-With images -
Scientists from Durham University, UK working with international partners have captured underwater landslides as they happen in the natural environment for the first time, revealing how these hidden events can move large amounts of sediment across the sea floor.
The research, carried out at the Homathko Delta in British Columbia, Canada, provides the first direct evidence of a process known as 'breaching' in naturally occurring landslides that take place entirely underwater.
Underwater landslides are common where rivers carry sediment into the sea.
Although they are hidden from view, they can pose a serious risk to seabed cables, pipelines and offshore wind turbine foundations. They also transport sediment, carbon and pollutants from coastal areas into the deep ocean.
Until now, scientists have relied on computer models, laboratory experiments and observations of landslides triggered by dredging to understand how these events develop.
During an 18-day research campaign, the team used underwater sonar to monitor the steep edge of the delta, taking images every second.
They also mapped the sea floor daily and placed a sensor further down the slope to measure changes in water clarity. The team recorded around 30 small landslides over 11 days.
Rather than collapsing suddenly, the sea floor gradually gave way, with the edges of the landslides creeping backwards up the slope at speeds of between two and 27 millimetres per second. Individual events lasted between 38 minutes and 100 minutes.
This slow, step-by-step movement is known as breaching. The findings show that a process previously observed in laboratory experiments and some human-caused failures also occurs naturally beneath the sea.
Study co-author Dr Matthieu Cartigny of Durham University, said: "This is the first time we've caught underwater landslides in the act in nature. Rather than one dramatic collapse, the seabed peeled away bit by bit for over an hour.
"Each event was small, but together they moved a fifth of the sand the river delivered. That matters for how sediment, carbon and pollutants reach the deep sea, and for the safety of offshore infrastructure."
The landslides generated powerful underwater flows of sediment-filled water, known as turbidity currents. These travelled at least two kilometres down an underwater channel and kept the water cloudy for between 12 and 14 hours.
Although each landslide was relatively small, together they removed around 61,000 cubic metres of sediment from the delta. This represents roughly 20 per cent of the sand delivered by the river during the same period.
The observations also revealed that the landslides did not always move steadily. Their retreat was faster and less regular than computer models had predicted and was sometimes interrupted by sudden collapses. This suggests that different types of underwater failure can occur together or influence one another.
All the recorded landslides happened during a period of high river flow and falling tides. This points to a possible role for changing river and tidal conditions in triggering the failures, although further research is needed to establish exactly how they begin.
The findings could help engineers better understand the risks facing offshore infrastructure.
