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BU marine geologist wins grant to study New Orleans dike construction

Hopes to use dredge material to build hurricane “speed bumps”

Earthen dikes would be vegetated by plants that grow in salty soil, such as those from the nearby marsh islands in Louisiana's inner Breton Sound.

As the people in Southeast Louisiana rebuild their homes and lives eight months after the devastation of Hurricane Katrina, a BU marine geologist and colleagues at the University of New Orleans (UNO) are studying the possibility of building new dikes from dredge material.

Duncan FitzGerald, a College of Arts and Sciences earth sciences associate professor, and three researchers from UNO’s Pontchartrain Institute for Environmental Studies, recently won a $130,000 grant from the National Oceanic and Atmospheric Administration’s Lake Pontchartrain Basin Restoration Program to explore how sediment removed from Louisiana’s inner Breton Sound can be used to build marsh dikes.

FitzGerald, Ioannis Georgiou, a UNO marine engineering assistant professor, Mark Kulp, a geology assistant professor, and Shea Penland, a geology professor, hope to use a three-dimensional computer hydrodynamic modeling study to determine where a network of earthen dikes could be placed to provide the greatest amount of wave protection. The study will take into account wind and weather systems.

“One of the reasons there was so much devastation from the hurricane was because of the storm surge,” says FitzGerald. “Lake Pontchartrain was 14 feet above normal. If you can lessen the amount of water that builds up, then you can decrease the vulnerability of New Orleans. And that can be done by increasing the size of the wetlands between the lake and the open gulf.”

Fitzgerald explains that Lake Pontchartrain is really not a lake — it’s an estuary where fresh water from rivers and bayous mixes with the Gulf of Mexico. Last August when water surged into it, levees broke and the city was flooded. But FitzGerald says sediment routinely removed from the Mississippi River Gulf Outlet channel may be helpful in constructing wetlands that would act as “speed bumps” to hurricanes. The dredged sediment, a mixture of mud and a small amount of sand, is unsuitable for most barrier reconstruction projects, but FitzGerald thinks it may be useful in building earthen dikes.

“Traditionally, when the channel is dredged, a pump dredge throws the material far away, like a snowblower, so the channel won’t infill,” he says. “The idea is not to use a dredging mechanism, but rather to pump the sediment through a series of pipes to spots in which the material could build up and be used to construct wetland areas.”

The mud, which is liquefied in the dredging process, could be pumped into a holding area to allow the sediment to dry. It would likely take several weeks for the material to consolidate, but FitzGerald believes it may prove highly suitable for dike construction.

The dikes, elevated to about three feet above mean high water — with the final height to be determined from computer simulations from storms — would be vegetated by hardy grasses and shrubbery to reduce wave erosion, says FitzGerald. Even if they did erode, regular maintenance dredging of the channel would provide more sediment to repair them.

FitzGerald, who periodically visits the Louisiana coast to study the influence of hurricanes and storm surges on wetlands, says the barrier islands and marshes that once protected the region from storms have been shrinking for years — and were dealt a major blow from Hurricane Katrina.
“The Chandeleur Islands were devastated,” he says. “A lighthouse that stood there for more than 100 years is completely gone. This island chain is historically New Orleans’ first line of defense against hurricanes, and there is little sand left and only vestiges of marshland. The Louisiana coast is extremely vulnerable to future hurricanes, and we’re looking at ways to increase wetland areas to provide a buffer from storms.”