Teaching an old dredge new tricks: Army’s oldest dredge creates new habitat

Flexible dredge pipe connects to the USACE Dredge Potter.

(Photo by U.S. Army Corps of Engineers - St. Louis District)

The endangered Least Tern may have a new island to call home on the Mississippi River thanks to the U.S. Army Corps of Engineers - St. Louis District, the Dredge Potter and a flexible, floating dredge pipe.

Historically, the Least Tern nested on sandbars along the Mississippi River. The establishment of navigational pools, repeated flooding and an increase in recreational activities along the river has led to the decline of the population. 

As part of a pilot project during this year’s dredging season, the St. Louis District reused dredged material to build small islands using a flexible floating dredge pipe. 

“The flexible floating dredge pipe provides the St. Louis District opportunities to create a diversity of aquatic habitats such as sand islands and shallow water habitats in areas where it may not be possible without the use of dredged material,” Brian Johnson, biologist with the St. Louis District said.   

Each year dredging is performed on the Mississippi River in order to keep it open to the Congressionally-mandated depth for river traffic. Historically, dredged material placement locations and options have been limited by both equipment constraints and cost.  

When feasible, dredged material is recycled for beneficial uses within the river. Reuse is the preferred approach by the Corps of Engineers, and the St. Louis District is always seeking out innovative and creative ways to accomplish this task. 

“Our partners and stakeholders have challenged us to find more ways to reuse dredged material in an environmentally friendly way,” Johnson said. 

USACE Dredge Potter crew assembles flexible dredge pipe.

(Photo by U.S. Army Corps of Engineers - St. Louis District)

The Potter crew spent most of the off season prepping the dredge for the use of the flexible floating dredge pipe.

“It took a lot of planning ahead and creative ideas to make this happen,” Lance Engle dredge manager with the St. Louis District said.

Traditionally, the use of standard dredging practices limited the opportunity to reuse dredged material as sandbar or island habitat because of the physical limits of the rigid metal disposal pipe that is used.  

“Normally dredged material is side-cast along the main channel border in a linear fashion, resulting in a long, narrow disposal bar that is limited in size, elevation, and location,” Engle explained.

A St. Louis District experiment with flexible floating dredge pipe in 2005 led to the idea of using it to create islands and sandbars on the Middle Mississippi River. Demonstration projects by the New Orleans and Mobile Districts helped develop the idea and prepare the dredge crew for the new challenge.

Flexible dredge pipe helps create more diverse habitats.
(Photo by U.S. Army Corps of Engineers - St. Louis District)

The pipe’s flexibility allows the Dredge Potter to place dredged material in a specific location as it moves. This allows material to build up to create sandbars and island habitats in various shapes, sizes and elevations in the Middle Mississippi River while maintaining the navigation channel. 

These types of habitats are essential for the nesting and spawning of various fish and wildlife species such as the Least Tern.  

“The new method allows creation of more diverse environments for wildlife than traditional dredging methods,” Johnson added.  “These islands are often remote, providing protection from predators, boats and humans.” 

The flexible floating dredge pipe allows the St. Louis District to keep the river safe and open for navigation in an environmentally sensitive way.

“In the future, we hope this will be the way we do business,” Johnson said. “Eventually we hope to use the Dredge Potter not only for navigational purposes but for environmental management purposes as well.”

Training the Untrainable Mighty Mississippi

Chevrons in the St. Louis Harbor

If you’ve seen the Middle Mississippi from the air or by boat, chances are you’ve seen the work of St. Louis District's, Rob Davinroy and his colleagues at the Applied River Engineering Center. Some of their creations jut out from the shoreline, perpendicular to the river’s flow. Others, like the arch-shaped chevrons, sit in the center of the channel, strategically placed to direct the river’s ever-flowing sediment, while others do their directing underwater along river bends.

Innovation is a major emphasis of this group, which uses state-of the art sonar and acoustic doppler devices to accurately map the river bottom or measure water speed and direction. That information is applied to models, created and tested in the center. Those in turn lead to new ways of maintaining a safe and navigable channel depth and in some cases habitat restoration for fish and other aquatic life.

“What we’re all about is trying to manage sediment, whether to make the river deeper or wider or create some diversity,” said Davinroy, the center’s river engineering chief. “It’s all about how sediment is falling out.” In all, the center’s staff keeps tabs on approximately 900 structures installed along 300 river miles. A majority were built over a century ago when unpredictable river depths and snags made steamboat travel an occasional death sentence.

As early as 1824, the federal government charged the then Engineer Corps of the Army to remove river obstructions. Increasing commerce in the mid-1800s led to even more federal involvement in navigation. The post-Civil War era saw an explosion in levees, wing dams, dikes and jetties.

“Sediment’s been an issue since the Corps launched a navigation program,” he said. “It’s only gotten worse with development. . . We’re on this treadmill, trying to keep up with the continual sediment that comes into the system.”

Bringing in equipment to dredge (or dig sediment out from) the river bottom is one option, but unlike the permanent structures, sediment starts to fill back in as soon as the dredging operation is completed.

Safety is the primary reason for redistributing sediment. On river bends, in particular, a buildup of sediment can narrow a passageway to the point that a tow grounds or crashes. The 1980s saw a couple of notable accidents and resulting spills involving tows pushing barge loads of oil, and that led to a particular focus on the safety of river bends, Davinroy said.

Least Tern, an endangered species
of bird, nesting preserved
by Bendway Weirs

In recent years, ecosystem impacts have become a larger focus of the river training structures, both in terms of projects designed specifically for their ecosystem benefits and development of navigation structures that benefit wildlife.

Chevrons were developed because they lead to better fish habitat behind the dike, creating miniature islands more favored by fish than the flat sandbars formed behind traditional structures, he said.

Bendway Weirs, located underwater and uniquely angled to improve bend safety, were developed as an alternative to shoreline structures because the endangered least terns were nesting on sandbars where projects otherwise would have been built.

“We knew we couldn’t build on the sandbar where they were laying eggs,” Davinroy said. “We didn’t have any other choice but to do something in the channel. We conducted research to find what worked and angled them upstream rather than downstream like most.”

The weirs dramatically improved tow safety. But they were found to have an unexpected benefit as well. Follow-up studies showed they were a magnet for macro-invertebrates and a favored playground for fish. In one survey, 217 fish were caught around one, representing 12 species. Chevrons similarly were found to attract diverse fish species, even in the middle of the St. Louis harbor. 

http://www.ourmississippi.org/