Developing the UK national centre of excellence for geohazards through quantification of field-scale turbidity current hazard

Our over-arching aim is to better understand the impact of powerful submarine flows, called turbidity currents, on pipelines and other seabed infrastructure used to recover oil and gas. Turbidity currents pose a serious hazard to expensive seabed installations, especially in deeper-water settings. These sediment flows are particularly hazardous because they can be exceptionally powerful (travelling at speeds of up to 20 m/s), and can flow for long distances (100s km), causing damage over vast areas of seafloor. Even weaker flows travelling at ~1-2 m/s can severely damage seafloor equipment, or break strategically important submarine telecommunication cables, while some flows have maintained speeds in excess of 5 m/s for hundreds of kilometres. This makes hazard mitigation by local re-routing of pipelines difficult. Where seafloor topography is rugged, many operators route pipelines within canyons; however, these are focal points for turbidity current activity. Mitigating against turbidity current geohazards, particularly within canyons, can have very significant cost implications for industry - additional deepwater pipeline routing costs ~ $3 million per km. Mitigation costs of $2 billion are predicted to route pipelines under the Congo Canyon, where turbidity current hazard is deemed to be high. Perhaps just as importantly, pipeline oil spills could lead to major reputational damage. Given concern over accidents to structures used to recover oil and gas, a focus on geohazards is also aligned with NERC's environmental responsibility.

The most remarkable aspect of turbidity currents is how few direct measurements there are from flows, in part because they damage monitoring equipment placed on the seafloor. Several lines of evidence point to the existence of a region of high sediment concentration at the base of turbidity currents. These dense basal layers are of key important because of: (i) their location just above the bed where most submarine infrastructure is located; and (ii) they carry most momentum due to their large density. Yet, sediment concentration has never been measured directly measured in these layers. Physical experiments, numerical modeling and ancient deposits provide valuable insights into these flows; but there is a compelling need to monitor full-scale flows in action. This project is timely because it will develop innovative field-based techniques for imaging near bed flow structure and vertical changes in sediment concentration in situ.
Aims: (1) Our first aim is to develop and field test a novel technique for remote sensing of dense near bed layers. (2) Our second aim is to better understand the nature of near bed dense layers. (3) Our third aim is to embed improved understanding of dense near-bed layers into numerical models used by industry to assess impact of turbidity currents on oil and gas pipelines. (4) The project will also help to establish an international centre of excellence for submarine geohazard research at the UK National Oceanography Centre.

Here we propose to make direct measurements of dense basal layers that form part of the turbidity currents occuring daily during the elevated summer river discharge on the Squamish Delta, located in Howe Sound, Canada. We will use an innovative four-point mooring to hold a vessel and suspended instrumentation payload stable above an active channel system, while we observe the dense basal layer with a Chirp sub-bottom profiler. The low frequency and broad bandwidth (1.5 -13.0 kHz) Chirp source guarantees penetration through dense near-bed layers, resolving layers with ~10 cm resolution. These field observations will help to understand the fundamental character of near bed layers, and the situations in which they form.