MODELLING EXTREME EVENTS (HURRICANES) AT THE SEAFLOOR IN THE NORTHERN GULF OF MEXICO (Invited Presentation)

A modelling of risks to subsea infrastructure of seabed sediment mass flows under extreme storm events has implications for stratigraphy on continental margins. This project under BOEM, coupled advanced modelling modules to investigate the phenomena. The period 2008-10 was used for test data, covering hurricanes Gustav and Ike, in the Mississippi to De Soto Canyons region. Currents, tides and surface waves were computed using the Regional Ocean Modeling System (ROMS) and river discharges using WBMsed. The Community Sediment Transport Model (CSTMS) calculated the concurrent regional patterns of sediment erosion-transport-deposition. Local sediment properties were provided from the dbSEABED database. The preferred paths of near-bottom sediment flows were based on a channel analysis of the bathymetry.

Locations and timings of suspended sediment gravity flow were identified by applying energy-based flow-ignition criterea. Wave-induced mass failure and subbottom liquefaction events were also assessed using geotechnical criterea. The downslope persistences, densities and velocities of the turbidity flows yielded by the ignitions were then calculated using high-Reynolds Number adaptations of LES/RANS-TURBINS models (Large-Eddy Simulation / Reynolds Averaged Navier-Stokes). A very important step here was the transfer of these advanced models from laboratory to geographic scales.

As known, much of the shelf sediment mantle is suspended and/or moved during hurricanes, consistent with the modeling results. Many short-lived gravity-flow ignitions occur on the shelf; many at the shelf edge will ignite into fast, erosive and persistent currents. Sediment patchiness and vagaries of hurricane path mean that the pattern of ignitions alters from event to event. A valuable experience in the project was devising workflows and linkages between these advanced, but independent models. The project opens a path to advanced modeling supporting stratigraphic analysis.