Geologic Faults Cause Structures In New Orleans to Sink, Study Says
Monday, April 3, 2006
NEW ORLEANS, April 2 -- Add one more possible threat to Louisiana's rebuilding: active geologic faults that are causing levees, flood walls, bridges, homes and other structures to sink.
A new study, published in the April edition of the Geological Society of America's Geology journal, charts a major fault, the Michoud fault, that runs through eastern New Orleans.
The study argues that the fault's downward movement "set the stage for the devastation of Hurricane Katrina by lowering elevations of the land and surrounding levee defenses."
In recent years, studies of subsurface faults, salt domes and other geologic characteristics have emerged as critical in the debate over what is causing Louisiana to lose vast tracts of land. The coast has lost about 1,900 square miles of wetlands -- an area about the size of Delaware -- since the 1930s.
The slumping of the Michoud fault, the study said, has caused as much as about 73 percent of the subsidence in sections of eastern New Orleans, an area that has seen some of the worst rates of land loss in south Louisiana. The land sank by as much as 1.7 inches from 1969 to 1971, according to Roy Dokka, a Louisiana State University geologist.
The study was the result of ongoing work by Dokka and the National Geodetic Survey to calculate land changes in south Louisiana, using global positioning system base stations and tide gauges.
Dokka's theories on how natural tectonic fault movements cause subsidence run counter to studies that have shown oil extraction and soil compaction as main reasons why the land is sinking.
This threat from deep underground has major implications. If faults are causing the sinking, there is very little humans can do to offset the subsidence other than build higher levees.
"It basically says that when levees cross faults, they need to apply more dirt on top of them to keep them safe," Dokka said.
But if oil drilling were the main culprit, then there is hope that the slowdown in onshore drilling will equate to less subsidence.
"Can faulting be induced? Absolutely, and he [Dokka] omits that completely," said Bob Morton, a geologist with the U.S. Geological Survey, who blames oil drilling for large amounts of subsidence.
Morton said sampling peat deposits is a more reliable method for determining long-term rates of sinking.
Based on peat calculations, the rate of subsidence was minimal before oil started being extracted in the 20th century, he said.
Dokka said oil was not drilled near the Michoud fault, so the sinking was not caused by "the usual suspects. . . . It has to be something deep."
A recent report by the National Academy of Sciences, "Drawing Louisiana's New Map," said subsidence needs more study.
Despite the disagreements, subsidence is a component in rebuilding plans. Al Naomi, a top U.S. Army Corps of Engineers project manager, said more attention is being paid to it and to fault studies.