Muddy Mystery
One of the most slippery and yet fundamental aspects of all that modeling and planning is figuring out subsidence — or just how fast the Louisiana coast is sinking and whether there is anything we can do about it.
Just as Katrina struck last year, scientists were in the midst of a stormy debate over subsidence, Twilley recalls. Rates of subsidence now range from just a few ten-thousandths of an inch per year to 170 times that rate.
Geologists know of at least three things that could be causing the ground to sink lower along Louisiana’s coast. One is the extraction of oil, gas and water from the ground — which was implicated last year in a US Geological Survey report. Another is the somewhat limited natural settling and compaction of river sediments that make up the ground.
Lastly, there are deeper "tectonic" changes involving the rising and falling of shifting large blocks of real estate along faults — the sort of thing that’s more common in fault-ridden places like California.
In April, Twilley’s LSU colleague Roy Dokka came out with a paper in Geology, in which he argued that faulting and what looks like a gigantic, slow-moving regional landslide is the cause of almost three-quarters of New Orleans' subsidence.
Dokka revised regional elevation data using precision Global Positioning System equipment and discovered a higher rate of subsidence over the last 50 years is almost all caused by this tectonic movement.
Then, in July, Geology published a paper by Tulane University’s Torbjörn Törnqvist, who used an entirely different approach. Törnqvist surveyed long-buried wetland peat layers in outlying areas to come up with entirely different and milder subsidence rates.
"I think there is a very balanced dialogue going on," said Twilley of the two studies. "I have a lot of respect for both of them."
It’s just the way science works, agrees Reed, and it’s not surprising, considering that the two studies are so different in methods and the places they examined, she said.
Discovering the truth about subsidence is going to take a lot more very tedious work and a great deal of time — all of which Dokka, Törnqvist and others are already investing at top speed.
Like the people of the CPR Authority, the Army Corps of Engineers, the levees boards and everyone else along the Louisiana coast, the subsidence researchers are urgently looking for answers so planners can make the right decisions and everyone can get to work before another Katrina-sized storm comes anywhere near.
"The problem is," said Twilley, "it takes a lot of time, and time we don’t have."
