1. ARCHITECTURE AND SEDIMENT PROCESSES ON THE FRENCH ATLANTIC CONTINENTAL SLOPE

The very high-resolution seismic data acquired on the `Tas de Pois' and `Beaugé' interfluves during the GITAN cruise allowed for the first time the recognition of four seismic units (U4, U5, U6 et U6b), all separated from the others by erosional boundaries (Figure 3). Calypso long-piston cores indicate a Miocene (sedimentary facies 1) to (Late) Pleistocene age (sedimentary facies 2 and 3) of these units (Figure 4). Their recognition of both the `Tas de Pois' and `Beaugé' interfluves highlight the regional significance of these units and suggest, by extension, similar sedimentary processes occurring along the French Atlantic continental slope.

Figure 3: Example of very high-resolution SYSIF data acquired on the `Beaugé' interfluve during the GITAN cruise. These data allowed for the recognition of four main seismic units. The yellow circles on the bathymetric map indicate the location of cores GIT-CS04 and GIT-CS05 (also visible on Figure 1). The lithology and stratigraphy of these cores is detailed in Figure 4 (Sabine, 2016; Sabine et al., in prep.).

The detailed observation of both the seismic units (Figure 3) and the sedimentary cores (Figure 4) indicates that the sedimentary architecture here described likely originates from strong hydrodynamical processes, including contour currents, i.e. bottom thermohaline current that flows parallel to the slope of the continental margin. As a result, we described the sedimentary units described on the French Atlantic continental slope as contourites, i.e. sediments deposited in the deep sea by contour currents (see Rebesco et al., 2014 for a thorough review). These sedimentary deposits in the Bay of Biscay are a valuable tools for paleoclimatic reconstructions (Toucanne et al., 2021). Based on a thorough comparison with the seismic data available in the Bay of Biscay, the sedimentary units described onto the `Tas de Pois' and `Beaugé' interfluves have been linked to the ones previously recognized on the continental shelf by Paquet et al. (2010). This provides for the first time a complete picture of the Neogene-Quaternary sedimentary architecture of the French Atlantic margin, from the continental shelf (<120 m water depth) to the deep slope (> 2000 m) (Sabine, 2016; Sabine et al., in prep.).

Figure 4: Sedimentology and stratigraphy of the GITAN cores (Sabine, 2016; Sabine et al., in prep.).

Interestingly, submarine slide scars described onto the `Tas de Pois' and `Beaugé' (Figures 1 and 3) interfluves are observed within the Neogene-Quaternary contourites detailed above. Detailed observations of the available seismic data indicate a polyphased history of these submarine slide scars, with an initiation during the Neogene and the Miocene in particular (23-5 Ma). Late Pleistocene submarine slides are observed in these (larger) slides scars of Miocene age. They are dated from ~18 ka (see cores GIT-CS01 and GIT-CS05; Figure 4) and likely occurred in response to the last occurrence of the Channel River activity and associated high sediment discharges (Toucanne et al., 2010). Nevertheless, these submarine slides are minor / anecdotal in comparison to the ones dating from the Neogene. This means that the morphology of the modern continental slope inherited from a long geological history and is not related to the last glacial period and associated lowstand conditions of the sea-level and high river runoff, expected to favor gravity processes and massive landslides in particular. On the basis on these results, we assume that submarine mass-movements on the French Atlantic continental slope do not represent a significant geological hazard, at least with regard to the present-day environmental conditions. In the ongoing climate changing conditions, predisposing factors could be activated in the future. Even if there is no evidence of ongoing new processes on the margin, it is wiser to consider in future research opportunities that sea-level changes and temperature rising might activate new predisposing processes for geohazard on the French Atlantic slope.

In this topic, recent works allowed the correlation between weather/storm-controlled near-inertial oscillations (up to 0.4 m.s^-1 over 3-13 days periods) and the triggering of sediment gravity flows in the Cap Breton canyon. Sedimentary deposits from these events were found in the GITAN interface cores. All these results reveal that sediment transfer in the Cap Breton canyon, and more largely in the Bay of Biscay, are related to an overall set of hydro-sedimentary processes (Guiastrennec-Faugas, 2020; Guiastrennec et al., soumis).

2. GEOTECHNICS AND RHEOLOGY

Geotechnical characterisation of sediments of the French Atlantic margin was based on both geotechnical and rheological analysis of samples from four long-piston cores (GIT-CS02, GIT-CS04, GIT-CS06 and ESSK16-KS08) collected during the GITAN and ESSK16-ATA Ifremer cruises. Geotechnical and rheological laboratory tests were carried out to better understand sediment history and mechanical behaviour. Based on a stratigraphic and sedimentological analysis (see above), the main geotechnical and rheological features have been presented to propose a characterisation of the geomechanical properties of the Bay of Biscay sediments on the continental slope, in order to characterise sediment flow initiation and propagation.

2.1 Geotechnical analyses

A series of laboratory tests has been carried out to compare the geotechnical properties of sediments of the different facies identified from the sedimentary analysis (see above). This investigation focused on the identification of trends in sediment concentration, plasticity, compressibility, vane shear strength and friction angle (Table 5).

