Grand Isle is sinking beneath the waves at a faster rate than any other area of the country and may be the most dramatic example of subsidence in the world, with estimates suggesting water levels there could rise between 4 feet and nearly 9 feet over the next century, according to data from the National Oceanic and Atmospheric Administration.
The readings from the Grand Isle station, which are used to gauge relative sea-level rise for the entire area, could signal New Orleans and surrounding parishes are sinking even more rapidly into the Gulf of Mexico.
Relative sea-level rise is calculated as a combination of rising water levels and sinking land.
The newly posted water-level data, which look at five years, represent a confirmation of what many who have looked into the issue have feared for years: that the region is rapidly sinking, Southeast Louisiana Flood Protection Authority-East Commissioner Stephen Estopinal said.
“The numbers are very grim, but we’ve always said the numbers are very grim,” said Estopinal, an engineer who has studied subsidence throughout his 40-year career.
Between 2007 and 2011, the average sea level at a monitoring station at Grand Isle rose about 1.32 inches, according to NOAA’s data.
Extrapolated to today, that would mean the water now is about 2.1 inches higher than it was in 2007, according to the agency.
That’s a rate more than four times that in Pensacola, Florida, which does not have the same rate of land subsidence and is seeing its water level rise by about 2.1 millimeters a year, close to the rate of general sea-level rise in the Gulf.
“Everyone else is moving at a snail’s pace; we’re zooming along, relatively speaking,” Estopinal said.
While dramatic on its own, the Grand Isle rate could indicate even greater problems for areas on the mainland, which has traditionally been seen as more prone to subsidence than Grand Isle.
The overall trend for the area has been for water levels to rise about 9.3 millimeters a year, with about 7 millimeters of that due directly to subsidence, said Tim Osborn, of NOAA’s Office of Coast Survey. The remainder is due to rising sea levels, though that increase has been part of a long-term trend and is not attributed to climate change, he said.
The only areas that exceed that rate are locations in Thailand and Japan where ground-water extraction and salt mining are directly contributing to subsidence, Osborn said.
The Grand Isle rate “is a very, very high rate of change,” he said.
A number of factors contribute to subsidence in Southeast Louisiana, including the levees along the Mississippi River, which prevent flooding but also keep sediment from rebuilding the land.
Drilling and dredging in wetlands by oil and gas companies, shipping channels and other factors also have contributed to land loss in the region.
While discussion of land loss and subsidence is often focused on rural communities closer to the Gulf, the issue of rising water levels will have significant impact on the New Orleans area and, in particular, the ability of the flood protection system designed by the U.S. Army Corps of Engineers after Hurricane Katrina to withstand future storms.
Water levels do not directly correlate with storm surge, and a difference of a few inches could, under the right conditions, translate to several feet of additional surge during a hurricane, Estopinal said. The exact impact of that rise is unclear.
“If you had a half-inch of subsidence, it doesn’t mean you’re going to have a six-inch increase in storm surge; you could have a four-foot increase,” he said.
Further complicating the task of flood protection is the fact that subsidence is not uniform across the region and can vary significantly even over short distances. The impact of that could be catastrophic if, for example, one portion of a floodwall is sinking at a faster rate than another.
It’s not clear how those structures would respond to such stresses or how the stresses could affect their ability to protect the region.
Estopinal has pushed for some time for the levee authority to do its own real-time monitoring of how quickly the region — and its flood protection system — is sinking, an issue that became more urgent in the fall when data from Michoud showed significantly higher than expected subsidence.
“That was a red-light warning that we may have some other kinds of movements going on,” he said.
The authority is working with the state’s Coastal Protection and Restoration Authority on developing that kind of a monitoring system, something Osborn said would also help more fully flesh out the readings gathered by his agency.
Commissioners with the flood protection authority plan to meet with Osborn on Thursday to discuss the issue and discuss the monitoring plans, which are aimed at determining how subsidence will affect the region and whether it will create weaknesses in the system.
“We need to know: Here’s where our threat is, here’s where we really have to spend our money to get our protection right,” Estopinal said.
Follow Jeff Adelson on Twitter, @jadelson.