The delta front seafloor, located downslope from South Pass of the Mississippi River, was numerically analyzed for resistance to cyclical, wave-induced bottom pressures generated by small to large-size hurricanes, including Hurricane Ivan. The results demonstrate that while small to medium-size storms may cause localized failures out to moderate water depths, intense Ivan-level storms can create region-wide large mudflows, and cause failures of the existing seafloor out to water depths greater than 400 ft.
The submerged delta apron of the Mississippi River exhibits many examples of seafloor instability triggered by powerful tropical storms, due to the existence of a thick, very weak sediment profile. In 1969, Hurricane Camille demonstrated the serious nature of the hazard posed to seafloor-founded facilities by intense hurricanes. The resulting damage to platforms and pipelines generated research into the geologic characteristics of delta seafloor failures. Methods were developed for sampling and testing these weak sediment profiles, and analytical tools were created for numerically evaluating seafloor stability during hurricanes. Since that time, platforms, wells, and pipelines have proliferated and spread from east to west across the delta front, and downslope into water depths then barely conceivable. The damage to these facilities caused by seafloor failures during Hurricane Ivan provides additional evidence of the power and reach of the largest storms.
This paper provides summary overviews of the nature of hurricane wave interactions with a yielding seafloor and the susceptibility of the delta seafloor to failure because of its geologic history. It then demonstrates how these factors combined during Ivan to produce regional seafloor instabilities that severely damaged portions of the hydrocarbon production facilities in the delta front of the Mississippi River.
Hurricanes generate surface waves that apply cyclic pressure to the bottom sediments. The magnitude of the bottom pressure depends upon the wave height, the wavelength and the water depth. A storm forms waves of many different wave heights and periods, and the greatest bottom pressures are caused by the highest waves with the longest periods.
Because the waves produced by hurricanes have many wave heights and periods, statistics are used to describe these variations. Typically the storm waves are characterized by the "significant wave height", which is the average of the onethird highest waves. For Ivan, the maximum measured significant wave height was about 53 ft. However, a few individual waves as high as 80 to 90 feet also apparently occurred. Wave period measurements can be summarized using the Average Period and the Peak Period (or Dominant Period). The Peak Period is the center of the energy band containing most of the wave energy. Values up to about 17 seconds occurred during Ivan.
When a storm approaches, the significant wave height, extreme wave height, and peak period increase until reaching the storm maximum. This "storm history" for a delta front location can last for days. Importantly, the storm wave bottom pressures affecting the seafloor sediments also have a long history of varying magnitudes and periods.