How are Mass Failures and Sediment Flows Triggered at Offshore River Deltas and What are the Implications for Deep-water Systems?

Abstract:

It has been proposed that triggering processes for landslides and turbidity currents at offshore river deltas include hyperpycnal flows, tidal effects, elevated bed shear stresses, and earthquakes, but these hypotheses are poorly tested in a quantified manner. As rivers and offshore turbidity currents are probably the two most volumetrically important sediment transport processes on Earth, it is important to understand how they are linked. In this study, a suite of statistical tests is applied to quantify significance of triggers, based on an unprecedented record of failures and flows. The submarine delta at Squamish, British Columbia provides an exceptional natural laboratory where 106 turbidity currents were monitored over 147 days during 2011. This unique dataset provides sufficient number of observations for statistical analysis of event timing, frequency and triggers. Elevated periods of river discharge are shown to be a significant control on when the turbidity current system ‘switches on’. The combined effects of river discharge and tidal elevation, interpreted to cause elevated bed shear stresses on the delta lip, are the most significant controls on flow timing. Of these two factors, river discharge is the dominant control on flow recurrence, as the seasonal discharge maximum correlates with more frequent flows. Each 1 m³/s increase in river discharge explains a 1% increase in offshore sediment flow likelihood, once a critical threshold is reached. The largest peaks in river discharge do not create hyperpycnal flows, but instead result in delta lip failures. Delta lip failures may also occur during periods of lower sediment input, which is related to the pore pressure regime and preconditioning of the slope by sediment accumulation.

Two types of events are documented at Squamish, which include landslides that create turbidity currents, and more enigmatic turbidity currents with no obvious initiation point. Initial results, from the first multi-platform monitoring of turbidity currents in 2015, show one such enigmatic event and we summarise the differences between the two event types. This study provides new insights into the dynamics and frequency of submarine deltas which has wide implications for hazard assessment and understanding deep-sea sediment transport.