Study: Louisiana coastline in deep water
Land sinking faster than thought — a third of inch a year
WASHINGTON — It’s common knowledge that the coast of Louisiana is quietly sinking into the balmy Gulf waters. But new research suggests we may have been underestimating how quickly it’s happening.
A new paper, published this month in the Geological Society of America’s bulletin GSA Today, includes an updated map of the Louisiana coastline and the rate at which it’s sinking into the sea, a process scientists call “subsidence,” which occurs in addition to the climate change-caused process of sea-level rise.
The new map suggests that, on average, the Louisiana coast is sinking at a rate of about 9 millimeters, or just over a third of an inch, per year — a faster rate than previous studies have suggested, the authors said.
“I think it’s a point worth making that we are finding here that what people recently have considered worst case scenarios are actually conditions that we already see right now,” said Torbjorn Tornqvist, a geologist at and a paper.
Subsidence is thought to be a natural process that has likely been occurring in the region for thousands of years.
But scientists think the process has been enhanced by a variety of human activities in the Mississippi Delta over the past century, including oil and gas extraction, as well as the building of levees and other actions affecting the flow of the Mississippi River, which carries mud and sediment down toward the Gulf and helped build up the delta in the first place. Tulane University co-author on the
“When we started building levees along the river, we made it much harder for the sediment to disperse across the delta and beyond the delta,” Tornqvist said.
The combination of subsidence and sea-level rise along the Gulf shore has made coastal Louisiana increasingly vulnerable to erosion in recent years. Last year, residents of the region’s rapidly sinking Isle de Jean Charles received a $48 million grant from the federal government to be used for relocation, giving them the grim title of the nation’s first “climate refugees.”
As sea levels continue to rise, the problem is only expected to worsen the Louisiana shore.
As a result, many researchers have attempted to quantify not only how quickly the seas are rising, but also how quickly the land is sinking.
They’ve used a variety of methods to do so — one technique involves tide gauges, which help measure changes in sea levels.
But previous methods have not always accurately captured all the changes taking place along the shore, Tornqvist said.
The new map relies on data, published earlier this year in another paper Tornqvist also co-authored, that along were assembled using a novel approach that the authors say is more accurate.
The new approach involves a combination of GPS data and special rods, driven deep into the sediment below the surface of the water, which help measure the land’s elevation.
“This is definitely a very accurate way of doing it,” said Alex Kolker, an associate professor at the Louisiana Universities Marine Consortium, who was not involved with the new paper. The new method “looks only at the land motions, whereas tide gauges can be influenced by both land movements and water movements.”
Using the new data, Tornqvist and his colleagues created a map of subsidence in coastal Louisiana. They found that subsidence rates throughout the region averaged about nine millimeters per year, although the rate varies significantly from one location to the next.
According to the authors, previous studies have generally suggested average subsidence rates of between one and six millimeters over the past few decades.
However, Kolker suggests that the nine-millimeter finding is perhaps not so surprising after all. Other studies have suggested high rates of subsidence in certain areas along the coast at various times, he pointed out, and said he thinks a coast-wide average of nine millimeters may not be “that far off from what many people expected.”
He said the paper’s greater value is the spatial variation it reveals, with different subsidence rates at different locations. This suggests that different parts of the shoreline may need more attention than others.
“Knowing that things are slightly different from one place to the other is more important than knowing the average, particularly as the state moves forward with its master plan for coastal restoration,” he said. “In Louisiana, the amount that the area is sinking directly impacts the amount of mud that you need to fill the area and to restore a marsh.”
But while the map reveals a high degree of variability from one location to the next, Tornqvist cautioned that there’s uncertainty about the subsidence rates in any specific place.
“This map should not be used for reading values for very specific locations,” he said. “It’s more intended to show the big picture.”