Scientists have reported what they say is the longest sediment avalanche ever measured in action, as it flowed underwater for two days off the coast of West Africa. 

An international team of researchers reports the detection of the formidable turbidity current, powerful underwater avalanches, that took place across January 14 to 16, 2020. The massive movement of sediment flowed with speeds of up to 8 meters (26 feet) per second.

In a preprint paper available on Earth ArXiv (yet to be peer-reviewed), the team reports that the current traveled for over 1,130 kilometers (700 miles) from the mouth of the Congo river, through the Congo submarine canyon, all the way down to a depth of over 4,500 meters (2.8 miles).

The flows led to the breaking of two seabed telecommunications cables, the SAT-3 (South Atlantic 3) and WACs (West Africa Cable System) which led to a reduction in data speed between Nigeria and South Africa – which in itself notified people that something was happening on the seabed. Part of the WACs closer to the coast was not affected, but the data collected showed that the current accelerated as it picked up more material.

“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,” first author Professor Peter Talling from Durham University told BBC News.

“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 cables had not suffered a fault for 19 years but after the January 2020 event, a second turbidity current caused more damaged to the SAT-3 on March 9, 2020. There are multiple factors behind the cause of these colossal submarine avalanches, although one seems to be crucial: major flooding.

December 2019 saw a major flood of the Congo river, something described as a one-in-50-years event. In Kinshasa, the capital of the Democratic Republic of the Congo, the river was at its highest since 1963, with a discharge of 72 million liters (19 million gallons).

The flood reached the river estuary on December 30, 2019, with the first turbidity current forming two weeks later and the second one forming 10 weeks later, coinciding with particularly strong spring tides. The team believes that the large flooding events enhance the risk of fast turbidity current for not just weeks but possibly up to several years.

Given the importance of underwater cables when it comes to global telecommunication, understanding these currents better will lead to faster intervention in repairing them and locations identified as at-risk can be avoided for future cables.