Morganza levee system design accounts for environment

By:  Chris LeBlanc,

May 2, 2015

While the protective impacts of the Morganza-to-the-Gulf levee system have been well noted, the environmental footprint of the alignment sometimes falls by the wayside.

However, levee officials and environmentalists say the lack of discussion on the environmental impacts of the levee alignment are an indication that its design is working.

The 72-mile semi-permeable chain of interconnected levees and flood gates that is under construction will bolster the mainland of Terrebonne and Lafourche parishes against storm surges and rising tides during hurricanes.

Most of the time, though, the use of water-control structures in the system will also allow water to flow somewhat naturally to mimic the natural processes of the estuaries, officials said.

Terrebonne Levee District Director Reggie Dupre said this harmony between the environment and levee construction was not a coincidence.

As part of the federal and state permitting process, Dupre said the Morganza engineers were required to extensively study the effect the levees would have on the environment.

In fact, Dupre said the construction of the alignment, which keeps saltwater outside and freshwater within some areas of the system, has been beneficial.

"It's what we've said all along, flood protection and environmental protection can go hand in hand," Dupre said.

Within the last couple of years, there is a resurgence of some of the wetland habitat lost by saltwater intrusion in areas such as the Pointe-aux-Chenes Wildlife Management Area and the Lake Boudreaux basin, Dupre said.

"We're starting to see a rebirth of some of the wetlands that turned to open water. We're starting to see some freshwater vegetation that is showing signs of coming back," Dupre said. "People are starting to catch freshwater fish again in the northern perimeters of the Boudreaux basin. Slowly we're starting to see signs of the environment rebounding."

Environmental specialists at the Barataria-Terrebonne National Estuary Program were also impressed with the levee system's relatively small environmental footprint.

BTNEP Program Director Susan Testroet-Bergeron tipped her hat to the designers of the Morganza project for their innovative look at estuarial processes.

"The people who designed the levees took into consideration what the plants need, what the animals need, how salinity changes, how to get water in and out of the wetlands on the northern side of the levee system. They were really trying to figure out how to make this mimic regular estuary ingress and egress," Testroet-Bergeron said.

Although it is progressive in its design, Testroet-Bergeron and BTNEP Water Quality Coordinator Andrew Barron were concerned that disruptions in the natural water flow in the wetlands could have a negative impact.

"Anytime you're going to add a levee system, you're going to disrupt that natural flow of water over the marshes and you're going to create some kind of impoundment," Testroet-Bergeron said.

She and Barron were also concerned about the Morganza alignment's contribution to land subsidence within the system.

When rainwater runs off through drainage arteries and into basins and estuaries, it picks up soil and sediment, Barron said. When pumps remove that soil-laden water, it naturally contributes to land subsidence.

"Think about the levee system with all of those gated waterways. It's like a big bowl. You're not going to allow that bowl to fill up with water during a storm water event, you're going to be pumping that down," Barron said. "All of our ground down here is actually held up by water and organic matter and mineral matter. So when you pump down the groundwater it's going to contribute to subsidence."

Dupre agreed that concerns of subsidence with new levees, as parts of forced drainage systems, are legitimate. But, he said, that's not how the Morganza system operates.

"Most of the Morganza alignment is a ‘leaky levee system' where you have gates and water control structures that allow the flow of water back and forth. But, by just restricting some of the saltwater and holding onto some of the freshwater from other sources like parish pump stations pumping inside of it, it seems to be benefitting the environment," Dupre said.

Dupre said the only pump stations along the alignment that would draw water outside the system are those connected with existing parish drainage systems.

"What we're doing is building a ‘leaky levee.' You're not putting up a levee and putting a pump station and draining an area and causing more subsidence. So the subsidence will be no more or less than it was before," he said.

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