Degree
Doctor of Philosophy (PhD)
Department
Department of Oceanography & Coastal Science
Document Type
Dissertation
Abstract
The Louisiana continental shelf harbors a remarkable geological feature—mud-capped dredge pits—that are intricately associated with Louisiana state’s ongoing coastal protection and restoration efforts. The significance of these pits extends beyond their immediate utility as they also offer unique insights into sediment dynamics and the interaction between anthropogenic activities and marine processes. Many studies have been performed in the dredge pits on the inner Louisiana continental shelf in recent decades, but our knowledge of temporal and spatial variations of sediment transport and post-dredge morphology is still limited. The objectives of this dissertation are to explore the sediment infilling process, the spatial and temporal evolution of the morphology of three mud-capped dredge pits (Raccoon Island, Sandy Point and Peveto Channel) on the inner Louisiana continental shelf by using (a) geophysical observation of bathymetry, side-scan and subbottom, (b) analysis of sediment on grain size, density, organic matter, and carbonate content, as well as (c) machine learning technique.
Six main conclusions are included in this dissertation. (1) Spatially, sediment near Ship Shoal was the coarsest and sediment deposited in the pit was the finest. Temporally, surficial sediments were generally coarser in July and November of 2021 and finer in March of 2021 and March of 2022. Spatial heterogeneity of surficial sediment interplays with the timing of Mississippi and Atchafalaya River flood season and hurricanes and together dictate the sediment texture. (2) Raccoon Island dredge pit experienced a half meter subsidence in 2021. It is highly possible that hurricanes Delta, Zeta, Laura, and Ida were responsible for potential sediment scouring and degassing. It implied that a filled-up dredge pit is not yet safe for new oil/gas pipeline or platform construction and long-term degassing and continuing consolidation must be considered in prediction. (3) In Peveto Channel dredge pit, the organic matter content is high, likely due to low hydrodynamic conditions and proximity to organic-rich wetlands and Chenier Plain. The “mud volcanoes” inside this pit are Chikishlyar type. (4) A massive pit wall collapse with an approximate area of 0.225 km2 occurred on the west side of the Sandy Point dredge pit. The largest outward migration of western pit wall was over 200 m (656 ft), which is comparable with the setback buffer distance of 500 ft for wells to 1,000 ft for pipelines, currently being used by Bureau of Ocean Energy Management. It was found that western pit walls directly hit by dominant currents experienced the largest outward migration. (5) Comparisons of three pits show that sand composition is controlled by distance to sandy shoal and paleo river channels and the extreme weather conditions are the driving forces for sand transport. The organic matter and grain size has a strong correlation in the sediment from Sandy Point dredge pit. Principal component analysis results indicate that two principal components account for more than 95% of the total variance. (6) A new Effective Geomorphology Classification (EGC) model was developed and provided the best prediction and validation accuracy among four machine learning models. The combination of SSS images and machine learning model (like EGC) will be a promising tool for monitoring geomorphic evolution and mudslides in dredge pit environment.
In conclusion, it seems that three mud-capped dredge pits on the inner Louisiana continental shelf are in different stages of life cycle. Sandy Point pit hasn’t been filled up yet, is in an ‘early’ stage and still experiencing active pit wall collapses. Raccoon Island pit was filled up around 2018 (5 years after dredging) and in a ‘middle’ stage but scouring and/or degassing formed a new half-meter depression. Peveto Channel pit was filled up in 2016 (13 years after dredging) and in a ‘late’ stage, and developed mud volcanoes. The dissertation findings can be used to better predict the complete life cycle of three pits, but none of the pits reached an equilibrium with ambient geological environment yet.
Date
7-16-2024
Recommended Citation
zhang, wenqiang, "Sediment Transport and Geomorphological Evolution of Mud-capped Dredge Pits on the Inner Louisiana Continental Shelf" (2024). LSU Doctoral Dissertations. 6576.
https://repository.lsu.edu/gradschool_dissertations/6576
Committee Chair
Kehui Xu