Table 5: Geotechnical characteristics of the main sedimentary facies repertoried on the French Atlantic continental slope.

This allowed pointing out that the sediment of facies 1 (Figure 4) mainly differs from the sediment of the two other facies by being of high plasticity and high compressibility, which explains why its strength tends to increase rapidly with depth below seabed. The sediment concentration and the friction angle alone did not provide unequivocal criteria to distinguish sediments of the different facies. However, visible correlations between sediment concentration and vane shear strength values allowed to distinguish two different trends within facies 2 (Figure 4). An overall linear increase in concentration and strength with depth characterizes the clay-dominated facies 2a. This contrasts with the high frequency fluctuations in concentration and strength ascribed to the presence of coarser sediments in facies 2b. The sediment in facies 3 (Figure 4) is strikingly different from the others as its strength and compressibility are anomalously high while its concentration is low.

2.2 Rheological analyses

A series of laboratory tests were carried out (using an Anton Paar MCR 301 Rheometer) to characterize the mechanical properties of sediment samples during the initiation and evolution of flow, which depends on sediment concentration, stress, deformation state, and constitutive material characteristics. In order to access these relationships and provide a more generic understanding (i.e. not test dependent), three types of tests were carried out: oscillatory, strain controlled and stress controlled tests.

Dynamic parameters (e.g. shear modulus, shear stress) are recorded during the tests (Figure 5) and critical parameters were extracted (e.g. yield stress, critical strain). All samples tested (on all facies and on all cores) show reproducible behaviours, as shown by adimensional analysis. Facies 3 shows higher standard deviations but can nevertheless be fitted into the general patterns. Mechanical transitions are related to sediment kinetics (i.e. deformation and strain rate), they occur in reproducible ranges of strain or strain rate (Figure 5), with linear elasticity occurring at deformations of ~1E-06-0.001, non-linear elasticity at deformations of ~0.001-0.1, liquefaction at deformations of ~0.1-1, and plasticity at deformations of ~1-100. Purely viscous flow can be observed at higher deformations (>10-100). This shows that flow initiation is related and determined by deformation.

Figure 5: (a) Map of the Bay of Biscay and of the studied areas (red stars) ; (b) examples of geotechnical devices ; (c) recording of an oscillatory test, featuring the storage modulus (G'), the loss modulus (G") and the shear stress (t) as functions of strain (g) and the main parameters extracted from the curve (G'0, G"0, g c1 and g c2). The yellow rectangle (I) represents the linear elastic part, the orange rectangle (II) represents the non linear elastic part, the dark pink rectangle (III) represents liquefaction and the blue rectangle (IV) represents the visco-elastic part.

Dynamic rheological parameters present a strong relationship (power law) with sediment concentration (Migniot, 1989). A power relationship, as provided by Migniot (1989), is applied to our data. Note that sediment concentration in our samples remain in a very thin range of values. Finally, a coherent link between concentration and yield strength of the materials in both rheological and geotechnical analysis is obtained.

2.3 Summary

The obtained site-to-site correlations allowed the identification of three main sedimentary facies in the northern Bay of Biscay, each of them reflecting the paleoclimatic conditions in the area. The analysis of the mechanical behaviour of the clay-dominated intervals of these different facies revealed similar trends at the different sites. It is confirmed by the good reproducibility for all samples in the rheological analysis, which allowed to point out a power relationship between the yield stress and the concentration. Besides, the mechanical transitions characterising flow initiation were observed to occur at identified and reproducible deformations. On the basis of these results, constitutive relations for the flow phase can be provided. These constitutive relations may be included in Navier-Stokes based numerical models simulating flow evolution (see below). All the results presented here are detailed in Howlett (2016).

3. GRAVITY FLOWS: PROPAGATION AND TRANSFORMATION

Numerical modeling of gravity-driven submarine landslides in the Bay of Biscay illustrates how sediment rheology in the post-failure phase might impact the water column and long waves generation (i.e. tsunami). The study of the propagation and the transformation of the considered gravity flows (e.g. Guiastrennec-Faugas, 2020) are based on results provided through the sedimentological and geotechnical characterisation of the French Atlantic margin (see above; Sabine, 2016; Howlett, 2016).

Figure 6: Maximal velocities for turbidity currents triggered by a landslides in the Capbreton canyon head.

Simulations have been carried out with Nixes-Tc numerical model (Figure 6) that integrates the constitutive rheological behaviour proposed in Howlett (2016; see results above). Nixes is a depth-integrated model for subaqueous  gravity-driven flows solving on a finite-elements grid the momentum and continuity equations together with a transport equation for the sediment concentration. Its rotational stress-closure extends the domain of validity to low Reynolds and highly concentrated flows. Simulations were made on a grid with 30 m resolution for the 3 landslides studied during the GITAN cruise.

