Monday, July 11, 2011

Levees & Trees

We recently posted on this, and, in an excellent article from the Sacramento Bee, by UCD Environmental Engineering Professor Jay Lund, the case is presented with balance and insight.

An excerpt.

“The U.S. Army Corps of Engineers' decision to enforce a policy of removing large vegetation from many miles of Central Valley levees has caused quite a stir.

“Both sides assert noble and worthy causes – environmental and recreation interests want to protect trees and bushes on levees, and public safety demands vegetation removal. Both sides are right.

“Sadly, California has set aside little room for both the environment and flood safety, so these important causes must fight over thin strips of levee that currently provide poor habitat and poor flood protection.

“Authorities can more effectively inspect levees when they are free of trees. Most levees fail before over-topping, and usually give warning (through slumping, animal burrows, or seeping water) before collapsing. That allows some time for repair and evacuation. Emergency repairs also proceed faster without vegetation. Limiting vegetation also might introduce fewer potential problems from decaying roots or uprooted falling trees.

“Worldwide, in countries such as the Netherlands and China, serious levee systems are cleared of trees.

“Nevertheless, the dilemma is real.

“Most levees prevent flooding for only a few days or weeks in the few years when major floods occur. More than 99 percent of the time, levees are used primarily for recreation and habitat. They are places where people can go up and see their rivers.

“Levees with trees are clearly more attractive, except when they need to be inspected. If the levee protects little of value, then perhaps a case can be made that reliability can be sacrificed for protecting a levee's habitat and aesthetics. Alas, urban levees, protecting homes and businesses from floods, are the ones that are the most valued for recreation and cooling shade.

“Levees can be built for both flood and non-flood purposes by widening, sheet-piling levee cores, improving drains, and other means, to allow vegetation without hindering flood protection. However, building multi-functional levees has financial costs.

“In urban areas, where multiple purposes are most valuable, land for widening levees is awkward and expensive to acquire.”