Lessons learned from monitoring of turbidity currents and guidance for future platform designs

Turbidity currents transport globally significant volumes of sediment and organic carbon into the deep-sea and pose a hazard to critical infrastructure. Despite advances in technology, their powerful nature often damages expensive instruments placed in their path. These challenges mean that turbidity currents have only been measured in a few locations worldwide, in relatively shallow water depths (≪2 km). Here, we share lessons from recent field deployments about how to design the platforms on which instruments are deployed. First, we show how monitoring platforms have been affected by turbidity currents including instability, displacement, tumbling and damage. Second, we relate these issues to specifics of the platform design, such as exposure of large surface area instruments within a flow and inadequate anchoring or seafloor support. Third, we provide recommended improvements to improve design by simplifying mooring configurations, minimising surface area, and enhancing seafloor stability. Finally we highlight novel multi-point moorings that avoid interaction between the instruments and the flow, and flow-resilient seafloor platforms with innovative engineering design features, such as ejectable feet and ballast. Our experience will provide guidance for future deployments, so that more detailed insights can be provided into turbidity current behaviour, and in a wider range of settings

A Cagey Approach to Speedy and Safe Seafloor Deployments

Researchers devised a simple way to deliver ocean bottom seismometers accurately to the seafloor to study ongoing seismic and volcanic activity near the islands of Mayotte

Cone penetration testing to assess slope stability in the 1979 Nice landslide area (Ligurian Margin, SE France)

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

Free Fall Cone Penetration Testing at Nice airport (SE France), 2009/12

Dynamic free fall penetrometer data from cruise Poseidon 386 and 429 to assess slope stability in the 1979 Nice landslide area

Underwater radio frequency antenna

The present invention relates to an underwater radio frequency antenna able to radiate in an underwater or equivalent propagation medium. It comprises -a hollow conducting tube (1) forming a resonant cavity, said conducting tube having an open end and a closed end,-means of excitation (2) of said resonant cavity which are able to be fed with signals and are arranged in such a way that the resonant cavity emits an electromagnetic radiation through said open end, -at least one layer (4) of di-electric material filling at least partially said resonant cavity so as to close the open end of the resonant cavity and render said cavity leaktight in relation to the underwater medium, said layer being able to resist the pressure in the underwater medium and to allow said electromagnetic radiation to pass through. (FR)La présente invention concerne une antenne radiofréquence sous-marine apte à rayonner dans un milieu de propagation sous-marin ou équivalent. Elle comporte - un tube (1) conducteur creux formant une cavité résonante, ledit tube conducteur ayant une extrémité ouverte et une extrémité fermée, - des moyens d'excitation (2) de ladite cavité résonante propres à être alimentés en signaux et agencés de manière à ce que la cavité résonante émette un rayonnement électromagnétique à travers ladite extrémité ouverte, - au moins une couche (4) de matériau diélectrique remplissant au moins partiellement ladite cavité résonante pour fermer l'extrémité ouverte de la cavité résonante et rendre ladite cavité étanche vis-à- vis du milieu sous-marin, ladite couche étant apte à résister à la pression en milieu sous-marin et à laisser passer ledit rayonnement électromagnétique

How distinctive are flood-triggered turbidity currents?

Turbidity currents triggered at river mouths form an important highway for sediment, organic carbon, and nutrients to the deep sea. Consequently, it has been proposed that the deposits of these flood-triggered turbidity currents provide important long-term records of past river floods, continental erosion, and climate. Various depositional models have been suggested to identify river-flood-triggered turbidite deposits, which are largely based on the assumption that a characteristic velocity structure of the flood-triggered turbidity current is preserved as a recognizable vertical grain size trend in their deposits. Four criteria have been proposed for the velocity structure of flood-triggered turbidity currents: prolonged flow duration; a gradual increase in velocity; cyclicity of velocity magnitude; and a low peak velocity. However, very few direct observations of flood-triggered turbidity currents exist to test these proposed velocity structures. Here we present direct measurements from the Var Canyon, offshore Nice in the Mediterranean Sea. An acoustic Doppler current profiler was located 6 km offshore from the river mouth, and provided detailed velocity measurements that can be directly linked to the state of the river. Another mooring, positioned 16 km offshore, showed how this velocity structure evolved down-canyon. Three turbidity currents were measured at these moorings, two of which are associated with river floods. The third event was not linked to a river flood and was most likely triggered by a seabed slope failure. The multi-pulsed and prolonged velocity structure of all three (flood- and landslide-triggered) events is similar at the first mooring, suggesting that it may not be diagnostic of flood triggering. Indeed, the event that was most likely triggered by a slope failure matched the four flood-triggered criteria best, as it had prolonged duration, cyclicity, low velocity, and a gradual onset. Hence, previously assumed velocity-structure criteria used to identify flood-triggered turbidity currents may be produced by other triggers. Next, this study shows how the proximal multi-pulsed velocity structure reorganizes down-canyon to produce a single velocity pulse. Such rapid-onset, single-pulse velocity structure has previously been linked to landslide-triggered events. Flows recorded in this study show amalgamation of multiple velocity pulses leading to shredding of the flood signal, so that the original initiation mechanism is no longer discernible at just 16 km from the river mouth. Recognizing flood-triggered turbidity currents and their deposits may thus be challenging, as similar velocity structures can be formed by different triggers, and this proximal velocity structure can rapidly be lost due to self-organization of the turbidity current

