Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Cuts to Mississippi levees could build new land in sinking delta

21.10.2009
Diverting sediment-rich water from the Mississippi River below New Orleans could generate new land in the river's delta in the next century that would equal almost half the acreage otherwise expected to disappear during that period, a new study shows.

For decades, sea-level rise, land subsidence, and a decrease in river sediment have caused vast swaths of the Mississippi Delta to vanish into the sea. The anticipated buildup of new land in a portion of the delta, as simulated by a computer model, could compensate for a large fraction of the expected future loss, protect upriver areas from storm surges, and create fresh-water habitat, the researchers say.

"What this model mainly shows is that we can, to a large degree, match future land loss by making these diversions," says David Mohrig, an associate professor at the University of Texas (UT) in Austin and a member of the research team. Wonsuck Kim, an assistant professor at UT-Austin led the study, which is reported in today's issue of Eos, the weekly newspaper of the American Geophysical Union (AGU).

The delta of the Mississippi River has been losing land to the sea at an average rate of about 44 square kilometers (17 square miles) per year since around 1940. The natural equilibrium between soil loss and sediment deposition has been altered by the levees that the U.S. Army Corps of Engineers built below New Orleans to prevent the Mississippi from flooding. The confined waters at the end of the river's course flow faster and drop their sediments over the continental platform, draining uselessly into the Gulf of Mexico.

"History recorded in the deposits of the river shows that the main channel of the Mississippi moved roughly every 1,000 years to a new lowland area," Kim and Mohrig note. "But the engineering of the levees has kept the river from entering lowland areas and depositing sedimentation."

The model looks at potential effects of an existing and contentious proposal to divert Mississippi River water through a pair of cuts made opposite each another in the levees 150 kilometers (93 miles) downstream from New Orleans. Nearly half of the river's flow would spill out through the cuts, taking sediment with it and depositing it to each side of the river channel.

Despite sea level rise, increased land sinking rates, and a drop in the river's sediment supply, the diversions would create an amount of new land equal to up to 45 percent of the area that would otherwise be lost to the sea in the coming century, the model predicts. Enough flow would remain in the main channel of the river to allow navigation there, the researchers report.

Other researchers studying coastal restoration had previously proposed creating these two diversions to allow water and sediment to exit the enclosed river and build two lobes of new land in adjacent shallow-water sections of Breton Sound and Barataria Bay.

But critics say that dams in the upper sections of the Mississippi River have reduced the water's sediment content so much that there isn't enough raw material left to rebuild the delta. Also, detractors argue that future sea level rise and the current high sinking rate of the delta would make restoration impossible.

"Until we put together this model, there was just a lot of debate that wasn't substantiated by anything but by intuition," says Mohrig. "We needed to move from having very soft impressions of what could be done to making predictions that can actually be tested."

The modelers use a conservative sediment supply rate, subsidence (i.e.,
sinking) rates from 1 to 10 millimeters (0.04 to 0.4 inches) per year, and rates of sea level rise that range from 0 to 4 mm (0.16 in) per year. In their calculations, the authors consider diverting only 45 percent of the water to ensure that the section of the river below the diversions remains open to navigation.

The model predicts that the two diversions would create between 701 sq km (about 271 sq miles) and 1217 sq km (470 sq miles) of new land over a century, partially offsetting land loss. Kim and Mohrig calculate the engineered new delta lobes would make up for 25 to 45 percent of the area expected to vanish throughout the delta between now and 2110.

"Diversions are really the only cost-effective way of building land that anyone is proposing," Mohrig says

The researchers verified their model by making it run a simulation of the evolution of another delta influenced by an existing diversion of the Mississippi River: the Old River Control Structures. These structures divert water from the Mississippi to the Atchafalaya River, which also empties into the Gulf of Mexico. The Atchafalaya River is currently building new land both in the Atchafalaya Delta and its subsidiary, the Wax Lake Delta, and Kim's model was able to predict the amount of land that has been built since 1980.

Torbjoern Toernqvist, an associate professor at Tulane University in New Orleans who has conducted research on the Mississippi Delta but not related to this study, says that the authors have been "too optimistic" in the sea level rise and subsidence rates they use in their model, since the current rates are already higher than the ones the researchers use in their best-case scenario.

"What they consider their worst case scenario should be considered the base case scenario -- the combination of 4 mm/yr of sea level rise and the subsidence rate of 10 mm/yr is the most likely scenario," Toernqvist says.

"That said, it is also true that even with the worst case scenario, [the model predicts] you can build substantial new land."

Kim and Mohrig acknowledge that there is debate about how fast the Louisiana coast is sinking. The highest rates of subsidence, caused by automobile traffic, pumping of underground oil and gas, and other human influences don't affect large swaths of the delta, they note."We're comfortable with the numbers" in the new study, Mohrig says.

He and Kim collaborated on the model with scientists from Louisiana State University, Baton Rouge; the University of Minnesota, Minneapolis; and the University of Illinois, Urbana-Champaign.

The National Science Foundation funded the research.

Peter Weiss | American Geophysical Union
Further information:
http://www.agu.org
http://www.agu.org/sci_soc/prrl/primages.html

More articles from Earth Sciences:

nachricht In times of climate change: What a lake’s colour can tell about its condition
21.09.2017 | Leibniz-Institut für Gewässerökologie und Binnenfischerei (IGB)

nachricht Did marine sponges trigger the ‘Cambrian explosion’ through ‘ecosystem engineering’?
21.09.2017 | Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GFZ

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: The pyrenoid is a carbon-fixing liquid droplet

Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.

A warming planet

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Rainbow colors reveal cell history: Uncovering β-cell heterogeneity

22.09.2017 | Life Sciences

Penn first in world to treat patient with new radiation technology

22.09.2017 | Medical Engineering

Calculating quietness

22.09.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>