Subaqueous, hurricane-initiated shelf failure morphodynamics along the Mississippi River Delta Front, north-central Gulf of Mexico
Seafloor instability along the Mississippi River Delta Front (MRDF) gained renewed attention with the landfall of Hurricanes Ivan (2004) and Katrina (2005). Traditional root causes for MRDF shelf failure were exacerbated by sea-state conditions associated with these severe tropical cyclones and their interaction with the seafloor. These conditions were characterized by large waves, long wave periods and wave-induced turbulence in the bottom boundary layer and throughout the water column. An evaluation of local and regional MRDF bathymetry data revealed substantial changes in seafloor elevation and the immediate subsurface sediment profile, hypothesized as the end result of cyclic wave-seafloor interaction, seafloor scour and failure, and the re-initiation of antecedent seafloor slides and subsequent sediment re-deposition. Observed bulk wave and bottom layer conditions during Hurricanes Ivan and Katrina, during which significant wave height and wave period exceeded 15 m and 12 sec, respectively, were used to calibrate a series of MIKE 21 numerical wave models. Once calibrated, hindcasts were generated for earlier MRDF hurricanes dating from 1965. Spectral frequency data indicated long-period, often bimodal MRDF wave effects up to 48 hours prior to storm arrival. Lithologic and geotechnical parameters indicate widely varying shear strengths and safety factors, with higher shear stresses coincident with the 25-m isobath. Safety factors decreased in tandem with hurricane approach both prior to and after peak conditions. One-dimensional sediment failure modeling, calibrated to past seafloor failures, indicated variable ranges of mudslide length, ranging up to several kilometers. A composite risk framework was constructed that employed various triggering, revealing and predisposition danger factors, a statistical analysis of elements at risk, and a vulnerability assessment to identify likely scenarios for future hurricane-initiated seafloor failure. A top tier of historical storms, including Hurricanes Ivan, Camille and the 1856 Last Island Hurricane, was risked as most prone to failure; a secondary tier included Hurricanes Katrina, Opal, Carmen and the 1915 New Orleans Hurricane. Five hypothetical future hurricanes of varying intensity were then used to help characterize potential MRDF seafloor response. Areas at highest risk included those characterized by steep slopes, rapid sedimentation rates, and lengthened temporal exposure to severe hurricane conditions.
Submarine mudslides, Deltaic processes, MIKE 21 numerical modeling
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