Results have been presented on the basis of the state variables of the model: current thickness, sediment concentration and horizontal components of the velocity. Other parameters have been computed on the basis of these variables, such as sediment fluxes and Reynolds numbers. Results have been analyzed on the basis of animation of time evolution of main variables and parameters, charts of the spatial distribution of same quantities and by diagrams plotting the time evolution of mean values of the flows in order to expose characteristic time scales of flow properties and transformation. Results have been obtained for the three studied slides along the margin of the Bay of Biscay, namely the slide of Arcachon, in the Aquitaine region, the one of Beaugé in the Armorican region and the slide called Tas de Pois. The main properties of the slides are shown in Table 6.

Table 6: Initial conditions and simulation parameters

Main features show that during starting and developing phases of the flows results are strongly affected by the type of viscosity considered. Eventually flows converge for similar and comparable conditions, when apparent viscosity is mainly dependent on turbulence intensity and entrainment of ambient water decreases the sediment concentration in the flows. The main difference is the transition time between initial coherent flows and the terminal fluidized state. Transition time scale is much lower for flows computed on the basis of a constant viscosity (which is the common practice). Viscosity modulation at low strain rates provides a slower development of the flows associating lower velocities but longer and persisting coherent states. Tsunamigenic impact, associated to the initial coherent state, is less instantaneous but prolonged (Figure 7).

Figure 7: Arcachon Slide: evolution of mean Reynolds number. Low Reynolds numbers (<500) means less turbulence and preservation of a coherent and blocky behavior of the slide. Red line: with a constant viscosity. Blue line: modulated viscosity.

For seismic tsunamic sources, when impact are not considered as an instantaneous movement but as time dependent process, generated tsunamis might be fairly different and these differences are often associated with stronger impacts. Hence, for slide-induced tsunamic sources, as in the present study, the initial coherent phase induces a barotropic impact on the water column. It is pertinent to expect that, as for seismic sources, longer evolution of the source might also induce stronger impacts on the generation of tsunamigenic long waves. A major perspective of the present work is the integration of a more complex but realistic rheology in the land-slides simulating packages of the generation models (in this case in the Tandem project).

4. TSUNAMI HAZARD ASSESSMENT FOR THE FRENCH ATLANTIC AND CHANNEL COASTLINES

The TANDEM (Tsunamis in the Atlantic and the English ChaNnel: Definition of the Effects through numerical Modeling) project (2014-2018) benefited from the above results, in particular the WorkPackage 4 focusing on the appraisal of coastal effects due to tsunami waves on the French coastlines (TANDEM Project member, 2018). The use of the sedimentological, rheological and geotechnical parameters described above allowed realistic simulations for landslides and tsunamis in the Bay of Biscay. Both the tsunami and landslide propagations have been computed by the Avalanche code developed at CEA. Several scenarios have been studied in Tandem in CEA (e.g. in Frere, 2016; Paris, 2017). Maximising sources have been defined in different zones on the Bay of Biscay. The first two zones are located near the Cap Breton canyon and its huge landslide scar (estimated volume of 70 km³). Such a landslide could generate a very destructive tsunami on the French coasts. However, the literature agrees on the fact that this scar is probably not the result of one big landslide, but rather several smaller ones of a volume of several km³ (Gonthier, 2006). The two other zones are located on the continental margin of the bay (e.g. Beaugé promontoire and its 3-4 km³ estimated landslide) and present steeper slope that could lead to faster landslides, and thus more hazardous tsunamis. The results can be summarized as below (TANDEM Project member, 2018):

- The different landslides-tsunamis scenarios for the Cap Breton area (processed towards La Rochelle in Figure 8) are expected to produce extreme water heights (Hmax > 2m) for very long return periods only (Figure 8). Even though landsliding can be triggered at several places along the margin, most probably due to long term sea-level variations (>10³ years), only the coastal sites in the rear of the scar would be impacted, and not all the Atlantic coastline;

Figure 8: Maximum water heigths on the neighbouring of La Rochelle (France) according to the Cap Breton sources and their estimated recurrence.

- The maximizing landslides-tsunamis scenario for the Beaugé promontoire area shows a first water wave with a period of 200 s followed by water waves with smaller amplitudes and shorter periods. The arrival times of the first water wave are 35 min for the Spanish coasts, 40 min for the Basque Country, 1h10 for the southern Brittany and 1h50 for the Gironde estuary. Water heights have been estimated, on average, around 10-50 cm in the Bay of Biscay. A focusing of tsunami energy is observed due to the bathymetry which concentrates the largest water heights from the landslide location to the southern Brittany (Figure 9) (Paris, 2017).

Figure 9: Beaugé promontory lanslide-tsunami scenario - maximal water level elevations (m) after 2h of propagation. Note that water waves are refracted by all submarine canyons along the continental margin.

These results provided by the TANDEM project shed new ligths on the tsunami hazards on the French Atlantic coast. All the results are available in the TANDEM final report (TANDEM Project member, 2018) and here: https://hal-anr.archives-ouvertes.fr/search/index/?q=*&anrProjectReference_s=ANR-11-RSNR-0023