Wednesday, August 29, 2007

Experience vs Science

When lab based scientific research reaches opposing conclusions to that of professionals working in the field, it makes it difficult to develop public policy.

This type of situation indicates more research is needed.

Scientists: Trees help, not hinder, levee safety
U.S. Army Corps of Engineers hears challenges to its removal policy at Sacramento hearing.
By Matt Weiser - Bee Staff Writer
Published 12:00 am PDT Wednesday, August 29, 2007

Though federal officials on Tuesday faced a deluge of evidence that trees do not threaten levees, they continued to tout their own policy that could require every mature tree to be cut down on Sacramento levees.

At a symposium on the issue in Sacramento, a parade of scientists summarized decades of research showing that trees may, in fact, improve flood safety when planted on levees.

The backdrop to Tuesday's meeting were the 32 Central Valley levee districts that in February failed a maintenance inspection by the U.S. Army Corps of Engineers. Most failed because their levees had too many trees and shrubs.

The corps, which is preparing a new national levee maintenance policy, currently says no vegetation larger than 2 inches in diameter should grow on a levee. But that standard has not been applied in California. In fact, the local district of the corps has worked for decades with local, state and federal agencies to plant more trees on levees.

The issue affects levee managers nationwide, but it is especially critical in California, where levees provide virtually the only remaining riverside wildlife habitat.

"By and large ... trees have a positive or beneficial influence on the safety of levees," Donald Gray, a geotechnical engineering professor at the University of Michigan, told the symposium.

The findings were included in a 1991 paper he co-wrote based on a study sponsored by the corps. "This report was vetted by all the corps districts before its publication," Gray said.

However, David Pezza, engineering and construction chief of the corps' civil works branch, said officials did not consider the study in their maintenance polices because "it didn't match what they saw in the field."

"We do a lot of research in support of our civil works program. But in that particular case, we did not find that science was relevant to what we were doing," Pezza said. "Vegetation is very hazardous to infrastructure when it's not done in an integrated way."