How the interaction between sediment density flows and the seafloor influences flow evolution

Abstract

Seafloor sediment density flows are the primary mechanism for transporting sediment to the deep sea. These flows are important because they pose a hazard to seafloor infrastructure and deposit the largest sediment accumulations on Earth. How sediment density currents interact with the seafloor is important, yet, poorly understood. This interaction is important because it governs how sediment density flows evolve along their trajectories; either by causing them to erode sediment and accelerate, or deposit sediment and to decelerate. This thesis explores how sediment density currents interact with the seafloor by exploring flows at different scales: from field scale monitoring to controlled laboratory experiments. Field-scale turbidity currents are investigated in the Squamish River Delta, British Columbia. Previous studies indicate that landslides are the predominant trigger of powerful flows in this and other locations. However, analysis of 93 repeat bathymetric surveys suggests that initially dilute non-landslide-triggered sediment density flows can be at least as powerful as landslide-triggered flows. For the first time, we show that settling from surface plumes can dominate the triggering of hazardous submarine flows. This thesis uses laboratory experiments to explore how fine cohesive sediments can be incorporated into sediment gravity flows by the disintegration of eroded mud clasts. Previous laboratory studies indicate mud clasts can only travel several hundred metres before disintegration, while field observations indicate clasts can travel tens to hundreds of kilometres. They demonstrate that sand armouring can enable the transportation of mud clasts over many kilometres. The results reconcile the contradiction between field observations and previous laboratory investigations. Finally, this thesis uses laboratory experiments to examine the sediment concentration and thickness of the basal layer of a sediment density flow. Field studies indicate the importance of dense basal layers in driving the overriding sediment gravity flow. However, the structure of these dense basal layers remains poorly understood, as no instruments are currently available to measure sediment concentration in such layers. Here we evaluate the Electrical Resistivity Tomography method and its applicability to enable us to monitor the sediment distribution within such layers in laboratory conditions.

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