April 21, 2014
Mississippi River Sediments Plentiful Enough To Rebuild Shrinking Delta
Brett Smith for redOrbit.com - Your Universe Online
While flood-control measures have kept the Mississippi River from disrupting commerce and transportation along the waterways of southern Louisiana, they have also reduced the amount of wetland-preserving sediment flowing into the river’s delta.
“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 author Jeffrey Nittrouer, assistant professor of Earth science at Rice University.
The Mississippi delta sinks annually as its soil settles and compacts. Flood-control actions intended to protect the lives and livelihood of millions of people have also taken away approximately one half of the annual availability of sediment that runs downriver.
To determine the status of sediment in the river, the study team reviewed sediment loads in the southern Mississippi and discovered that while the overall quantity of sediment has reduced, the quantity of sand captured by upstream dams is offset by “mining” of new sand in the lower Mississippi.
“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,” he added.
The new study expands on previous research conducted by Nittrouer on the land operations that occurred during significant flooding in 2011. In one of the biggest floodwater diversion of the past century, the Army Corps of Engineers opened the Bonnet Carré Spillway, a massive channel that sent floodwater straight to Lake Ponchatrain.
Nittrouer and his peers discovered that although the 42-day diversion through the 7,000-foot-wide channel removed under 20 percent of the water going downriver, it redirected approximately 40 percent of the river’s sediment load into Bonnet Carré. In examining how this happened, the study team uncovered factors that the corps should look at in designing sediment diversion jobs 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.”