During Hurricane Ivan in 2004, a number of Gulf of Mexico (GOM) pipelines and platforms were believed to have been impacted by mudslides in the region of Ivan's path. This project, proposed under two phases, will provide hazard information for the design and placement of new pipelines and structures by determining the applicability of developing regionally consistent hazard maps that delineate relative susceptibility of GOM offshore regions to future submarine mudslides, including identification of past and future probable locations of underwater slope failures.
The project consists of a pilot test to map the sea floor bottom using high-resolution bathymetric and seismic data to delineate past mudslide failures, sediments susceptible to failure, and areas of relative stability. An important part of this mapping is to determine the relative ages of sediment and past failures in order to evaluate where future failures are most likely to occur, and equally important, likely to not occur.
Project completed as follows:
It was originally envisioned that landslide susceptibility maps for mudflow hazard in the Gulf of Mexico would incorporate four main input components: 1) mudflow density, 2) slope percent, 3) material strength, and 4) 'gassy' mud thickness. Of these factors material strength, as determined from available borehole and geophysical data, was found to be highly variable within mudflow deposits. Sediment strength varied significantly, both laterally and with depth. In addition, Hooper and Suhayda (2005) showed that sediment strength likely varies with time with local strengthening of mud deposits following degassing associated with mudflow deposition. Therefore, it was difficult to derive diagnostic characteristics for recent or active mudflows.
The researcher's initial approach focused on delineating and characterizing areas of failure. That is, by characterizing the geotechnical properties of mudflow sediments, the hypothesis was that it would be possible to identify similar sediments susceptible to future failure. However, it was discovered that material strength lessens after soil movement and therefore areas that had not recently failed were less susceptible to failure in the future. There appeared to be areas of the sea floor that are relatively stable over time. The researcher worked with available historical bathymetry to delineate the areas of the sea floor that have changed the least over time. The researcher's evaluation of mudflow susceptibility and the numerically-based evaluation of mudflow hazard by Dr. Wright and Gilbert of OTRC, relied on compiled boring and laboratory data to provide sediment properties and the means to evaluate relative potential for failure. Comparison of sea floor conditions prior to and after Hurricane Ivan illustrated the degree and type of change that can occur in high soil movement areas such as the Mississippi delta, however due to the length of time between the pre-Ivan surveys and Ivan the data did not allow the researcher to determine which mudslides, if any, were due to Hurricane Ivan.