The amount of sand carried by the Mississippi River toward Louisiana has not decreased since dams were built upriver and it’s not expected to decrease significantly for at least 600 years, according to a new report.
That’s good news for Louisiana coastal restoration efforts, said authors of the report published this month in Nature Geoscience.
Although the report agrees with an earlier 2009 report that the total amounts of sediment flowing down the Mississippi River are much lower than they were historically, the new report focuses on one component of that sediment — sand.
“It seems the sand load is not decreasing,” said Enrica Viparelli, assistant professor in the Department of Civil and Environmental Engineering at the University of South Carolina. In fact, the study estimates there will be a decline in sand amounts coming down the river of only about 17 percent within the next 600 years.
Researchers looked at sand the river contains from Cairo, Ill., to Tarbert Landing, Miss., and found it is possible for the river to mine and scour the existing sand in the system to make up for any sand contained behind upriver dams.
“It’s almost like a bank account,” said co-author Jeffrey Nittrouer, assistant professor in the Department of Earth Science at Rice University. “In a sense, the sand that’s in the river itself basically is like a big sand savings account.”
This study doesn’t contradict a study done in 2009 by Michael Blum and Harry Roberts, of LSU, that finds the sediment in the Mississippi River has been reduced by 50 percent because of dam construction elsewhere in the basin.
The new report agrees that overall sediment being carried by the river is reduced, but when that sediment is separated into sand and mud, the sand won’t decrease for some time.
“A lot of previous research has focused on the mud load in the Mississippi River,” Nittrouer said, and that makes sense because the sediment carried by the river is about 85 percent mud and clay.
Although sand makes up only about 15 percent of the sediment carried by the river, it’s important in building the foundation of new land , which is exactly what Louisiana is planning, he said. Mud is reduced and sand isn’t because mud, which usually is suspended in the water, moves through the system much faster than sand, which generally drags along the river bed.
The report studies what resources are available to help buffer the effects of land loss Louisiana is experiencing.
“I hope our science community is moving in a direction of what can we do with what we have at hand,” Nittrouer said. “We’ve been able to show that sand is stable, and it’s a sustainable resource.”
As the state moves forward with diversions and dredging sediment from the river for marsh creation, there have been questions as to just how much material will be available in the future, said Chip Groat, president of The Water Institute of the Gulf.
Although overall sediment has decreased since being trapped behind upriver dams, current research is showing there is a consistent supply of sand getting to the lower Mississippi River.
“I think that really confirms there’s a steady supply,” Groat said. “It’s still a lot of sediment.”
The challenge, he said, will be to find the best way to get that sand out of the river and into building coastal land. The institute, the state and the U.S. Army Corps of Engineers are studying how much sediment is in the lower river and how it flows to understand how sediment diversions should be built, Groat said.