Officials at the U.S. Army Corps of Engineers said Thursday that they have no intention of changing a policy that bans trees on levees, despite a new study by Corps researchers that shows trees can boost flood safety in some situations.

The Corps formally released the new study Thursday. From extensive field research in Sacramento and other sites across the nation, it concludes that trees at the base of levees can improve levee safety by binding the soil together with their roots; and it found that tree roots pose little risk of creating a path for seepage that could undermine levees.

But the study also found that trees on top of levees may pose risks, and that unanswered questions remain, such as whether exposed roots aggravate erosion.

Corps officials pointed to these unknowns in holding fast to the policy.

"Our current standard is the best way to address the uncertainties that trees may introduce to a levee system," said Tammy Conforti, national manager of the Army Corps Levee Safety Program.

The Corps policy allows only grass to grow on levees. Local levee agencies would lose federal disaster aid if they don't comply.

The policy was imposed nationally starting in 2007. It came despite decades of policy by the Corps in California that allowed and encouraged trees on levees.

Those trees now make up most of the remaining riparian habitat in the Central Valley. They also provide much-loved shade and scenery.

"This highly anticipated report should remind policymakers at the Corps of Engineers that a blanket policy prohibiting vegetation on levees is misguided and ignores scientific evidence," Rep. Doris Matsui, D-Sacramento, said in a statement Thursday.

Under that kind of pressure, the Corps has hinted it is willing to be flexible. It recently told levee agencies they may get more time to comply if they can demonstrate higher priorities, such as fixing seepage problems.

On Thursday, Conforti said the Corps is drafting a new "stand-alone document" to allow exceptions based on new research.

This would augment the vegetation policy, not replace it, she said, and exceptions could be either site-specific or national in scope.

She said the Corps also is creating regional teams of levee and wildlife experts to investigate solutions to safety and habitat concerns. California will have such a team.

"It does seem like they are leaving the door open for trying to be more flexible," said George Qualley, an engineer at the California Department of Water Resources who oversees levee-vegetation issues. "We've suggested things they could change and hopefully they'll consider our suggestions."

LEARN MORE

The Corps study can be found online at: http://wri.usace.army.mil/woody_vegetation_research.html

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