C185

This study involves the examination of wave-induced submarine mudslides caused by recent major hurricanes in the Mississippi Delta region of the Gulf of Mexico, including the development of a model that can be used to analyze and predict these mudslides. Reports of mudslides caused by Hurricane Ivan in 2004 and Hurricane Katrina in 2005 were collected and studied to determine locations of mudslide activity. A simple limit equilibrium model was adapted to calculate a factor of safety against mudslide occurrence given the soil shear strength, the water depth, the slope angle, and the wave height and period at a site. The limit equilibrium model was verified using a more rigorous deformation model, and parametric analyses were performed to determine the sensitivity of the model to changes in input parameters. The limit equilibrium model was validated by comparing model predictions with reported occurrences and non-occurrences of mudslides in Hurricanes Katrina and Ivan as well as Rita, Lili, and Andrew. Finally, the limit equilibrium model was incorporated into a risk assessment methodology to quantify and analyze the risk associated with offshore mudslides in the Mississippi Delta.

The following major conclusions are drawn from this study:

• Mudslides caused by recent large hurricanes occurred in or very near to the mudslide prone area delineated by Coleman et al. (1980). Most mudslides occurred in the mudflow lobe areas within the mudslide prone area, while a few occurred in the mudflow gully area.
• In most parts of the Mississippi Delta region of the Gulf of Mexico, mudslides can only be caused by very large storm waves and are therefore infrequent events. Only three major storms (Camille, Ivan, and Katrina) caused significant and widespread mudslide activity in the past 40 years.
• Hurricanes Ivan and Katrina probably caused similar amounts of mudslide activity.
• Wave period is an important factor in mudslide vulnerability. Waves in Hurricanes Ivan caused significantly more mudslide activity than other storms of its magnitude due to its very long wave periods.
• Slope angle is not a significant factor in mudslide vulnerability except in the deeper parts of the mudslide prone area (water depths greater than about 300 feet).
• Site-specific analyses of mudslide vulnerability are challenging due to substantial variations in soil shear strength over distances as short as a few hundred feet; analyses need to incorporate these possible variations in shear strength to be realistic.
• Mudslides are localized features, on the order of several thousand feet in lateral extent and about 50 to 150 feet deep. The areal extent and depth of mudslides are related to the lengths and widths of the storm waves that cause them. Mudslides are not likely to lead to large-scale, regional mudflows due to the very flat slopes in the mudslide prone area and the large amount of local variation in soil shear strength.
• The return periods for mudslides impacting facilities range from less than 10 years to greater than 1,000 years and depend strongly on location. The risk for mudslides increases as the water depth decreases, the slope of the bottom increases, and the amount of infrastructure in a particular area increases. Project-specific risk analyses should incorporate all available site-specific information on metocean conditions, bathymetry and soil properties.