Underwater avalanche continued for two whole days

Scientists are reporting what they are saying is the longest sediment avalanche but measured in motion.

It occurred underwater off West Africa, in a deep canyon main away from the mouth of the Congo River.

Something in extra of a cubic kilometre of sand and dust descended into the deep.

This colossal circulation saved shifting for two whole days and ran out for greater than 1,100km throughout the ground of the Atlantic Ocean.

The occasion would have gone unrecorded have been it not for the truth that the slide broke two submarine telecommunications cables, slowing the web and different knowledge site visitors between Nigeria and South Africa within the course of.

And additionally due to the prescient motion of researchers who had lined the size of the Congo Canyon with devices able to measuring present and sediment velocities.

“We had a series of oceanographic moorings that were hit by the event, which broke them from their seafloor anchors so that they popped up to send us an email,” stated Prof Peter Talling from Durham University, UK.

“This thing gradually got faster and faster. Because it erodes the seabed as it goes, it picks up sand and mud, which makes the flow denser and even quicker. So, it has this positive feedback where it can build and build and build,” he instructed BBC News.

The Leon Thevenin was referred to as in to restore the Congo cable breaks in 2020 and subsequent injury in 2021 – all the results of turbidity currents

The underwater avalanche – extra correctly referred to as a turbidity present – was initiated on 14 January final 12 months. It’s solely being reported now as a result of scientists wanted time to recuperate the sensors and absolutely analyse their knowledge.

The crew says two components mixed to prime after which set off the prodigious circulation.

The first was an exceptionally giant flood alongside the Congo River in late December 2019. A 1-in-50-year prevalence, this delivered huge portions of sand and dust to the pinnacle of the underwater canyon. But this was nonetheless two weeks earlier than the slide.

Congo River

The flood in December 2019 noticed 70,000 cubic metres of water per second move Kinshasa. This deluge was accountable for pushing sediment to the pinnacle of the offshore canyon

What adopted in January, nonetheless, have been some unusually huge spring tides.

“The turbidity current we think was triggered at low water, at low tide,” stated Prof Dan Parsons from Hull University.

“As the loading of the ocean above declines, so you get a change in the pore water pressure within the sediment – and that’s what allows it to fail.

“But first you must load the cube by delivering the sediment. Then the tidal signature can kick every little thing off.”

The analysis shows the turbidity current reached the shallowest of the team’s velocity profilers at 22:31 GMT on 14 January 2020, and arrived at the final instrument almost 24 hours later at 21:01 GMT on 16 January. By that stage the slide had reached an ocean depth of more than 4,500m.

Sample cables

Submarine cables carry more than 99% of all data traffic between continents. They are key arteries in the modern information economy

The team had an early assessment of the speeds involved simply by noting the times when their sensors surfaced.

The recovery of the profilers, though, enabled the rough velocity calculations to be properly calibrated.

This showed the flow initially travelled at 5.2m/s in the upper canyon, but then continuously sped up to 8m/s by the time it reached the end of the channel.

Prof Talling and colleagues have detailed the event in a White Paper aimed particularly at the international submarine cable sector.

There are important lessons on how to mitigate risks.

It’s obviously not possible to predict with certainty when a damaging turbidity current will occur, but knowing something about the conditions that trigger sediment failures could influence the global positioning of cable repair ships.

The occurrence of another flood along the Congo River like the one experienced in December 2019 would make it wise to keep a vessel on standby in the region, for example.

The two cables that broke in mid-January 2020 – the South Atlantic 3/West Africa (SAT-3/WASC) cable and the West Africa Cable System (WACS) – were repaired within a few weeks. But there have been additional breaks since as further sediment has tumbled downslope.

One interesting finding concerns why some cables get severed and others do not. This may relate to differences in erosion rates along the flow path.

In some areas, the turbidity current will dig deep into the seafloor, while in others, large amounts of sand and mud are dumped.

The Congo study is the first to document this patchwork effect of erosion and deposition.

“This is new data for the cable business and is getting used to design new routes on this and different canyons – to keep away from the areas which can be most certainly to expertise deep erosion (instantly upstream of steep steps within the canyon that appear to be underwater waterfalls, often called ‘knickpoints’) as this may depart the cable extra weak to break,” explained Dr Mike Clare, a marine geoscientist at the UK’s National Oceanography Centre and who advises the International Cable Protection Committee.

Cable repair

There is a fleet of cable restore vessels positioned across the globe prepared to reply to outages just like the January 2020 Congo occasion

It’s arduous to overstate the significance of the worldwide submarine cable community.

More than 99% of all knowledge site visitors between continents goes by way of these connections, together with each day cash transfers to the worth of trillions of {dollars}.

The analysis within the Congo Canyon has wide-ranging participation, together with from IFREMER (Institut Français de Recherche pour l’Exploitation de la MER) in France and GEOMAR Helmholtz Centre for Ocean Research in Germany. The mission is co-led from Durham and Angola Cables.

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