Louisiana's coastline has been losing wetlands at a rate of 16.57 square miles a year during the past 25 years, equal to the loss of a football field of coast every hour, according to a study released today by the U.S. Geologicial Survey.
That's five square miles a year faster than measured by the USGS between 1985 and 2004, the last time such a study was done.
The increase is largely the result of wetlands lost during Hurricanes Katrina and Rita in 2005 and Gustav and Ike in 2008, said Brady Couvillion, a USGS geographer and lead author of the new survey.
That rate is still far below the 42 square miles a year measured during the 1970s in a 1993 study by Army Corps of Engineers officials.
The new study found that a post-Katrina and Rita estimate of 217 square miles of wetlands lost to the two storms - made in October 2005, only days after the storms - was premature, Couvillion said. Only about 70 percent of those wetlands were really missing.
"There were some areas that were just flooded temporarily and they were included in those 217 miles," he said. "And when the flooding receded, we picked them up as land again."
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That's in part the result of how the land is mapped: by satellite.
"Weather conditions on the particular day you look at the satellite image can affect how much land we assess that we currently have," he said.
In 2008, though, Gustav and Ike shredded another 94.69 square miles of valuable wetlands.
In all, Louisiana has lost 1,883 square miles between 1932 and 2010, according to the study. That's an area as large as the state of Delaware.
"If that loss were to occur at a constant rate, it would be equal to losing more land than the island of Manhattan every year," Couvillion said.
Preliminary measurements for the years 2009 and 2010 indicate what could be the beginning of a positive trend: a loss of only 3 square miles of wetlands.
That trend could result from the continued natural healing of some damage from the four hurricanes, including the reestablishment of flotant or quaking marshes, Couvillion said.
It also could represent gains resulting from spending millions of federal and state dollars on small to moderate coastal restoration projects, he said.
"If you take out the fact that we've had four of the most devastating hurricanes in the last century happen in the last six or seven years ... you will see that the land loss rate has declined," said Phil Turnipseed, director of the USGS National Wetlands Research Center in Lafayette.
Turnipseed believes that promising trend is partly because many areas of marsh that have been most susceptible to erosion or subsidence have already disappeared.
But he thinks key human contributors to erosion in the coastal marshes - the oil and gas exploration and production industry - also have become more environmentally sensitive in their operations.
The map created as part of the study shows lost wetlands in 32 color shades identifying land gains and losses over the years.
"We've known for a long time that we're losing coastal wetlands," Couvillion said. "We've better isolated not only where the losses have occurred, but when they occurred, and that helps us understand why they've occurred. You can see patterns of why it was lost and you know that has implications for future management and protection and restoration."
Before 1973, the few studies measuring wetland losses were based on photographs from planes and ground surveys.
Then researchers began using satellite images to monitor the changes more frequently. They caught a dramatic increase in the wetlands loss rate in the 1970s that coincided with the initial outcries about the coast's future from scientists, environmentalists and state officials.
That rapid loss rate was likely from several causes, Turnipseed and Couvillion said. Subsidence in wetlands in the Barataria Basin quickly followed the rapid drawdown of oil and gas beneath them. But part of the loss was likely the result of what Couvillion refers to as "sediment deprivation."
"We built levees and sediment is no longer allowed to get out there and sustain these marshes and help them keep up with sea level rise and the subsidence that may have been in part related to oil and gas extraction," he said. The loss of sediment also is liked to the rapid development of dams for hydropower and other purposes on the Missouri and upper Mississippi rivers, which captured sand and dirt that would otherwise have traveled to Louisiana.
"We built these levees long before the '70s, but you have a latency period and they're not immediately lost when they get cut off from their sediment sources," he said. "They held on as long as they could, but it looks like a lot of the collapse occurred in the '70s."
Fast-forward to the 1980s, and the rate of wetlands loss begins to dip and rise. Part of that is due to natural variability or satellite images taken when high tides or storms covered wetlands, Couvillion said.
The next two dips are the result of the hurricanes, before a significant rise representing the last two years.
"We're continuing to study these areas to see if that trend of increasing wetland areas continues into the future," he said.
But the data from the study also are being fed into computer models being used by USGS, state and corps scientists to predict future subsidence rates as part of the state's 2012 update of its master plan for coastal restoration and hurricane protection.
"What you've seen in the past is people will paint the coast blue, and it gives you no idea of the types of uncertainties that went into their model," he said, referring to maps showing the effects of the combination of subsidence and sea level rise caused by natural and human-induced global warming.
The uncertainties include the half-dozen sea-level rise scenarios that have been proposed by scientists, based on varying estimates of the speed of global warming. But they also include estimates of how quickly wetlands die when they're inundated by rising water, and differences in the rate of sinking soils at different locations along the coast.
The federal-state team will apply those uncertainties to sets of restoration and hurricane protection scenarios being proposed by the state.
"That will convey to the public a series of best case and worst case scenarios," Couvillion said. "The main driving force behind the effort is determining where we can best locate projects in order to maintain as much marsh as we possibly can."