Marine infrastructure projects are becoming larger and more complex as technology improves. Existing and proposed structures are often located in areas subject to marine geohazards that may include active faulting, submarine landslides, shallow gas, turbidity currents, and complex subsurface stratigraphy. Large amounts of data must be acquired, stored, analyzed, interpreted, and presented to evaluate these geohazards during all stages of project work from feasibility to construction. Utilizing a GIS database is the most efficient way to complete these tasks. The Sanitation Districts of Los Angeles County (LACSD) are evaluating the feasibility of a new tunnel and ocean outfall offshore the Port of Los Angeles, California. This project serves as a case study on the essential role of the GIS database in storing and analyzing various types of data, allowing integrated marine geohazard evaluation of potential tunnel alignments and diffuser locations. A GIS database containing data from previous work in the area served as the starting point for the project GIS database as the feasibility phase began in 2006. Geological, geophysical, and geotechnical data were added to the project GIS database during this feasibility phase. The project area lies in a region characterized by complex geology due to interaction between the Pacific and North American Tectonic Plates. Seismicity and geologic structure, as well as fault data for the Palos Verdes and Cabrillo faults which traverse the project area were incorporated into the GIS database. Geophysical data in the project GIS database included multibeam bathymetry and seismic reflection data with interpreted geologic contacts. Geotechnical GIS data consisted of logs from borings, Cone Penetrometer Tests and vibracores performed in various portions of the project area. These data were integrated using GIS to evaluate marine geohazards in the project area. Geologic contacts interpreted in the seismic data were ground truthed with the geotechnical data, exported to GIS, gridded and contoured. Areas characterized by faulting, shallow gas, hard rock, possible gas vent craters, potentially unstable slopes, and sediments possibly prone to liquefaction were mapped by integrating the GIS data. This mapped information was provided to the LACSD to assist in tunnel alignment/diffuser evaluation, cost/risk analysis, preliminary diffuser and tunnel design, tunneling equipment selection, and assessing construction methods and risks. As the project moves toward design in later phases, new data will be obtained and added to the project GIS database. Therefore, this GIS database will continue its role as an essential tool in helping to determine the final tunnel alignment and diffuser locations for this critical infrastructure project. Introduction Large marine infrastructure projects have become more prevalent around the world in recent years as technology progresses and over-water construction capabilities improve. Railroad tunnels and bridges have been built across large bodies of water. Hydrocarbons are being extracted from below the seafloor in increasingly greater water depths. Pipelines have been constructed to transport oil, gas and other materials over long distances on and below the seafloor. These existing structures, and larger proposed marine infrastructure projects, are often located in areas characterized by marine geohazards that can include active faulting, submarine landslides, shallow gas, turbidity currents, and complex subsurface stratigraphy.
This study illustrates a ground model approach used to characterize the complex site conditions within the Atlantic Shores Offshore Wind Farm Lease Area. The ground model is the result of the integration of geophysical, geological, geotechnical, and benthic (environmental) data to evaluate marine geohazards and summarize seafloor and sub-seafloor conditions. The ground model is provided in a Geographic Information Systems (GIS) format which can be interactively used for Wind Turbine Generator siting, design, and construction. Various components of the ground model including seafloor sediments and morphology, subsurface geologic features, stratigraphy, soil provinces, and soil profiles are presented and discussed. Soil province variability across the Lease Area has been successfully delineated in detail through the ground model methodology presented in this paper. Geotechnical properties of various identified soil units are generally favorable for a variety of potential foundation types and installation methods. This is one paper in a collaborative series that demonstrates the value of an integrated geoscience approach considering regulatory requirements and project design essentials.
Offshore wind developers obtain extensive geophysical, geotechnical, and habitat data during Site Characterization activities. Integration and delivery of this information to a diverse group of stakeholders and Government agencies is required. We present an integrated benthic habitat mapping approach tailored to regional geology and ground conditions and discuss how various data was utilized to deliver multiple components of the permitting process. Multiple data sets were integrated and presented via a web-based GIS platform to aid delivery, visualization, and communication. Our unified approach to benthic habitat mapping and delivery of products to stakeholders was instrumental in successfully coalescing multiple performers to develop their individual deliverables in a cohesive and rapid manner. This approach reduced risk to schedule and budget, without sacrificing data density or quality. Four annual (2019–2022) benthic surveys were acquired to support Site Characterization and subsequent permitting processes. High-Resolution Geophysical data were collected concomitantly with the 2020 benthic survey data and used to refine subsequent 2021 and 2022 benthic survey designs. Benthic survey data consisted of grab sample tests (grain size), macrofaunal taxonomy, sediment profile and plan view imagery (SPI-PV), video imagery from each grab station, and towed video transects. Acoustic data products were processed and interpreted to create polygons of seafloor sediment coverage over the ASOW study area and ground-truthed with physical sampling, video, and digital still imagery to refine and validate acoustic data into a mappable model of essential fish and benthic habitats. Seafloor morphology and seabed sediment interpretations were coalesced into a benthic habitat model that displayed substrates consisting mostly of mobile sand sheets, with interspersed areas of gravelly sand and discrete patches of gravel. Overlying the substrate model was a range of benthic features and morphologies, including sand ridges, sand waves, megaripples, ripples, areas of depressional marks, hummocky seafloor, interbedded surficial sediments, irregular seafloor, and localized relief features. From these data, classified maps of Coastal Marine Ecological Standard (CMECS) substrates and fish habitats were made. Additional CMECS classification of benthic biotic components were mapped, showing the taxonomic communities that are present in each substrate. Seabed sediment modeling and morphological trends were dynamically studied and compiled into an interpreted and GIS-friendly dataset that enabled rapid online transfer to subject matter experts tasked with quantifying the benthic ecosystem across the development area. The methods and modeling that were produced by expert refinement of geophysical data to reflect the physically observed habitat structures allowed for dynamic minimum mapping unit variability while also isolating and identifying key areas of interest for benthic researchers and regulators. This mapping process led to an efficient and unified approach for all teams, saving project time and expense.
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