Trees can fortify levees, study finds

SACRAMENTO, Calif. – For years, California flood control officials have pleaded with the U.S. Army Corps of Engineers to reconsider a policy that bans trees on levees, urging the agency to heed years of independent research that found little risk to flood safety.

Now, a new study by the Army Corps itself has found that trees actually strengthen levees in some situations.

The conclusion could force the agency to reconsider its policy.

The controversial levee maintenance policy, which emerged in the wake of Hurricane Katrina, allows only grass on levees, on the grounds that trees could damage levees if they fall down or that their roots may promote seepage that would undermine a levee.

The new study, completed in July by an Army Corps research unit in Mississippi, has not yet been publicly released.

The team of more than 15 researchers, all Army Corps employees, concluded that trees growing at the base of a levee – called the “toe” – actually improve flood protection.

“Trees near the toe increased the factor of safety because of the reinforcing effects of the roots and the increased counterweight effect of the tree to slope movement,” the authors state.

The benefits diminish when trees are higher up a levee, to the point that safety is “slightly reduced” by trees atop a levee.