COCODRIE, LA. — To build a levee or to enlist wetlands to defend against a hurricane's storm surge, it's critical to know how fast – and why – the land is sinking under your feet.
Next year, surveyors head into the field under a National Oceanic and Atmospheric Administration program to help pin down current rates of subsidence, according to Julianne Blackwell, chief of observation and analysis at NOAA's National Geodetic Survey.
The results could play a key role in resolving a debate about the leading causes for southern Louisiana's high subsidence rates. If the causes are largely man-made, such as the widespread use of levees or oil-drilling or removal of groundwater, then humans can undo them. If the causes are predominantly natural, then the only long-term protection against threats from sea-level rise and hurricane storm surges may rest in a border-to-border barrier system along the coast, notes Roy Dokka, a geologist at Louisiana State University. And the barriers must be built high enough to account for subsidence anticipated over their design life.
In 2004, a NOAA report Dr. Dokka coauthored showed that between 1920 and 1995, the coast from Texas to Alabama was sinking. The highest rates were along the Louisiana coast, and the pace was far faster than previous studies had indicated.
He points to a metal "benchmark" here in Cocodrie, on the Louisiana coast. It's a disc-capped steel rod that many years ago was driven deep into the ground until it hit something immovable. Then, the disk was at ground level.
"When we first came out here and looked in 2002, that benchmark was 200 millimeters off the ground," he says.
On this day, he measures it again. The disk now hovers some 259 millimeters above the ground. The land has subsided roughly two inches in four years. At that rate, this patch of Louisiana would drop another four feet – and dip below sea level – in 100 years.
Dokka holds that subsidence rates for southern Louisiana are too high to attribute all of the problem to trends in surface sediment or to human activities, such as pumping out groundwater. He holds that deeper, tectonic forces – the weight of accumulated sediment depressing the crust – and local faulting play a much larger role than previously believed. The danger: Wetlands restoration, which is important for many other reasons, nevertheless may be oversold as a defense against sea-level rise and storm surge if the land keeps sinking.
To be sure, Dokka's appears to be a minority view at the moment among scientists studying Louisiana's coast.
Based on his own work, Tulane University geologist Torbjörn Törnqvist suggests that the Mississippi Delta is tectonically stable and that the bulk of the subsidence problem occurs in soil layers at depths shallow enough to allow for human restoration efforts to help significantly.
The different viewpoints could make it difficult for policymakers to design the long-term flood protection that the region needs.
"I feel bad for people who have to develop policies," he says.