Description: Highlights: • Comprehensive morphological, seismic, sedimentological, and geotechnical data sets. • Dynamic CPTU provides a powerful, time- and cost-efficient in situ technique. • Development of a regional sub-seafloor strength model offshore Nice airport. • 2D finite element slope stability assessment gives slope failure depth of 〉 50 m. Abstract: In the landslide-prone area near the Nice international airport, southeastern France, an interdisciplinary approach is applied to develop realistic lithological/geometrical profiles and geotechnical/strength sub-seafloor models. Such models are indispensable for slope stability assessments using limit equilibrium or finite element methods. Regression analyses, based on the undrained shear strength (su) of intact gassy sediments are used to generate a sub-seafloor strength model based on 37 short dynamic and eight long static piezocone penetration tests, and laboratory experiments on one Calypso piston and 10 gravity cores. Significant strength variations were detected when comparing measurements from the shelf and the shelf break, with a significant drop in su to 5.5 kPa being interpreted as a weak zone at a depth between 6.5 and 8.5 m below seafloor (mbsf). Here, a 10% reduction of the in situ total unit weight compared to the surrounding sediments is found to coincide with coarse-grained layers that turn into a weak zone and detachment plane for former and present-day gravitational, retrogressive slide events, as seen in 2D chirp profiles. The combination of high-resolution chirp profiles and comprehensive geotechnical information allows us to compute enhanced 2D finite element slope stability analysis with undrained sediment response compared to previous 2D numerical and 3D limit equilibrium assessments. Those models suggest that significant portions (detachment planes at 20 m or even 55 mbsf) of the Quaternary delta and slope apron deposits may be mobilized. Given that factors of safety are equal or less than 1 when further considering the effect of free gas, a high risk for a landslide event of considerable size off Nice international airport is identified.

Description: The upper shelf of the landslide-prone Ligurian Margin (Western Mediterranean Sea) off Nice well-known for the 1979 Airport Landslide is a natural laboratory to study preconditioning factors and trigger mechanisms for submarine landslides. For this study low-stress ring shear experiments have been carried out on a variety of sediments from 〉50 gravity cores to characterise the velocity-dependent frictional behaviour. Mean values of the peak coefficient of friction vary from 0.46 for clay-dominated samples (53 % clay, 46 % silt, 1 %) sand up to 0.76 for coarse-grained sediments (26 % clay, 57 % silt, 17 % sand). The majority of the sediments tested show velocity strengthening regardless of the grain size distribution. For clayey sediments the peak and residual cohesive strength increases with increasing normal stress, with values from 1.3 to 10.6 kPa and up to 25 % of all strength supported by cohesive forces in the shallowmost samples. A pseudo-static slope stability analysis reveals that the different lithologies (even clay-rich material with clay content 〉=50 %) tested are stable up to slope angles 〈26° under quasi-drained conditions.

Description: Submarine groundwater discharge (SGD), the flow of fresh and saline groundwater from the seabed into the coastal ocean, has been intensively investigated in the recent years. This research has usually been restricted to shallow water and intertidal areas, whereas knowledge about groundwater seepage in deeper water is mainly limited to point sources from karstic aquifers. In this study we observed submarine groundwater seepage and a subterranean estuary in sediments at water depths of 20-44 m located within the Ligurian Margin, western Mediterranean Sea. Here, a catastrophic submarine landslide occurred near the Nice airport (French Ligurian coast) in the fall of 1979 after a period of heavy rainfall. During two research cruises, gravity cores were recovered in and around the area of the landslide scar. Pore water samples collected from sediment cores indicated sediments containing freshwater within the landslide scar. Pore water profiles of selected ions, such as chloride, ammonium, manganese, sulfate and barium were used to assess transport and reaction processes within the sediment. A 1-dimensional transport model indicates in most cores upward pore water velocities of 2.3-8.8 cm/yr. This study shows that submarine groundwater seepage along the French Mediterranean coastline can occur at water depths reaching 44 m.

Description: Between 28.04.2006 and 17.05.2006, bathymetric data was acquired in the Eastern Mediterranean Sea during the R/V POSEIDON cruise P336. The expedition was dedicated to studying slope instability and related geohazardous processes at the northern Cretan Margin and the slopes of the Aegean volcanic islands. The cruise aimed at an understanding of parameters controlling sediment physics, and ultimately geohazards such as landslides and other mass wasting events. Bathymetric mapping with the multibeam echosounder (MBES) ELAC SEABEAM 1050 was used to survey approximately 2450 km² of the study area and identify landslide and mass wasting deposits. Based on hydroacoustic and seismic data, these deposits were characterised by in-situ measurements, such as Cone Penetrating Testing and sub-surface heat flow measurements, and sampling, such as gravity coring and laboratory studies on board. CI Citation: Paul Wintersteller (seafloor-imaging@marum.de) as responsible party for bathymetry raw data ingest and approval. Description of the data source: During the P336 cruise, the multibeam echosounder (MBES) ELAC SEABEAM 1050 was used for continuous mapping of the seafloor. The transducer array is fixed in a Mills cross configuration and transmits a frequency of approximately 12 kHz with an angular coverage sector of up to 150°. The signal is formed into 191 beams, where the beam spacing can be set to equidistant or equiangular. During this cruise, the opening angle was set to 60-70°, depending on the state of the sea, which restricted the coverage to gain a more continuous beam spacing on the seafloor. For further information on the system, consult: http://www.angelopodesta.com/documenti/SeaBeam_1050-1055.pdf Responsible person during this cruise / PI: Ingo Kock (Ingo.Kock@grs.de), Melanie Reichelt & Sebastian Krastel-Gudegast (sebastian.krastel@ifg.uni-kiel.de) Chief Scientist: Achim Kopf (akopf@marum.de) CR: https://elib.suub.uni-bremen.de/ip/docs/00010106.pdf CSR: https://www2.bsh.de/aktdat/dod/fahrtergebnis/2006/20130246.htm

Description: Two matlab scripts which simulate water emission from the subducting plate and in the inner accretionary prism.