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Recently, over 500 people traveled to a symposium in Sacramento, CA featuring two sides of a fierce debate. On one side, a parade of scientists summarized findings that trees landscaped on levees can actually improve the levees?EUR??,,????'??? strength and aid in flood safety. On the other hand, federal officials continued to maintain their policy that every mature tree must be cut down on the area?EUR??,,????'???s levees. In February a maintenance inspection by the U.S. Army Corps of Engineers failed 32 Central Valley districts in California mainly because their levees had too much trees and plant life. The issue is very significant to many Californians because they feel that the levees provide almost the only remaining riverside wildlife habitat. ?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 at the University of Michigan, at the symposium. His findings were included in a 1991 paper which he co-wrote. Though it had been sponsored by the corps., he said it was ?EUR??,,????'??vetted by all the corps districts before its publication.?EUR??,,????'?? David Pezza, engineering and construction chief of the corps?EUR??,,????'??? civil works branch, said that 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 Corps policy comes from a document called the FEMA 534 Technical Manual for Dam Owners. Co-author Bill Bouley of the U.S. Bureau of Reclamation writes that ?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??,,????'?? Over 500 people traveled to the symposium, from as far away as Holland. Several scientists collaborated to use a computer model to show that trees could offer more erosion protection than a uniform carpet of grass, which is preferred by the U.S. army corps. They also feel that tree roots actually improve the shear strength of the soil, and concluded 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
Recently, over 500 people traveled to a symposium in Sacramento, CA featuring two sides of a fierce debate. On one side, a parade of scientists summarized findings that trees landscaped on levees can actually improve the levees?EUR??,,????'??? strength and aid in flood safety. On the other hand, federal officials continued to maintain their policy that every mature tree must be cut down on the area?EUR??,,????'???s levees. In February a maintenance inspection by the U.S. Army Corps of Engineers failed 32 Central Valley districts in California mainly because their levees had too much trees and plant life.
The issue is very significant to many Californians because they feel that the levees provide almost the only remaining riverside wildlife habitat.
?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 at the University of Michigan, at the symposium. His findings were included in a 1991 paper which he co-wrote. Though it had been sponsored by the corps., he said it was ?EUR??,,????'??vetted by all the corps districts before its publication.?EUR??,,????'??
David Pezza, engineering and construction chief of the corps?EUR??,,????'??? civil works branch, said that 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 Corps policy comes from a document called the FEMA 534 Technical Manual for Dam Owners. Co-author Bill Bouley of the U.S. Bureau of Reclamation writes that ?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??,,????'??
Over 500 people traveled to the symposium, from as far away as Holland. Several scientists collaborated to use a computer model to show that trees could offer more erosion protection than a uniform carpet of grass, which is preferred by the U.S. army corps. They also feel that tree roots actually improve the shear strength of the soil, and concluded 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
Francisco Uviña, University of New Mexico
Hardscape Oasis in Litchfield Park
Ash Nochian, Ph.D. Landscape Architect
November 12th, 2025
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