Atchafalaya River may rebuild wetlands
Published: Thursday, October 25, 2012 at 9:20 p.m.
Last Modified: Thursday, October 25, 2012 at 9:20 p.m.
A study lead by a University of Pennsylvania scientist examined the region's 2011 flood to offer new insights about how diversions off the Mississippi River might help restore Louisiana's wetlands.
Scientists found that during the flood, sediment-laden water jetted down the controlled and leveed Mississippi River, shooting far out into the Gulf and out of the system. In contrast, river water coming down the slow-moving Atchafalaya River was allowed to flood the basin when a flood-control outlet called the Morganza spillway was opened for the second time ever.
The action flooded nearly 7,500 square miles of land, damaged crops and homes, and took several lives.
But it also offered scientists a rare look into how floods in the delta may have looked before river control structures locked the river into place.
“While this was catastrophic to the people living in the Atchafalaya Basin, it was also simulating — accidentally — the sort of natural flood that used to happen all the time,” said Douglas Jerolmack, an assistant professor in the Earth and Sciences Department at the University of Pennsylvania and lead scientist on the study.
“We were interested in how this sort of natural flooding scenario would differ from the controlled floods contained within levees that we normally see in the delta,” Jerolmack said.
Scientists said the research might help the state's efforts to recreate land-sustaining river floods by opening up diversions off the Mississippi River. This is among the only large-scale experiments conducted to look at how those diversions might function.
“We were interested in taking advantage of the 2011 flood to understand something about the hydrodynamics, the way the water moves out of the river outlets and how much sediment might be carried out with that flow and into the marshes,” said Karen McKee, a recently retired U.S. Geological Survey research ecologist who worked on the project.
The research, which was reported in Nature Geoscience, included scientists from five universities in the United States and Italy, the Geological Survey and Italy's National Research Council.
The team began by examining satellite images showing the plume of sediment-laden water pouring from the mouths of the Atchafalaya and Mississippi rivers. They calculated the amount of sediment in the plumes based on the ocean color in the satellite images. Field samples taken from a boat in the Gulf of Mexico helped to corroborate these calculations. Boat sampling also allowed them to gather data on the speed of the plume and the extent to which river water mixed with ocean water.
From the satellite images, researchers observed that the Mississippi River unleashed a jet of water into the ocean. In contrast, the waters diverted into the Atchafalaya Basin spread out over 100 kilometers, or 62 miles, of coastline, the sediment lingering in a wide swampy area.
McKee led a team that travelled by helicopter to 45 sample sites covering nearly 125 miles of coast to sample sediment cores.
They observed more sediment deposited in more locations across the Atchafalaya Basin. Below the end of the Mississippi River levees in the Bird's Foot Delta, there were also substantial amounts of river sediment, McKee said. But almost no sediment reached the river-starved parishes in Terrebonne and Lafourche, which was expected.
Scientists believe the Atchafalaya's slower flow allowed direct deposit of sediment into the marshes. And what sediment ended up in the Gulf was in the near shore current in shallow water, where it might have an opportunity to be deposited back into the marshes during a storm or high tide.
The sediment carried by the Mississippi was shot out into the Gulf so far that it left the system completely, McKee added.
“You have this intentionally flooded Atchafalaya Basin, and when those flood waters hit the coast they were trapped there for a month, where tides and waves could bring them back on shore,” Jerolmack said. “Whereas in the Mississippi channel, where all the waters were totally leveed, you could see from satellite images this sort of fire hose of water that pushed the sediment from the river far off shore.”
Jerolmack said the Atchafalaya is showing us that “this is the sort of diversion that you would need in order to create effective sedimentation and marsh building.”
Nikki Buskey can be reached at 857-2205 or email@example.com.
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