Assessment of Mud-Capped Dredge Pit Evolution Offshore Louisiana
Kehui Xu1,2,*, Michael D. Miner3, Samuel J. Bentley1,4, Chunyan Li1,2, Nazanin Chaichitehrani1,2, Jeffrey
Obelcz1,2, Jiaze Wang1,2
(1) Coastal Studies Institute, Louisiana State University, Baton Rouge, LA
(2) Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA
(3) U.S Department of Interior, Bureau of Ocean Energy Management, New Orleans, LA
(4) Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA
*Corresponding author: K. Xu, 2165 Energy, Coast and Environment Building, Department of
Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803 USA.
(kxu@lsu.edu)
The shelf offshore Louisiana is characterized by a dominantly muddy seafloor with a paucity of
restoration-quality sand proximal to shore. Large sand shoals are present but their offshore distance has
been cost prohibitive for most projects. However, discrete sand deposits associated with ancient rivers
that incised the shelf during lower sea-level positions occur closer to shore. The channels were filled with
sandy sediments as sea-level rose, and ultimately buried by recent mud deposition. These shelf channel
sands have been targeted for coastal restoration projects resulting in significant cost savings over more
distal deposits. Several recent projects targeted shelf paleo-fluvial deposits comprising relatively deep
channel sands underlying a muddy overburden. The muddy seafloor and deep nature of the pits are quite
different from sand-only environments because they are usually narrow and deep (e.g., 10m below
seafloor to channel base). Moreover, excavation produces a deep pit that has walls composed of muds
overlying sands. Because of contrasting characteristics of cohesive mud vs. non-cohesive sand and
potential modern fluvial mud supply, long term pit evolution is poorly understood relative to their more
common sand-only counterparts.
Alterations to seafloor topography from dredging shelf sediment resources can potentially affect oil and
gas infrastructure or other resources of concern (i.e. historic shipwrecks) located proximal to dredge pits.
Mitigations are applied to protect adjacent areas but the basis for assigning buffer distances is somewhat
subjective and the effectiveness of mitigations has not been evaluated. Site-specific data required to make
accurate predictions and empirical measurements to test and validate predictive models were only
available for Peveto Channel offshore Holly Beach, Louisiana. Our study will build on initial studies to
better understand this problem by filling data gaps, validating predictive models developed during
previous studies, and evaluating the effectiveness of mitigations applied to existing dredge pits.
Here we present new geophysical and geological data (bathymetry, sidescan, subbottom, and shallow
sediment cores) and physical oceanographic observations (hydrodynamics and sediment dynamics)
collected at Raccoon Island (dredged in 2013) and Sandy Point southeast (dredged in 2012) dredge pits in
Louisiana. These field data collections along with pre-existing data provide a time-series to capture
evolution at Raccoon Island and Sandy Point southeast post-excavation. Conceptual morphological
models will be developed for dredge pit evolution and testing effectiveness of setback buffers protecting
pipelines, habitats, and potential cultural resources. Our results will increase decision making ability
regarding safety and protecting environmental and cultural resources, and better management of valuable
sand resources in Outer Continental Shelf.
Brief biography of presenter Kehui Xu:
Dr. Kehui Xu is a geological oceanographer at Louisiana State University whose research has been
focused on delivery, transport and deposition of fluvial sediment in continental margins using
observational, laboratory and modeling approaches. He has been studying the Mississippi deltaic coast
since 2007, and has been leading or participating in multiple projects funded by both federal (NSF,
BOEM, NOAA) and state (Sea Grant) agencies. His research areas include geological oceanography,
coastal morphodynamics, observation and numerical modeling of sediment transport along bottom
boundary layer, sedimentary geology and coastal processes. He has published ~30 articles which received
over 1,800 citations.