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Technology
Assessment & Research (TA&R) Program
| Project Number |
550 |
| Date of Summary |
April 5, 2006 |
| Subject |
A Pilot Study for Regionally-Consistent Hazard Susceptibility Mapping of
Submarine Mudslides, Offshore Gulf of Mexico |
| Performing Activity |
William Lettis & Associates, Inc. |
| Principal Investigator |
Christopher Hitchcock |
| Contracting Agency |
Minerals Management Service |
| Estimated Completion |
Completed |
| Description |
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. |
| Progress |
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. |
| Report |
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AA (29
pages) |
A Pilot Study for Regionally-Consistent Hazard Susceptibility Mapping of
Submarine Mudslides, Offshore Gulf of Mexico", Final Report, submitted by
Christopher Hitchcock for William Lettis & Associates, Inc., March 1, 2006. |
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