HOLDEN, Mass. — We were standing on the levee of the Mississippi River, about an hour west of Boston.
Of course, the actual Mississippi River is a half-continent away. We were in fact in a vast, warehouse-size laboratory above a scale model of a bend in the river in Louisiana, painstakingly recreated at 1/65th scale, right down to the simulated sand.
The model is one of the most striking parts of an ambitious project to rebuild Louisiana’s vanishing coast, which is rapidly being lost to rising seas and sinking land. Engineers want to be sure their design for a river “diversion” — an enormous mechanism for restoring eroded wetlands — will work.
A diversion is a set of gates in a river levee that can be opened and closed. The escaping river water is supposed to sweep sand, sediment and clay into nearby wetlands being annihilated by climate change and other environmental disasters.
The cost of the model, some $4 million to build and test, is a bargain, said Dan Gessler of Alden Research Laboratory, which built it. “You’re paying for insurance, basically, to make sure you got it right,” he said. Considering the cost of the finished project, roughly $1.4 billion, and the enormous potential cost of correcting design flaws that a working model can reveal, “It’s very, very cheap.”
It is also fun. At the scale of the model, I would be more than 300 feet tall — a heady experience for a short person. Dr. Gessler stepped across a bridge that put him over the middle of the river and squirted a stream of bright red dye into the water to watch how the currents flow.
In the middle of the river is the deep navigation channel that serves as a major thoroughfare for shipping. The dye in the middle continues to flow downstream, while the dye close to the diversion flows obediently into the long chute that, in real life, would then enter an eroding stretch of bays and estuaries south of New Orleans known as the Barataria basin. The dye experiment shows that the ships in the navigation channel are not likely to be pulled toward the diversion.
Since the 1930s, Louisiana has lost more than 2,000 square miles of land, roughly the size of Delaware, to subsidence, sea level rise and the loss of sediment caused by construction of the levees along the length of the Mississippi. Those levees have reduced flooding and saved countless lives over the decades, but they have also penned up the nourishing sediment that used to flow out with the spring floods and renew the Delta. That, and other damage to the wetlands that includes channels cut for oil and gas exploration, have led to ongoing losses of about a football field of land every 100 minutes.
In response, the state has developed a $50 billion, 50-year master plan for coastal restoration and protection. The first was published in 2007 and Louisiana has re-evaluated and upgraded its plans every five years since then.
Officials have developed an ambitious roster of projects to protect areas of the state and rebuild wetlands, which can buffer the effects of hurricanes. But planners have come to realize that they cannot restore all of the land lost over the decades, and have had to become strategic about what areas can be protected and how.
The Mid-Barataria diversion, and a similar structure farther north that will open eastward into another body of water, are among the most expensive, audacious — and promising — proposals. Land can be built through dredging and pumping sediment into place, and Louisiana has done some of that in some areas, including efforts to beef up the state’s barrier islands.
But that process is prohibitively expensive. The Mid-Barataria diversion could restore as much as 60 square miles over its first 50 years, the coastal authority says, building land in critical areas where people live, work and play.
The funding for the diversion comes from settlement money the state received from BP over the 2010 Deepwater Horizon spill.
Brad Barth of Louisiana’s Coastal Protection and Restoration Authority and the program manager for the project said that diversions are nothing short of revolutionary for those who manage the mighty waterway. For more than 100 years, flood protection was the priority, placed far above any consideration of the health of the wetlands.
Building diversions means opening up those levees, “a complete about-face of how we’re using the resources of the lower Mississippi River,” he said. Putting in diversions is an acknowledgment, then, that “we can no longer accomplish things by damming things off with flood protection.”
In building the model, the designers are trying to monitor water currents, pressure and flow around the structure, and to determine how much sand the diversion could be expected to deliver through its long chute to the basin. The other components of land building — silt and clay — are suspended fairly evenly in the water and will flow out with the current.
But sand is heavier. So the engineers needed to test the conditions required to lift the sand and carry it into the chute with the minimum amount of fresh water spilling into the brackish estuaries and bay.
The scale model, about 200 feet long and 70 feet wide, had to account for the differences between the weight of actual sand at the speed the real river flows, and the much slower rate of flow in the model. The researchers needed sand for their model with a specific size and density to match the behavior of the real thing.
The answer: “We had to come up with fake sand,” Dr. Gessler said. They designed a stand-in and had a plastics company manufacture it: a white acrylic with the consistency of sugar, ordered in vast amounts and costing about a thousand dollars a ton.
As the 200,000 gallons of recirculating water flow through the model, the researchers experience the unexpected in real time.
Dr. Gessler pointed to a spot at the downstream end of the diversion where water is backing up over the wall of the channel and, as he put it, “corkscrewing” on its way into the channel. That turbulence could be a problem, putting stress on the final construction, he said. “We couldn’t see this in the numerical model,” he said. The discovery could lead to a design tweak.
Instruments are placed around the model, with nozzles connected to long white plastic tubes that run back to racks of sample bottles. During the many tests of the model, as water flowed at different rates, water entered the bottles, and the amount of sand and water in each was recorded.
There is a second model of the chute itself, with tiny pebbles scaled to match the corresponding size of the heavy stone, known as riprap, that will line the actual channel. And, after the tests on this Mid-Barataria model are completed, the model will be broken down and a new one representing the second diversion will be built.
One thing that is not being tested by the model: a proposed liquefied natural gas shipping terminal that would sit just upstream from the diversion and could affect river flow. The controversial project is still under consideration by state agencies, and the work to determine whether the project would hinder the diversion is being performed on computational models, Mr. Barth said.
Some people in the commercial fishing and oyster industries oppose the diversions, which will inevitably bring fresh water into saltier portions of the bays and estuaries and change the balance of salinity. The project’s designers are planning to minimize those effects by opening the gates only at certain times of the year, and only at water volumes that will provide the best balance of fresh water and land-building sand and sediment.
Mr. Muth of the wildlife group said that the fishers’ concerns miss the bigger picture. Even without action, he said, “The end game is very little estuarine habitat left.”
Inaction is not an option, he said. “The sea is going to win the fight.”
Fish and oyster habitats might have to move as the salinity shifts, but “there will always be places where the appropriate saltiness are there,” he said. To rebuild land, “We only have one effective tool, which is the Mississippi and the sediment it carries downstream,” he said.
“We have to unleash the power of the river, the river that built the Delta in the first place.”