A new study by the Army Corps of Engineers says that trees growing on levees can strengthen the flood-control structures in some circumstances, but indicated that results vary by soil type, climate conditions and levee design.
“These results cannot be generalized to apply to every levee system,” said Maureen Corcoran, an Army Corps research geologist.
Trees and their root systems can either increase or decrease levee safety, she said, and must be analyzed on a case-by-case basis.
The city of Coeur d’Alene learned this year that hundreds of mature ponderosa pine trees growing along Rosenberry Drive, also known as “the dike road,” don’t comply with the corps’ national levee standards. The dike road separates Lake Coeur d’Alene and the Spokane River from North Idaho College and the Fort Grounds neighborhood.
The city is among dozens of Western communities grappling with noncompliant trees. The Coeur d’Alene City Council recently directed the city engineer to start repairing other deficiencies in the levee, which is more than 60 years old, while continuing talks with the corps options other than removing the trees.
On Thursday, Army Corps officials said they weren’t ready to abandon the agency’s existing vegetation standard for levees, which promotes only grass on levees.
More than 14 million people nationwide live and work behind levees, said Tammy Conforti, manager of the corps’ levee safety program. Allowing vegetation creates uncertainty in how levees will hold up during flooding, she said.
However, the Army Corps will review the study results for inclusion in its vegetation variance policy, which Conforti said is undergoing revisions. The Puget Sound Partnership, a Washington coalition, is pushing for a variance that will allow trees on levees to enhance floodplains and improve habitat for salmon streams, while retaining levee integrity, said spokesman Michael Grayum.
The two-year Army Corps study included field work along the Skagit River in Western Washington and in California’s Sacramento Valley. In some instances, trees planted at the levee’s base strengthened the structure, particularly in sandy soil conditions.
When wind gusts hit 40 mph, however, trees were considered a detriment to levee stability, regardless of their position on the levee.
The study only looked at live trees. It didn’t consider how root decay would affect levee integrity.