoL during an onshore storm: an approach based on acoustic glider an d nu mer ical modelling
Chapter 6 General conclusion


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