Every engineering control
has its drawbacks. As communities upstream of the Mississippi Delta continue to
emplace dams and other flood control measures to prevent community flooding,
less sediment is pulled from the lands upstream. Flood control measures have
eliminated about half of the annual supply of marshland sediment to the
Mississippi Delta. The existing soils continue to compact and sink without
sediment replenishment. But researchers have found that the river’s supply of sand,
the key ingredient used by engineers for rebuilding, will remain constant for
many centuries.
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“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 2012 land-building processes study 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-widesafety valve to divert 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. Nittrouer and colleagues were able to
show what factors the Corps should consider in designing sediment diversion
projects for wetlands replenishment.
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Mississippi River Delta image via Shutterstock.
