Researchers from our top-rated Geography Department working with international partners have recorded underwater landslides as they happen, providing the first direct evidence of how these events develop.
The findings, from the Homathko Delta in British Columbia, Canada, could improve understanding of offshore risks and how material reaches the deep ocean.
Watching the sea floor
Underwater landslides are common where rivers deposit sediment into the sea.
They can damage seabed cables, pipelines and offshore wind turbine foundations, while carrying sediment, carbon and pollutants into deeper waters.
Until now, scientists had not watched a landslide taking place underwater in nature. Their understanding came from computer models, laboratory experiments and dredging-related landslides.
During an 18-day research campaign, the team used sonar to image the delta's steep front every second.
They also mapped the sea floor daily and placed a sensor 700 metres downslope to measure water clarity.
A slow-moving process
The researchers recorded around 30 small landslides over 11 days.
Rather than collapsing at once, their edges gradually moved backwards up the slope.
This process, known as breaching, happened at speeds of two to 27 millimetres per second. Each event lasted 38 to 100 minutes.
The movement was less steady than models suggested and was sometimes interrupted by sudden collapses.
All events occurred during high river flow and falling tides, suggesting these conditions may help trigger them.
Together, the landslides removed around 61,000 cubic metres of sediment, which is roughly 20 per cent of the sand delivered by the river during the same period.
Flows travel for miles
Each landslide sent a flow of sediment-filled water, known as a turbidity current, down an underwater channel.
These flows travelled at least two kilometres and kept the water cloudy for 12 to 14 hours.
The findings show that frequent, small landslides can move material offshore. This matters for understanding how carbon and pollutants reach deeper waters.
The research could help engineers plan for potential risks to seabed cables, pipelines and offshore wind developments.
