Corps prohibits trees on levees; so where’s the state?
by Len Bahr, PhD*
On August 4 2012 Mark Schleifstein reported in The Times-Picayune that sections of the Mississippi River levee between the Corps New Orleans District headquarters on Leake Avenue and Audubon Park are being elevated by two feet, requiring temporarily closing some baseball fields and picnic areas known locally as “The Fly.”
Schleifstein reported that the elevated levee segments will conform to the new corps policy on levee vegetation. This caught my eye and reminded me of one of my favorite subjects…the possible use of trees to stabilize and sustain hybrid levees for certain specific applications.
Virtually no one alive still blames Mother Nature for the August 29 2005 catastrophe spawned by Hurricane Katrina. Independent forensic analyses showed that the tragedy was the consequence of a levee system that failed under hydraulic forces weaker than its advertised design capability.
Levee failures in SE Louisiana took the lives of over 1,600 victims and incurred $150 billion in damages, which convinced the U.S. Congress to pony up almost $15 billion to upgrade levees, install pumps and build surge barriers.
The sordid Katrina episode called levee integrity into question throughout the country and prompted the corps to undertake a survey of the status and condition of all federal levees, from DC to Sacramento. This survey revealed that, notwithstanding a traditional corps blanket prohibition of woody vegetation on all federal levees, some ‘leftist’ California levees had become overgrown by trees.
These ‘forested’ levees had created wildlife habitat for ecologically significant migratory birds and contributed esthetic qualities that were locally popular. These and other attributes are not discussed in civil engineering schools. In my technical opinion, the intertwined roots of these trees probably greatly increased the integrity of the levee sediments as well, although to my knowledge this binding property has never been quantified.
Despite objections from the State of California, on April 10, 2009 the U.S. Army Corps of Engineers formally adopted a standard one-size-fits-all-policy prohibiting trees on levees. This policy goes under the rubric: Guidelines For Landscape Planting and Vegetation Management of Levees, Floodwalls, Embankment Dams, and Appurtenant Structures. The policy document was accompanied by a draft policy guidance letter: “Process for Requesting a Variance from Vegetation Standards for Levees and Floodwalls–25 Fed. Reg. 6364-08″ (PGL).
The State of California officially challenged the policy on environmental grounds. Here are some comments from a California review of that policy:
In the wake of Hurricane Katrina, the U.S. Army Corps of Engineers (Corps) embarked upon a process of reviewing and improving their levee standards with the goal of improving public safety. As part of this process, they reinvigorated and clarified national policy that would require the removal of all woody vegetation over 2 inches in diameter from levee systems throughout the country. This was done even though vegetation did not cause any of the numerous levee and floodwall failures in New Orleans.
On April 15, 2010, the California Department of Water Resources (DWR) and California Department of Fish and Game (DFG) submitted extensive comments on the ETL and POL, explaining how the Corps’ vegetation management policy will reduce public safety in California, result in extensive and unnecessary environmental damage, and remove the Corps’ responsibility to assist state and local maintaining agencies in ensuring the integrity of California’s levee system.
California agrees with the Corps that public safety is the highest priority for flood management. California also agrees upon the importance of appropriate vegetation management on levees. Despite these common goals, California asserts that the Corps’ strict enforcement of the ETL and PGL will adversely impact public safety. This unintended consequence is due in large part by attempting to address complex technical, financial, legal and institutional problems with a highly prescriptive, one-size-fits-all approach to vegetation management.
No comments on this blanket levee tree prohibition policy have ever issued from the Louisiana Governor’s Office of Coastal Activities (GOCS) or the Office of Coastal Protection and Restoration (OCPR) or the Coastal Protection and Restoration Authority (CPRA). During the current recovery period from Hurricane Isaac I hear a growing call for putting more of the limited coastal restoration ‘eggs’ into the traditional levee basket, i.e., to construct costly, high maintenance earthen levees, rather than expanding ecosystem restoration, as called for in the Louisiana Comprehensive Master Plan for a Sustainable Coast.
I have previously argued strenuously (and fruitlessly) for consideration in carefully selected areas of a modest cost ‘hybrid’ levee design, borrowing from the function of forested chenier ridges in SW Louisiana. These sustainable levees, called Chenieres Faux, would be smaller and lower than traditional earthen levees and purposely colonized with dense populations of native trees known to be extremely durable in the face of hurricane winds and water currents.
Chenieres Faux would be designed to be resistant, but not impervious to storm surge. In other words, overtopping by surge waters would be fully expected. Consideration of forested hybrid levees would of course require a special dispensation from the blanket prohibition on levee trees.