Assessing spatio‐temporal variability of free gas in surficial cohesive sediments using tidal pressure fluctuations

From a geohazard assessment perspective, the distribution, content and dynamics of free gas in surficial sediment was addressed by imaging and monitoring the upper 15 m beneath the shelf offshore Nice, France. Based on high resolution seismic data covering three sites where pore pressure was recorded over three and a half years, the presence of free gas was determined in the upper 2.75 to 14.75 m of cohesive, silty clay. Seismic velocity changes delineate two layers with gas volume fraction ranging from 0.12 % to 1.89 %. By considering the tidal response recorded by eight pore pressure sensors, gas volume fractions were estimated to vary from 0.26% to more than 9.4% on a spatio-temporal scale which cannot be achieved with seismic data. To depict spatio-temporal patterns three types of free gas occurrence (FGO) were distinguished. The one uniquely characterized by sawtooth fluctuations in overpressure of 27% to 45 % of the hydrostatic effective stress was recognized as an occurrence where bubbles grow and rise. The other two types showed long-term overpressure trends indicative of a situation whereby bubble growth has ceased. Type 1 FGOs are distinguished from type 2 by their gas volume fraction lower than 9.4 % and ratios of overpressure to hydrostatic effective stress lower than 0.3. Values higher than this threshold are considered sufficient for shear failure to initiate from the steep shelf edge (> 20°). Beyond site-specific insights, the distinction of FGO from their overpressure levels yields testable implications for the dynamics of methane in sediments. Plain Language Summary Increased awareness of the role of gas bubbles in surficial sediments with regard to the stability of submarine slopes have stressed the need to quantify their distribution, content and evolution with time. This was addressed by imaging and recording pressure fluctuations of shallow subsurface marine sediments. Both methods agreed in delineating the broad distribution of gas in these clayey deposits. However, they provided contrasting estimates of gas content which could be related to their distinct sensitivity to local changes. The analysis of the results obtained from the pressure records led to the recognition of three types of gas accumulations which cannot be discerned by imaging the subsurface. One type distinguishes from the others by showing episodic fluctuations in gas content and pressure ascribed to the growth and rise of bubbles. The other two types show trends in pressure suggesting that bubble growth has ceased. Of these two types, the one which is characterized by the highest gas content and pressure level is also considered to have the potential to initiate local shear failure in sediment. On a broader perspective, field evidences reported in this study provide constraints and testable implications for models addressing the transfer of methane to the atmosphere

Novel sensor array helps to understand submarine cable faults off West Africa

Seabed telecommunication cables can be damaged or broken by powerful seafloor flows of sediment (called turbidity currents), which may runout for hundreds of kilometres into the deep ocean. These flows have the potential to affect multiple cables near-simultaneously over very large areas, so it is more challenging to reroute traffic or repair the cables. However, cable-breaking turbidity currents that runout into the deep ocean were poorly understood, and thus hard to predict, as there were no detailed measurements from these flows in action. Here we present the first detailed measurements from such cable-breaking flows, using moored-sensors along the Congo Submarine Canyon offshore West Africa. These turbidity currents include the furthest travelled sediment flow (of any type) yet measured in action on Earth. The SAT-3 (South Atlantic 3) and WACs (West Africa Cable System) cables were broken on 14-16th January 2020 by a turbidity current that accelerated from 5 to 8 m/s, as it travelled for > 1,130 km from river estuary to deep-sea, although a branch of the WACs cable located closer to shore survived. The SAT-3 cable was broken again on 9th March 2020 due to a second turbidity current, this time slowing data transfer during regional coronavirus (COVID-2019) lockdown. These cables had not experienced faults due to natural causes in the previous 19 years. The two cable-breaking flows are associated with a major flood along the Congo River, which produced the highest discharge (72,000m3) recorded at Kinshasa since the early 1960s, and this flood peak reached the river mouth on ~30th December 2019. However, the cable-breaking turbidity currents occurred 2-10 weeks after the flood peak and coincided with unusually large spring tides. Thus, the large cable-breaking flows in 2020 are caused by a combination of a major river flood and tides; and this can provide a basis for predicting the likelihood of future cable-breaking flows. Older (1883-1937) cable breaks in the Congo Submarine Canyon occurred in temporal clusters, sometimes after one or more years of high river discharge. Increased hazards to cables may therefore persist for several years after one or more river floods, which cumulatively prime the river mouth for cable-breaking flows. The 14-16th January 2020 flow accelerated from 5 to 8 m/s with distance, such that the closest cable to shore did not break, whilst two cables further from shore were broken. The largest turbidity currents may increase in power with distance from shore, and are more likely to overspill from their channel in distal sites. Thus, for the largest and most infrequent turbidity currents, locations further from shore can face lower-frequency but higher-magnitude hazards, which may need to be factored into cable route planning. Observations off Taiwan in 2006-2015, and the 2020 events in the Congo Submarine Canyon, show that although multiple cables were broken by fast (> 5 m/s) turbidity currents, some intervening cables survived. This indicates that local factors can determine whether a cable breaks or not. Repeat seabed surveys of the canyon-channel floor show that erosion during turbidity currents is patchy and concentrated around steeper areas (knickpoints) in the canyon profile, which may explain why only some cables break. If possible, cables should be routed away from knickpoints, also avoiding locations just up-canyon from knickpoints, as knickpoints move up-slope. This study provides key new insights into long runout cable-breaking turbidity currents, and the hazards they pose to seafloor telecommunication cables