Along levees and floodwalls throughout the New Orleans area, a team of foresters is digging shallow trenches for a novel study of root systems to help determine the fate of hundreds of trees growing within sight of the region's flood-protection system.
Their work will be used by the Army Corps of Engineers in deciding how much more land to clear-cut in order to protect the levees from failing when trees are uprooted during a storm, a scenario in which disturbed roots could open easy erosion paths for water.
Shortly after Hurricane Katrina thrashed the region 21 months ago, the corps began cutting all trees and shrubs growing in the levees or canal banks themselves in East Jefferson and New Orleans and, on the land side, within 6 feet of a levee toe or 15 feet of a floodwall.
Now, corps officials are trying to determine how much more land should be cleared, an emotionally and environmentally sensitive issue in a community where many of the trees in jeopardy are on private property and where tree canopy defines neighborhoods, affects property value and sustains wildlife.
"We don't want to overreact and just whack down trees unnecessarily. We want to know, what is the appropriate distance," corps project manager Mike Stout said. "We'd like to base this decision on science. We need to know how the roots of different trees behave in certain situations, and I'm not sure we know that now."
To get that science, the corps turned to Louisiana State University forestry specialists Jim Chambers and Tom Dean, who will document what actually happens underground when roots meet up with compacted levee soils. They think it is the first study of such a wide area.
Using a methodology they developed for this study, Chambers and Dean are eyeballing tree roots by excavating trenches about 10 feet long, 3 feet deep and 2 to 3 feet wide.
Most of the trenching is being done on the roots of trees already doomed: those at the levee toes or within the 6-foot clear zone. The roots are exposed and studied, then corps contractors move in to take out the trees and stumps and fill the holes.
"I think this will be the largest database ever collected in this area (of research). We just don't get to deal with the root systems of so many large trees," said Chambers, who holds a graduate degree in forest ecophysiology and chaired Gov. Kathleen Blanco's Science Working Group on Coastal Wetland Forest Conservation and Use. Dean holds a graduate degree in forest ecology, and both men are professors in the LSU School of Renewable Natural Resources.
"This is probably the largest project I've ever dealt with involving the root systems of mature trees," Chambers said, adding that the real trick is deciding where to start digging.
"Large trees are very difficult to deal with, and root measurements are extremely difficult because you don't know where they grow," he said. "Roots kind of grow randomly, so it's sort of like finding a needle in a haystack and doing it blindfolded."
Their methodology calls for digging parallel trenches several feet apart to track individual roots and to see what happens when they bump up against a section of levee along Lake Pontchartrain, the outfall canals and the Mississippi River.
Do the roots die? Do they grow faster? Or do they burrow deeper or start climbing, perhaps following the slope of a levee toward its crown or floodwall?
Only midway through the process of collecting data, Chambers said it's too soon to draw final conclusions. But already, he said, the team is seeing a little bit of everything, and some of it is eye-popping.
Although most trenches are fairly shallow, Chambers said the crew began its field work at the base of the London Avenue Canal by digging down to about 8 feet in one spot.
"We discovered some exceptionally large roots down there growing into real heavy clay soils commonly below the water table," he said. "They varied in size, but were roughly 4 inches to 6 inches in diameter.
"We don't know if those actually entered the levee or went under it because we don't excavate to that kind of depth within the levee section."
He would not speculate on whether such large root systems are likely to be similarly embedded in other areas.
Chambers said the roots encountered in that spot belonged to bald cypress, live oak and sycamore trees, which in varying degrees are more tolerant of wet soils than most other species.
"Tree roots, depending on the species, may encounter levees and continue to grow in several ways," he said.
Although most trees are easily smothered when a moderate amount of dirt is added atop their roots -- even dirt fill in a residential yard can do it -- Chambers said the more water-tolerant species have the amazing ability to capture oxygen from above ground and pull it below ground to sustain life.
As a rule, the trenching begins about 9 feet from a tree. If a large number of roots, or roots of a certain size, are encountered, the crew moves 6 feet back and digs a second trench. If they again find sizable roots, yet another trench is dug.
"The goal is to get to the point that we encounter roots no more than one-quarter-inch in diameter," he said. "Sometimes we have to just quit because we get too close to a floodwall to use our equipment."
Forensic experts investigating multiple hurricane-protection system failures during Katrina don't agree on what role, if any, trees in or near the levees contributed to those failures. But some large trees were uprooted near the site of at least two levee breaches.
The analysts do agree that trees and even shrubbery can pose a risk on several levels. Once trees are overturned, disturbed root systems can open paths for water to damage and help undermine levees and floodwalls. Even in nonemergencies, trees, bushes and vines can hide a developing problem or restrict access to flood fighters.
But how close is too close?
"People say trees could have been a contributing factor to those breaches, but we don't know. We'll never know," said Jerry Colletti, assistant chief of the corps' New Orleans District operations division. "But what Mike Stout's tree team is doing with this root study is getting some science.
"I don't know what the survey will show, but it will be real data coming from an independent outside view," he said. "We need some scientific guidance, and we're getting it."
Colletti and Stout said they expect the corps will use the findings to help establish post-Katrina tree policy not only in New Orleans but throughout the country.
Chambers said the research team has already dug more than 100 trenches along the lakefront and the Orleans Avenue and London Avenue canals. They are scheduled to begin trenching this week along the 17th Street Canal.
Most residential yards that back up to the New Orleans side of the canal are especially thick with trees, but Chambers said the trenching would take place only within the 6-foot zone.
Some homeowners are disputing the corps' tentative right-of-way boundary along portions of the canal's east bank. But even if they didn't, Chambers said it is too difficult to track roots in a thicket of trees.
"When yard trees are too close to levee toe trees, it interferes with our ability to analyze the data. You can't tell one root system from the other," he said. "We look for solitary trees."
Once finished on the 17th Street Canal, Chambers said, the group will do some trenching in a couple of abandoned Mississippi River levee sections near Baton Rouge, where circumstances will allow them to dig deeper and collect more data.
The tree root project is being done under a $193,000 contract that the corps awarded to JESCO Environmental & Geotechnical Services Inc. of Jennings. Chambers and Dean are subcontractors, as is CEC Environmental of Baton Rouge.
Stout said JESCO provides general coordination, equipment and operators, while CEC provides additional labor.
Chambers said he expects to give the study findings to JESCO in several weeks.
Sheila Grissett can be reached at email@example.com or (504) 717-7700.