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Trees Offer Superb Erosion Control09-28-07 | News

Trees Offer Superb Erosion Control

Many Californians are opposed to the deforestation of the levees on the California Delta, arguing the levees provide some of the only remaining riverside wildlife habitat.

Recently, over 500 people traveled to a symposium in Sacramento, Calif. A parade of scientists summarized findings that trees landscaped on levees can improve the levees?EUR??,,????'??? strength and aid in flood safety. Federal officials, however, maintain every mature tree must be cut down on the levees. The issue is very significant to many Californians because they feel that the levees provide almost the only remaining riverside wildlife habitat. In February 2007, a maintenance inspection by the U.S. Army Corps of Engineers (ACOE) failed 32 Central Valley districts in California, mainly because their levees had too many trees and plants.

?EUR??,,????'??By and large ?EUR??,,????'??? trees have a positive or beneficial influence on the safety of levees,?EUR??,,????'?? said Donald Gray, a geotechnical engineering professor from the University of Michigan who spoke at the symposium. Gray is also a former associate editor for Landscape Contractor National and Landscape Architect and Specifier News. His findings were included in a 1991 paper that he co-wrote. The paper was was sponsored by the ACOE, but he said it was ?EUR??,,????'??vetted by all the corps districts before its publication.?EUR??,,????'??

David Pezza, engineering and construction chief of the ACOE?EUR??,,????'???s civil works branch, said officials did not consider Gray?EUR??,,????'???s study because it ?EUR??,,????'??didn?EUR??,,????'???t match what they saw in the field ?EUR??,,????'??? Vegetation is very hazardous to infrastructure when it?EUR??,,????'???s not done in an integrated way.?EUR??,,????'??

Much of the ACOE policy comes from the FEMA 534 Technical Manual for Dam Owners. Co-author Bill Bouley of the U.S. Bureau of Reclamation writes: ?EUR??,,????'??When trees grow, they tend to corkscrew their way into an embankment and that tends to loosen the soil ?EUR??,,????'??? Tree roots do not stabilize soil mass. On the contrary, tree root penetration loosens the soil.?EUR??,,????'??

Several scientists collaborated to use a computer model to show that trees could offer more erosion protection than a uniform carpet of grass, the preferred ACOE method. Those scientist believe tree roots improve the shear strength of the soil, and that the maintenance standards should favor shrubs and woody trees. Others pointed out the consequences of removing the trees, such as root decay leading to erosion, and a higher population of burrowing rodents.

Joe O?EUR??,,????'???Connor fought the Army Corps a few years ago to preserve several oak trees in his neighborhood, but lost to their standard levee design and the trees were cut down.

?EUR??,,????'??Safety is No. 1, always,?EUR??,,????'?? O?EUR??,,????'???Connor said. ?EUR??,,????'??But there?EUR??,,????'???s no need to damage the parkway if it?EUR??,,????'???s not going to produce a beneficial effect.?EUR??,,????'??

Source: Sacramento Bee