By Mr Michael William Petersen (USACE)December 9, 2011
ST. LOUIS, Mo. -- Every winter as the water level in the Mississippi River get lower, rock structures in the river start to emerge: long dikes of limestone jut from the banks, and arch-shaped chevrons point upstream. These river training structures are a part of the innovative engineering that allows for safe passage of commerce on the Mississippi River, saving taxpayer money and creating habitat in the river.
The U.S. Army Corps of Engineers has a Congressionally-mandated mission to ensure that our inland waterways are navigable. The Corps' St. Louis District combines a tradition of river engineering with award-winning innovation to accomplish its mission and keep the Mississippi River open.
The Mississippi River is in an alluvial valley, which means the riverbed is made up primarily of moving sand and prone to changes in depth, like shoaling. The ability of waterborne commerce to move on the river plays an important role in the nation's economy: more than 300 million tons of barge cargo moves on the river annually, including sixty percent of America's agricultural exports.
Shannon Hughes has worked on the Mississippi River for more than 22 years, starting as a deckhand and working his way up to port captain for Kirby Inland Marine. In his career, Hughes has seen the difference river training structures have made for the river transportation industry, reducing the need for a tricky practice known as flanking.
Flanking is a maneuver used by towboat pilots headed downstream into a bend. The tow slows to match the speed of the current and goes into a river bend at an almost sideways angle. The strong current in the heart of the bend then straightens the front of the tow downstream.
"There's a lot of stress involved in flanking," Hughes said. "If you don't do it right, you've got bad consequences. You can hit the bank."
The construction of bendway weirs on the Mississippi means less risk to towboats, their crews and cargo.
"The underwater weirs have made a big improvement in some of the hard turns. We've had a lot fewer problems in the areas that didn't have the weirs before," Hughes added. "The whole Upper Mississippi is kept in much better shape as far as shoaling since I started in the wheelhouse in 1994."
River training structures, including dikes, chevrons and bendway weirs, are used to keep the dynamic river navigable in the most efficient and environmentally sound manner.
"It takes a combination of structures, revetments, and dredging to keep sediment moving through the system and maintain the nine foot navigation channel on the Mississippi," said Eddie Brauer, river engineer at the Applied River Engineering Center in St. Louis.
The structures work by using the river's energy to move sediment out of the navigation channel, reducing the need for dredging. While dredging remains an important part of the Corps' navigation mission, there are associated limitations that make river training structures a financially and ecologically sound alternative.
"Dredging is expensive; there's no way we could dredge the entire channel all the time," Brauer said. "It's also very intrusive to fish habitat when we dredge up material from the bottom, not to mention the placement of dredged material."
A dredge mechanically lifts sand from one area to another without making a permanent change to the river's energy and flow. The dredge has to be used in the same area over and over again because the river continually replaces the sand that is removed.. The river training structure takes the place of the dredge and moves this continuous supply of sand out of the channel by using the river's own energy.
Similar structures have been used in the Mississippi River since the 1830s to deal with the ever-changing nature of the river. While designed to help provide a navigable channel, the river training structures have a secondary benefit: creating or improving habitat for wildlife.
"The Middle Mississippi is a sand-bed river, which comparatively doesn't have a great deal of biological diversity," said Tom Keevin, biologist and chief of the Environmental Compliance Branch for the Corps in St. Louis. "When you put rock in, it creates a hard substrate in the river, which is habitat for bottom feeding invertebrates. It also provides a place for fish to go and feed."
In cooperation with natural resource agencies, the Corps has conducted numerous studies that demonstrate the positive ecological effects of river training structures. Recent studies include fish and macro-invertebrates in bendway weir fields, and numerous fish sampling studies associated with chevrons, to include a recent study of St. Louis Harbor.
With the growing amount of information available, the St. Louis District will conduct a voluntary environmental assessment of the river training structures for 2012. Keevin anticipates the assessment will be helpful to the agency and the public, both to summarize and gather the results of dozens of studies in one place and get public involvement in an important part of the Corps of Engineers' missions on the Mississippi River.
"The environmental assessment will give another opportunity for the public to be part of the decision making process," Keevin said. "That's really the heart of the National Environmental Protection Act."
Along with decades of study, river training structures have also received awards and accolades. The St. Louis Harbor chevrons earned the 2010 Chief of Engineers Design Team of the Year Award for Environmental projects by USACE Headquarters, and most recently the 2011 Project of the Year from the St. Louis chapter of the American Society of Civil Engineers.
The Applied River Engineering Center's website has more information, including studies, reports and videos, on how innovative river engineering fuels the American river industry in a sustainable and cost effective way.