Environment
 

Historic study to find out what the Mississippi River really carries to help coastal restoration

This image from spring 2001 shows the plume of sediment pouring from the Mississippi River into the Gulf of Mexico. More sediment would make it downstream if so much water weren't already diverted by the Old River Control Structure upstream of Baton Rouge and other outlets, natural and manmade.

NASA

This image from spring 2001 shows the plume of sediment pouring from the Mississippi River into the Gulf of Mexico. More sediment would make it downstream if so much water weren't already diverted by the Old River Control Structure upstream of Baton Rouge and other outlets, natural and manmade.

If there’s one underlying assumption that’s been used to justify the $50 billion cost of Louisiana’s Coastal Master Plan, it’s this: We actually have a chance to prevent southeast Louisiana from drowning in the Gulf because the Mississippi River carries the mud and sand necessary to keep pace with sea-level rise.

Turns out that assumption was a best guess.

In fact, the definitive evidence for the state’s claim that the river carries hope for the coast is just now being collected. A small army of state and federal researchers has embarked on  the first-ever comprehensive study of the lower river.

The Mississippi River Hydrodynamic and Delta Management Study is a massive, $25 million, five-year project begun in 2011 that seeks to provide detailed information about the building materials that today’s river can offer restoration projects. In particular:

  • Where and how those projects should be managed
  • How effective they can be in building and protecting land in the adjoining basins
  • How they will impact the U.S. Army Corps of Engineers’ traditional river responsibilities: navigation and flood control

The restoration effort relies centrally on diversions, places where the levees lining the Mississippi River will be breached to allow sediment-rich waters to flood and replenish adjacent basins, creating coastal marsh and new land. Currently, nine such diversions are planned between New Orleans and the Gulf.

“What we learn from this is absolutely critical to the success of the master plan,” said Paul Kemp, an LSU geologist and coastal researcher who served on a stakeholders’ panel that was a consultant to the state’s planning effort.

“The modeling for the master plan was amazing work done using the best information available at the time,” but that information was gathered back in the 1960s, Kemp said.

“So, now we’ll find out what kind of river we have today,” he added.

Sediment capacity lowered

Planned since 2004, the study lately has taken on new urgency, as a result of evolving insights into the river’s capacity.

A 2011 study found that much of the sediment and water the river carries into Louisiana never makes it past New Orleans, disproving the assumption that most of what the river gathers from its vast drainage system eventually is sluiced out the river’s mouth and into the Gulf of Mexico.

In fact, only 19 percent of the total suspended sediments, including just 1.4 percent of the suspended sand, that passes the mouth of the Atchafalaya River north of Baton Rouge, exits the Mississippi into the Gulf, the study found.

A significant portion of the Mississippi River sediment downstream of the Atchafalaya is trapped on land between the riverbank and the levees during times of high water. Even more drops to the bottom of the channel before it reaches New Orleans.

That last finding was attributed to one of the study’s most surprising and significant discoveries: Less than half the water the Mississippi carries past the Atchafalaya makes it to the Gulf — 46 percent. Since lower water volume equates to less power for moving sediment, the finding cast new light on the potential of manmade and natural spigots drawing from the main channel.

“The results of the present sediment budget suggest that only a relatively small proportion of the upstream sediment load is available for coastal restoration approaching the Gulf,” the study’s authors concluded.

The study was conducted between 2008 and 2010 by some of the scientists now involved in the new hydrodynamics study, including Mead Allison, director of physical processes and sediment systems at The Water Institute of the Gulf, the Baton Rouge-based research organization that assists the state Coastal Protection and Restoration Authority.

“We were not saying there isn’t still enough in the river for coastal restoration — this is a huge system and there’s still plenty enough mud to work with,” Allison stressed.

Charting differences downriver

The goal of the research, he said, was to see if the conventional scientific wisdom about how much sediment the river carries in its upstream reaches holds true on its last leg to the Gulf.

“We wanted to see if there were forces at work that changed the way sediment is moved in that lower part of the river — the section where the diversions would be operating,” Allison said.

Clearly there are.

