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On-going projects

Bute Inlet

The
Challenge

Sediment-laden density flows are responsible for creating some of the largest deposits of sediment on Earth. Scientists first became aware of these sea-floor flows almost 100 years ago when underwater communication cables were broken by flows triggered by an offshore earthquake. The subsequent growth in the number of subsea telecommunication cables and other infrastructure has increased the level of hazard to the global economy posed by the flows. Additionally, it is becoming increasingly clear that density flows play a crucial role in the transport of organic carbon to the deep ocean where it is either buried or recycled as nutrients.

The mechanisms that cause density flows to initiate and travel large distances over the sea-floor remain poorly understood as monitoring the events has, until recently, proved too difficult due to the highly energetic and episodic nature of the flows. However, recent advances in technology have demonstrated that the flows and their impact on the shape of the seafloor can be successfully monitored, giving scientists a real opportunity to explore how the flows develop and whether our current models of their behaviour, developed mostly from small-scale laboratory experiments, are correct.

The
Approach

Lead researchers

dan-parsons ye-chen steve-simmons

Project partners

National Oceanography Centre

Natural Resources Canada

Canadian Hydrographic Service

University of Durham

University of Newcastle upon Tyne

During 2018, an international team of scientists, including experts from the Energy and Environment Institute, collaborated on an intensive programme of monitoring density flows and measuring their impact on the floor of Bute Inlet, British Columbia. Six moorings were deployed along the channel that runs along the floor of Bute Inlet. On these moorings were mounted ultrasonic acoustic flow-meters that sent pulses of sound through the water column in the channel every few seconds to measure the speed of the water near the seafloor. These measurements continued for a period of six months, capturing the speeds within the body of the density flows as well as important information about the density of the sediment that drives the flows down the channel.

High resolutions surveys of the river deltas that feed into the Inlet and the channel that runs along the Inlet floor were carried out using the University of Hull’s state-of-the-art echo-sounder at the beginning and end of the monitoring period. These surveys provided vital evidence of the changes to the shape of the deltas and channels that were caused by the initiation and passing of the density flows. These data, combined with lower resolution surveys of a larger region of the Inlet; sediment samples taken from within the channel; and sub-surface acoustic profiling, are providing us with an extensive data-set that really helps us to begin to unlock the secrets of how these flows behave.

The Impact

The data collected in 2018 coincided with a large river discharge and sediment input to Bute Inlet from the melting snowpack high in the mountains that surround Bute Inlet. This resulted in an unprecedented number (> 80) of density flows being observed at the mooring closest to the river deltas, some of which potentially merged with one another. The largest of the flows observed during the monitoring period ran out through the full array of instruments over a distance of 40 km from the river mouth and is largest flow that has been directly observed in Bute Inlet to date. Together with surveys from previous years, these data give valuable new insights into the long-term evolution of the channel system. The high resolution surveys inform us of how channel features, such as crescent-shaped bedforms and knickpoints, evolve under the influence of the density flows. Analysis of the carbon content of the sediment is providing new information regarding the transport and burial of organic carbon by the flows.

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