GIS Database Applications to Evaluation of Marine Geohazards: New Los Angeles County Ocean Outfall Project

Abstract

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.