“That work significantly changed how we have to look at the lower-most river,” said Kemp. “And that makes the hydro study now even more vital,” he added, calling it the most important river study since the 1960s — “and certainly critically important for the master plan. ”

Allison agreed.

The hydrodynamics study is designed to provide the engineers and scientists molding the state’s coastal master plan with the raw numbers to determine the efficacy of restoration projects that depend on the river’s volume and sediment capacity.

“We will end up with a set of very well-calibrated numerical models that can basically test any restoration scenario,” Allison said.

“We’ll know if we open a diversion at Point X how much sediment it will capture, how we should operate it in various ways at various times to achieve the objectives, and what impacts operating it might have on the river and on other diversions.”

He called such insights “absolutely critical” to the success of restoration effort but not a reason to slow down the planning process.

Researchers said it was not unusual that the coastal authority did not wait for the latest river data before planning projects for the 2012 master plan and making predictions for their success. Events beyond their control played a part in that timeline, they said.

Funding lags research

While the study of river hydrodynamics was first proposed in 2004, it wasn’t funded until 2011. So the scientists working on the 2012 edition of the master plan put together preliminary planning models that prepared the way for the more detailed operating models to be developed when the latest data on the lower river became available. That work was cheered in peer reviews, even though it was based on river data that, in some cases, were decades old, researchers said.

“The planning-level models the state used were and are very much state-of-the-art, and it was very impressive work,” Kemp said. “There were over 100 researchers from all over the country and the world involved in what was a pretty remarkable effort.”

Many studies have been conducted on the river below New Orleans in the past, but those typically have focused on one site important to an individual project.

“This is the first time to look at the entire system,” said Ehab Meselhe, director of natural systems modeling and monitoring at The Water Institute of the Gulf. “We will want to know how the river acts and how these projects perform, not just in the short term, but over decades.”

The state and federal government are true partners in this project, splitting the cost 50/50. More than 50 researchers and technicians from state and federal agencies are shouldering the workload. The Coastal Protection and Restoration Authority leads the state effort, while the U.S. Army Corps of Engineers heads the federal group that includes the U.S. Geological Survey.

Researchers are using stationary monitoring devices to collect information on water and sediment flow, as well as boat-borne instruments to develop accurate pictures of the river’s shape, including its bottom all the way to the mouth of Southwest Pass, the main shipping channel.

Cherie Price, a corps engineer and planner, said the agencies would continue compiling detailed accounts of the river’s transport of water and sediment over time to help project planners make accurate, decades-long forecasts. The ability to adapt to unusual river conditions has become more critical in the age of climate change, she said, noting that the river experienced a flood in 2011 followed by a drought in 2012.

Just as importantly, Price said, the study will show the impact of the diversion projects on the river and its other users.

“And we will be looking at the cumulative effect of multiple diversions on the river — the first time that will occur,” Price said.

‘Budgeting’ overall demands on river

That information is critical to the master plan because the river must also serve other important needs. Navigation, for example, is important not only to the economy of New Orleans but to 31 states, and cities and industries along its course also depend on the river for fresh water.

The idea of a “budget” that includes all draws on the river is only now beginning to be studied but is expected to be a major factor in the operation of the master plan, said Garret Graves, head of the Coastal Protection and Restoration Authority.

Price said the delta-management part of the hydrodynamics study will focus on the impact of restoration projects on adjoining basins.

“The hydro study determines what resources are available for restoration; the delta-management study will take that information, and they will determine the quantity and quality of sustainable wetlands that can be built with large-scale diversions,” Price said.

“We will also be evaluating how large-scale diversions” — those passing more than 50,000 cubic feet per second of river water through the levee opening — “will impact habitat shifts, marsh integrity, flooding, fisheries and other resources in the basins.”

Many of those issues have been hotly debated by commercial fishers who face displacement if an influx of fresh water moves their catch to water that’s more salty.

The delta-management side of the study has yet to start, however. The U.S. Army Corps’ New Orleans office awaits approval of its funding share because the project time-frame of five years is longer than new corps’ guidelines that require studies to be completed within three years.

Price said she expects the waiver to be granted from corps headquarters by the end of the month.

“We feel we still have time to get this part done in three and a half years,” she said.

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