16 May 2016
By Alan Buis
New 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 finds.
The observed rates of sinking, known as subsidence, were generally consistent with, but somewhat higher than, previous studies conducted using different data.
The new study, published in the Journal of Geophysical Research: Solid Earth, a publication of the American Geophysical Union, was the most spatially-extensive, high-resolution study to date of regional subsidence in and around New Orleans. Scientists at NASA’s Jet Propulsion Laboratory (JPL), Pasadena, California; UCLA; and the Center for GeoInformatics at Louisiana State University, Baton Rouge, collaborated on the study, which covered the period from June 2009 to July 2012.
The highest rates of sinking were observed along the Mississippi River upriver of New Orleans and around major industrial areas in Norco and Michoud, Louisiana, with up to 50 millimeters (2 inches) a year of sinking. The team also observed notable subsidence in New Orleans’ Upper and Lower 9th Ward and in Metairie, where ground movements could be related to water levels in the Mississippi.
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 40 millimeters (1.6 inches) a year of sinking behind the structure and up to 40 millimeters a year at nearby industrial facilities.
While the study cites many contributing factors for the regional subsidence, 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 study’s results will be used to improve models of subsidence for the Mississippi River Delta that decision makers use to inform planning, according to the authors.
“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,” said Cathleen Jones, a scientist at JPL and lead author of the study. “The more recent land elevation change rates from this study will be used to inform flood modeling and response strategies, improving public safety.”
To fully and accurately measure and predict future subsidence in and around New Orleans, it’s necessary to better understand 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. Jones said 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 a technique known as interferometric synthetic aperture radar (InSAR). InSAR 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.
InSAR data must be carefully interpreted to disentangle these phenomena, which operate at different time and space scales. 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 said another key advantage of this study 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 said.
In addition to the UAVSAR data, researchers from the Center for GeoInformatics (C4G) at Louisiana State University (LSU) provided up-to-date GPS positioning information for industrial and urban locations within southeast Louisiana. This information helped establish the rate of ground movement at these specific points.
C4G maintains the most comprehensive network of GPS reference stations in the state. The Louisiana network consists of more than 50 Continuously Operating Reference Stations, or CORS sites, which acquire the horizontal and vertical coordinates at each station every second of every day. A wide range of people rely on CORS data, from geoscientists to surveyors, engineers and farmers, according to Joshua Kent, Geographic Information System manager at C4G.
“We define all the parameters to reduce the ambiguities,” Kent said. “This enables us to distill a location down to millimeter-level precision.”
—Alan Buis is a media relations specialist at JPL. This post originally appeared as a press release on the JPL website.