Levees vs. Wetlands Get Spotlight
Subsidence a Lurking Villain
In the aftermath of Hurricane Katrina, the rhetoric about the pressing need to save Louisiana’s rapidly disappearing wetlands escalated considerably. After all, one of the many valuable roles the wetlands play is to help protect low-lying population centers from some of the ravages of hurricanes.
Vanishing wetlands, however, are just a symptom of a far bigger disease crippling the coast, according to Roy Dokka, Fruehan endowed professor of engineering at Lousiana State University and former Adolphe G. Gueymard professor of geology.
The crippling disease?
It’s subsidence; and it’s way more extreme -- and extensive -- than previously thought, according to Dokka.
Coastal Louisiana has subsided approximately two- to-four feet since 1950. Furthermore, the problem extends inland from the coast for hundreds of miles, especially along the Mississippi River valley, Dokka noted when testifying before a U.S. congressional subcommittee in October 2005.
Both natural and human-induced activities are recognized to play a role in lowering the land surface relative to sea level since the last sea level low stand. Levee building, which restricts the natural accumulation of sediment that occurs when rivers overflow their banks, has received its share of the blame. Prior to extensive levee construction, the deposited sediments helped provide a natural offset to subsidence.
“Some people say you can fix the wetlands by pouring water out into the coast like nature,” Dokka said. “That’s true, but people can’t live in south Louisiana without hurricane levees, because of the low and ever-sinking landscape.
“Fundamentally, there’s no remedy; if you live there, you better have a levee in front of you,” Dokka said. “But we need to do both, because what would Louisiana be without its wetlands?
“To understand the subsidence, you have to understand the geology,” Dokka noted. “The group that understands the geology of the area best is the oil industry, because what you see in the subsurface is exactly what’s going on today.
“But the average coastal scientist doesn’t know what the global oil industry knows about the evolution of the Gulf of Mexico.”
The Big Picture
In fact, Dokka asserts that key aspects of the coastal work that has been done in the region is flawed because it has failed to integrate what’s known from the geological history of the Gulf.
Any discussion of subsidence requires a look at the impact of the Mississippi River, among other phenomena.
The river carries massive volumes of debris from inland areas, which are deposited in its delta region where the river enters the Gulf. The enormous weight continues to depress the earth’s crust and mantle.
Furthermore, the pile of sediment is weak and unstable, and large segments have slumped southward along south-dipping or sloping faults. This accumulation of gargantuan quantities of sediments also has led to the mobilization of underground salt, according to Dokka.
“The Mississippi delta constitutes a massive load on the earth’s crust and is causing it to bend,” he said. “That’s an idea that was first noted in the literature in 1870 by one of the renowned people in geology, and the idea was later picked up by Richard Russell (the noted late geographer who co-founded the LSU geography and anthropology departments in 1928 and later founded the LSU Coastal Studies Institute).
“In fact, if you want to understand what’s happening in the coast, go back and read Russell’s work in the ‘30s,” Dokka said. “That’s all been forgotten, and now it’s all about the wetlands.”
There’s plenty more to worry about than the causative elements per se.
In fact, a whole new debate has emerged concerning subsidence rates, following the July 2004 publication of NOAA Technical Report NOS/NGS 50: Rates of Vertical Displacement at Benchmarks in the Lower Mississippi Valley and the Northern Gulf Coast. The report was co-authored by Kurt Shinkle at the National Geodetic Survey and Dokka.
The effort entailed measurement of modern gulf coast subsidence based on first order geodetic leveling measurements on benchmarks and tidal records published by NOAA.
To assess the accuracy of the National Spatial Reference System in the region, Dokka and Shinkle computed vertical motions on 2,710 benchmarks throughout Louisiana, Mississippi and coastal areas of Alabama and Florida. The fixed datum used in Technical Report 50 is the North American Vertical Datum of 1988 (NAVD 88).
The conclusion from the precise geodetic measurements: Subsidence rates in southern Louisiana are significantly higher than previously thought, with the mean rate pegged at 0.43 inches per year.
Additionally, it was concluded subsidence also is occurring far beyond the Mississippi River delta wetlands.
Shinkle and Dokka noted the data demonstrate subsidence has multiple human-induced and natural causes that include a large tectonic component and, locally, a substantial fault component.
Nature vs. Nurture
Bob Morton, research geologist at the U.S. Geological Survey in St. Petersburg, Fla., doesn’t necessarily buy Dokka’s conclusions that much of the subsidence is due to natural geologic events including both tectonic and depositional processes, such as crustal down-warping, salt movement, gravity slumping, sediment loading and compaction, along with eustatic sea level rise.
For instance, Morton said subsurface fluid withdrawal (read: oil and gas) produces the same results as sediment loading.
“I’m not saying all wetland loss is associated with oil and gas production and recent extraction,” Morton said, “but a pressure drop in the reservoir is the same as adding new sediment on the surface. A change in the state of stress in the subsurface is quite capable of causing compactional subsidence in the reservoir and also movement along the faults.”
Morton cited two lines in the NOAA study that crossed the structural grain of the Gulf Coast region in one area where he said the data indicated the highest rates of subsidence occurred over oil and gas fields in that particular locale.
Dokka emphasized, however, the data in Technical Report 50 do not support oil and gas extraction as a substantial contributor to subsidence, even at type areas.
Despite considerable behind-the-scenes exchanges between the U.S. Geological Survey and NOAA about the methods and numbers associated with Technical Report 50, Morton acknowledged the numbers put out must be accepted on face value.
“NOAA is still the official government agency responsible for elevation and elevation change in the U.S.,” he noted.
Technical Report 50 is fundamentally a geodetic report rather than an interpretive effort, and Dokka said the geodetic method used is as good as it gets.
The noted Mississippi River delta expert and LSU Boyd professor, James Coleman, concurs.
“I haven’t seen the results (of the study),” Coleman said, “but setting those benchmarks up was the right thing to do and hadn’t been done before. The tracks he did were good tracks in the right place, and the benchmarks he picked were good and in areas both in and out of oilfields.
“Basically, it just showed a general overall subsidence of the whole coast,” Coleman said, “like everyone’s been saying for years.”
Making it Legal?
Dokka attributes any criticism of the report to the fact that the implications of the documented high rates of subsidence are downright scary to many people.
Consider the rebuilding of storm-ravaged New Orleans, for instance. How high should the levees actually be built to protect the inhabitants well into the future if the land they’re built on continues sinking at a rapid rate?
There’s another scary scenario just lurking in the shadows.
Dokka noted politics already have determined public policy where wetlands and coastal issues are concerned. Indeed, it’s widely known that state and federal agencies have long been jockeying for -- and receiving -- federal funds for myriad projects focused on saving and reconstituting the wetlands, touting this approach as a kind of be-all, end-all solution to coastal flooding and other problems.
He cautioned that professional geological organizations and the industry as a whole must help educate the public on the geological reality of the Gulf of Mexico -- before lawsuits begin coming down the line.
“You can’t sue God,” Dokka said, “but you can sue the oil companies. The oil companies have money, and this is all about money ultimately.”