A Mini Mississippi In Mass. May Help Save New Orleans From Rising Seas

Apr 29, 2021
Originally published on April 29, 2021 10:02 am

Mark Twain called it the “magnificent Mississippi.” He said the river “will always have its own way; no engineering skill can persuade it to do otherwise.”

Garret Dunn is doing his best, anyways.

He’s in the river on his hands and knees, rebuilding a wall. It’s part of a structure called a “sediment diversion,” designed to pull water — and a whole lot of sand — out of the river and into Louisiana’s Barataria Basin. It’s part of Louisiana’s big (and controversial) plan to rebuild and maintain an area of drowning wetlands nearly the size of Delaware.

Since the 1930s, Louisiana has lost 2000 square miles of land due to  both rising seas and engineered levees. The levees — installed to preventowns from flooding and help ships navigate — also keep the Mississippi from dumping sediment and nutrients onto coastal lands. Without the Mississippi mud, the wetlands have withered, leaving southern Louisiana communities like New Orleans with less buffer between them and the sea, and vulnerable to increased storm surge and flooding from climate change.

Dunn’s work is a crucial part of a plan to rebuild some of the lost land. But he’s not actually in Louisiana. He’s in his hometown of Holden, Massachusetts, in a dark, warehouse-sized building on the campus of Alden Research Lab – the oldest continuously operated hydraulics lab in North America.

“There’s not very many labs in the United States that can do this kind of work,” says Dan Gessler, the lab’s senior vice president, “especially at the scale that was required here.”

Since 2017, the lab has been working with Louisiana’s Coastal Protection and Restoration Authority to iron out the details of the sediment diversion’s design.

The Alden lab is responsible for ensuring that the diversion will work as well in real life as computer models say it should. And to figure this out, they’re running tests on a scale model of a section of the Mississippi River, built on the lab’s campus just outside of Worcester.

When the lab was founded in 1894, Gessler says, researchers worked outdoors with water from a nearby pond. But today, most work happens inside Alden’s more than 20 buildings, where teams of engineers all study the same basic issue: how water interacts with the built environment.

The engineers test water pumps, vents, and systems that keep fish out of power plants. They’re even working out how water flows through the next generation of nuclear reactors.

The lab’s largest project is the Mid-Barataria Sediment Diversion — the one that shrunk down the mighty Mississippi to fit inside a building the size of a football field. 

The lab has gone to great lengths to ensure that everything – the water levels, the diversion, the synthetic plastic sand, the barge parked slightly upriver – is exactly 1/65th its real world size. Gessler said that’s as small they could go without the model losing its real-world significance.

“We have the expertise,” Gessler says. “We’ve been working on rivers for a very long time.” 

“The only thing we’re not correctly modeling is our water temperature,” saysDunn. “That water is cold. I have to imagine the Mississippi is a little warmer.” 

Dunn is the project’s testing engineer. It’s his job to climb in and out of the water a dozen times a day to disassemble, readjust, and then test different angles of the diversion’s wall, to see which version pulls the most sediment. 

He said it’s hard not to feel like Godzilla next to the miniatures.

Dunn runs his tests from a computer on dry land, though he keeps his soggy rubber green overalls on. With each experiment, he releases streaks of Christmas colors into the river. Red dye runs at the top of the water, and green dye down near the river’s bed. The idea is to visualize the flow of sediment. 

The lab is more interested in the green dye, Gessler said, because that’s the water that will carry the sediment out.

All of this has already been tested in computer simulations. But there are “certain aspects to this that the computer models don’t do quite as well,” Gessler says.

He points to a whirlpool spinning at the edge of the diversion’s wall as an example.

“That little thing right there is not captured,” he says. 

It may seem unimportant, Gessler says, but that swirl means the diversion captures less sediment. The physical tests will help the lab determine how to eliminate the whirlpool.

The lab should finish testing in August, Gessler says. Construction of the real diversion is planned to begin in 2023, according to the Louisiana Coastal Protection and Restoration Authority.

The $1.5 billion project will be funded by settlement money the state received from the 2010 Deepwater Horizon oil spill, according to Brad Barth, director of the Coastal Protection and Restoration Authority’s sediment diversion projects.

Barth says the ultimate goal of all the engineering work is to reestablish the Mississippi’s natural connection to surrounding wetlands that, until the past century, fueled a healthy coastal ecosystem. 

“We really want Mother Nature to do it’s thing.” 

This article was originally published on WBUR.org.

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