Centuries of sand to grow Mississippi Delta
The new study, which appears online this week in Nature Geoscience, is encouraging news for scientists and government officials who are working to shore up southeastern Louisiana's rapidly disappearing wetlands. The delta sinks each year as its soil settles and becomes more compact. While floodwaters from the untamed Mississippi River formerly provided a steady supply of sediment to counteract this subsidence, engineers have fought for nearly a century to contain the floods, which threaten the lives and livelihood of millions. Flood-control measures have eliminated about half of the annual supply of sediment that flows downriver, but the new study finds that sand—they key ingredient for rebuilding marshlands—is still abundant.
"It's true that the total amount of sediment has diminished, but river sediment contains both fine-grained mud and course-grained sand, and our research found that upstream dam construction has not reduced the amount of sand in the lower Mississippi and won't for at least 300-600 years," said study lead author Jeffrey Nittrouer, assistant professor of Earth science at Rice University.
Nittrouer and co-author Enrica Viparelli, assistant professor of civil and environmental engineering at the University of South Carolina, analyzed sediment loads in the lower Mississippi and found that while the total amount of sediment—both sand and mud—has diminished, the amount of sand trapped by upstream dams is offset by "mining" of new sand downstream.
"When clear water is released from the floodgates at upstream dams, it churns dormant sand that has long been deposited and carries it downriver," Nittrouer said. "This 'mining' of ancient sand makes up for the sand that is trapped by upstream dams, and our numerical models suggest that the sand load in the lower Mississippi River channel will not decline for at least 300 years. Looking even further into the future, we found that 600 years from now, the lower Mississippi River's sand sediment load will have declined by less than 20 percent from today's levels."
Nittrouer, whose research focuses on the sediment transport, hydrology, basin evolution and stratigraphy of lowland river systems, has studied the Mississippi River for the past decade. His previous work included a 2012 study of the land-building processes that took place during the historic flooding of 2011. In one of the largest floodwater diversions of the past century, the U.S. Army Corps of Engineers opened the Bonnet Carré Spillway, a 7,000-foot-wide "safety valve" that diverts floodwater directly to Lake Ponchatrain.
Nittrouer and colleagues found that even though the 42-day diversion siphoned off less than 20 percent of the water flowing downriver, it diverted about 40 percent of the river's sand load into Bonnet Carré. In analyzing how this occurred, Nittrouer and colleagues were able to show what factors the corps should consider in designing sediment diversion projects for wetlands replenishment.
"Our previous work showed how large volumes of sand could be deposited in specific locations, and our latest research shows that significant volumes of sand will be available for land-building for several centuries," Nittrouer said. "Each of these are important because studies at Wax Lake Delta and other sites have shown that sand—even though it makes up less than 20 percent of the overall river sediment load—is the key ingredient for land-building."
Provided by Rice University