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Mon November 11, 2013
For Strengthening Levees, Bermuda Grass Is Hard To Beat
The idea that grass can armor anything is hard to believe.
But on a recent visit to the Lake Pontchartrain levee, LSU agronomist Jeff Beasley explained how plain old, garden variety grass has earned a reputation with the US Army Corps of Engineers as one of the best armoring materials to keep the huge mud walls of a levee from collapsing during a storm.
"You know how we reinforce concrete with rebar?" says Beasley. "We can do the same with these levees."
In fact, Hurricane Katrina showed the world why armoring the protected — or land side — of a levee is deadly serious business. When a storm surge overtops a levee, water rushing 20 to 25 feet down the protected side has the force of a river and can scour holes in the surface. If it digs past even a small patch of the of the armoring — in this case the grass — it can quickly eat away at the mud interior, causing a collapse.
Bob Turner is an engineer and regional director of the local flood protection authority. "It’s like unraveling a sweater by pulling a string," Turner says. "Once you get rid of that grass at the bottom, it’s like unraveling the soil, and it just comes apart."
Prior to Katrina, the grass armoring on some levees that were overtopped had not been properly maintained. Investigators said if those levees had not collapsed, the flooding in some parts of New Orleans that reached roof levels, would only have been ankle deep. With that much riding on the grass doing its job, alarm bells went off at local flood authorities when they learned some of the Corps contractors had trouble getting the stuff to grow on its new levees. They reached out to LSU's Beasley for help.
While locals may find it hard to believe anyone could have trouble getting grass to grow in this sub tropical climate, Beasley said levees and lawns are two different challenges.
"You have to go back to how you construct a levee. First of all you’re using clay and compressing it up, up, up. So now you have embankments and compressed or compacted soils."
There were other challenges: salty spray for the nearby lakes, periods of drought, and the blistering summer heat. In fact Bermuda grass was chosen because it does well in all those conditions. Beasley says that with help and vigilance, the contractors and local authorities can help the Bermuda stand up to the challenge.
"Grasses are one of those things that have to be managed; they are biological," Beasley says. "The good news is, most people have a lawn and they realize it’s not that tough, but there are things you have to do."
And if they are done correctly and consistently, plain old Bermuda grass could help save lives and property when that super storm comes.
This story was reported by Bob Marshall of The Lens, and produced by Janaya Williams.