In a new paper, scientists have reported what they say is the longest sediment avalanche measured so far in action and the only monitored flow to continuously self-accelerate for over 1 000 km (620 miles). The event occurred off the coast of West Africa, in a deep canyon leading from the mouth of the Congo River. The event was triggered by severe flooding and unusually large spring tides.
The underwater avalanche kept moving for two whole days and ran out for more than 1 100 km (684 miles) across the floor of the Atlantic Ocean.
The event would have gone undocumented if not for the fact that the slide broke two submarine telecommunication cables, causing slow internet and data traffic between Nigeria and South Africa in the process.
Researchers were also able to line the length of the Congo Canyon with instruments that can measure current 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," said study author Peter Talling from Durham University in the 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."
The event, more properly called a turbidity current, was initiated on January 14, 2020. Scientists only reported the underwater avalanche as they needed time to recover the sensors and fully study their data.
According to the team, two factors combined to prime and then trigger the prodigious flow.
The first was an exceptionally large flood along the Congo River in December 2019, which was a one-in-50-year occurrence. It delivered large quantities of sand and mud to the head of the underwater canyon two weeks before the slide.
Then some unusually big spring tides followed in January 2020.
"The turbidity current we think was triggered at low water, at low tide," said author Dan Parsons from the 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 have to load the dice by delivering the sediment. Then the tidal signature can kick everything off."
The analysis showed the turbidity current reached the shallowest of the team's velocity profilers at 22:31 UTC on January 14 and arrived at the final instrument at 21:01 UTC on January 16. By that time, the slide had reached an ocean depth of more than 4 500 m (14 800 feet).
One interesting finding concerns why some cables get severed and others do not.
"This is new information for the cable industry and is being used to design new routes in this and other canyons - to avoid the areas that are most likely to experience deep erosion (immediately upstream of steep steps in the canyon that look like underwater waterfalls, known as 'knick points) as this will leave the cable more vulnerable to damage," said Dr. Mike Clare, a marine geoscientist at the UK's National Oceanography Centre and who advises the International Cable Protection Committee.
"Novel sensor array helps to understand submarine cable faults off west Africa" - Talling, P. J., et al. - Earth ArXiv - DOI:10.31223/X5W328
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.
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