LAKE CHARLES, La. (KPLC) - We know Louisiana is rapidly losing its coastline, but our state— according to scientists at Louisiana State University — is being threatened in another way. Louisiana is sinking.
“Subsidence, as it relates to Louisiana, is the sinking or the consolidation of the surface of the state," Cliff Mugnier, chief of geodesy at LSU’s Center for Geoinformatics, said.
Over the past year, Mugnier’s department, along with the National Oceanic and Atmospheric Administration’s Geodetic Survey completed gravity measurements around the state, which show a change in our elevation.
“The main thrust of our research here at the center is in how much Louisiana is sinking or subsiding. There are a number of different ways to measure or determine that," Mugnier said.
At each place where gravity is measured, LSU’s gravity survey crew completes a thorough set up — from collecting relative gravity data, to measuring the absolute gravity at a specific location.
To measure the acceleration of gravity, they use an absolute gravity meter — a 2-foot vacuum chamber.
“We run a set of one hundred drops, so it’s one hundred drops per hour. We end up letting it run for a minimum of a day, but generally, two to two-and-a-half days," Ben Fernandez, a geodetic surveyor for LSU’s Center for Geoinformatics, said. “Each drop is a measure of the acceleration of gravity. The set of one hundred - it ends up averaging and comes up with a mean, standard deviation.”
So, how does this data explain our elevation?
The closer you are to the Earth’s core, the stronger the gravitational pull; measuring the gravity, and crosschecking it with GPS heights can give an idea of how far we are.
The first time Lake Charles was surveyed was in 2003, and, compared to the most recent results from the geodetic survey, Lake Charles has sunk 16 millimeters in 16 years.
“Southwest Louisiana is subsiding, we think, primarily because of the amount of deposition since the last Ice age. That kind of deposition, geologically, is called Holocene,” Mugnier said.
Holocene is the system of deposits laid down during that period of time.
“The thicker the Holocene, the more subsidence one experiences. There’s a lot of Holocene in Lake Charles, and even more Holocene in Terrebonne Parish," Mugnier said. “It just depends on how the rivers in the past ten-thousand years or so have geologically built Southern Louisiana.”
But, what exactly does this mean for the Lake Area?
“Sixteen millimeters seems like a small amount. But, when you convert that to inches, there are 24.5 millimeters in an inch," Mugnier said. "When we consider that as a constant, more or less rate of subsidence, over ten, twenty, fifty, or one hundred years, that accumulative amount of subsidence, means that, in less than one hundred years, all of Calcasieu Parish will be at or below sea level. Less than zero elevation. In other words, join the people like in New Orleans.”
Mugnier said residents in Lake Charles would likely see impacts from subsidence during storms.
''It will be at or near sea level. But between now and then, it’s highly likely that the State of Louisiana will be putting up some flood protection devices like levees and the like to protect Lake Charles. So, it won’t be on the coastline, but they will be in greater danger of a hurricane," Mugnier said.
He said subsidence is something we can’t really prevent, but maybe something we can slow down.
“It’s a function of god’s universe, mere people can’t do a single thing about subsidence," Mugnier said. “Other than to modify their way of life and to have to move to the whims of nature as it occurs, either through catastrophic storms or through slow actions such as subsidence. But, essentially, civilization so far is completely helpless.”
Bren Haase, the executive director of the Coastal Protection and Restoration Authority, or CPRA, agreed it’s not preventable.
“So, not directly. Not directly to reduce subsidence, subsidence is always going to be occurring.”
But, Haase said it is something the CPRA is working against.
“Really, the way to combat subsidence is to have healthy coastal wetlands that can contribute to the vertical elevation of our coastal wetland through the input of vegetative matter essentially. Our marsh grasses grow and they die," Haase said. “They die in place, of course, so that provides some structure to the soil to help increase its elevation, but, primarily, through the input of sediment. And so, a large part of what we do at CPRA is just that.”
Haase said there are a number of CPRA projects that work to combat both coastal erosion and subsidence. The authority just announced additional funding for the Rockefeller Shoreline Protection Project, which aims to build up wetlands along coast areas.