Groundwater Pumping a Major Cause of New Orleans Subsidence
Engineering360 News Desk | June 01, 2016New Orleans and surrounding areas continue to sink at highly variable rates due to a combination of natural geologic and human-induced processes, a new study using NASA airborne radar has found.
The study cites many contributing factors for the regional subsidence, but the primary contributors were found to be groundwater pumping and dewatering (surface water pumping to lower the water table, which prevents standing water and soggy ground).
The research was the most spatially extensive, high-resolution study to date of regional subsidence in and around New Orleans, measuring its effects and examining its causes. Scientists at NASA’s Jet Propulsion Laboratory (JPL), UCLA and the Center for Geoinformatics at Louisiana State University collaborated on the study, which covered the period from June 2009 to July 2012.
The highest rates of sinking were observed upriver along the Mississippi River around major industrial areas in Norco, and in Michoud, with up to two inches per year of sinking. The team also observed notable subsidence in New Orleans’ Upper and Lower 9th Ward and in Metairie. At the Bonnet Carré Spillway east of Norco—New Orleans’ last line of protection against springtime river floods overtopping the levees—research showed up to 1.6 inches per year of sinking behind the structure and up to 1.6 inches per year at nearby industrial facilities.
Top: Subsidence rates around Norco, Louisiana, and the location of flood protection levees (white). Bottom: Location of water wells active in 2012, local industry and the Bonnet Carre Spillway. Image credits: NASA/JPL-Caltech, Esri. Accurately measuring and predicting future subsidence in and around New Orleans requires an understanding of the various natural and human-produced processes contributing to the sinking. Those include withdrawal of water, oil and gas; compaction of shallow sediments; faulting; sinking of Earth’s crust from the weight of deposited sediments; and ongoing vertical movement of land covered by glaciers during the last ice age. According to JPL scientist and study lead author Cathleen Jones, the comprehensive subsidence maps produced by this study, with their improved spatial resolution, help scientists differentiate these processes.
The maps were created using data from NASA’s Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR), which uses interferometric synthetic aperture radar (InSAR) to compares radar images of Earth’s surface over time to map surface deformation with centimeter-scale precision. It measures total surface elevation changes from all sources—human and natural, deep seated and shallow. UAVSAR’s spatial resolution makes it ideal for measuring subsidence in New Orleans, where human-produced subsidence can be large and is often localized.
Jones says another key advantage of this methodology is that UAVSAR enabled better resolution of small-scale features than previous studies. “We were able to identify single structures or clusters of structures subsiding or deforming relative to the surrounding area,” she notes.
The study's results will be used to improve models of subsidence for the Mississippi River Delta that decision makers use to inform planning.
“Agencies can use these data to more effectively implement actions to remediate and reverse the effects of subsidence, improving the long-term coastal resiliency and sustainability of New Orleans,” Jones says. “The more recent land elevation change rates from this study will be used to inform flood modeling and response strategies, improving public safety.”