Concerns about Vegetation on Levees
Vegetation can block visual inspections – Vegetation can limit visibility for inspection of the levee structure. Vegetation growing on the levee or at its base can block the view of levee inspectors and can be physically in the way of any maintenance response during a flood-fight. Inspections during a flood event are focused on the landside of the levee. Any seepage through the levee is visible on the landside. The ACOE has protocols for vegetation management on and at the base of levees. Planting designs have been proposed that allow for efficient visual inspection from the top of the levee to its base. For example, plantings on the side of a levee in rows designed as an inverted chevron (VVVVV) will allow clear views from the levee top down the levee side.
(Above) Vegetation on levees can be planted in rows that will allow the levee to be visually inspected for damage or weakness.
Dense vegetation can affect a flood-fight. Trees and shrubs on or near a levee could be in the way during a flood fight - they will be physically in the way of equipment and sand-bags.
Tree roots will rot and cause piping. This has never been substantiated with photographs. Shields and Gray (see Science of Vegetation on Levees) found that old root channels on a Sacramento River levee had filled with sand.
Vegetation will provide burrowing rodents with cover. Burrowing rodents could weaken levee integrity by excavating networks of tunnels and cavities. Burrowing rodents typically live in family-groups or colonies of many individuals. They will excavate their burrow systems in the surface two to three feet of the levee, similar to the roots of plants, though collapsed beaver dens will leave deep holes into the levee. Unlike roots, most rodent burrows are open channels arranged in a network through the surface of the levee. Two species of burrowing rodents can be found near California’s levees: the Botta’s pocket gopher (Thomomys bottae) and the California ground squirrel (Spermophilus beecheyi). Each species can develop networks of burrows covering many square feet. How deep into a levee they penetrate is not known. Both pocket gophers and ground squirrels prefer open grassland habitat – not vegetation dominated by woody trees and shrubs, where they can observe predators approaching. These species are rarely observed in woody vegetation, especially dense woody vegetation, where predators could approach undetected and burrow excavation would be difficult because of large woody plant roots.
(Above) “A keystone species for the state's grasslands, the California ground squirrel provides food and shelter to numerous other species. Golden eagles, coyotes, and rattlesnakes depend on squirrels for food, while rare amphibians like the California tiger salamander spend most of their lives in the squirrels' burrows. Illustration by Tim Gunther.” Illustration and caption from baynature.org.
(Above) Botta’s pocket gopher Thomomys bottae and California ground squirrel Spermophilus beecheyi.
(Above) A California levee shows the exposed soil castings of gophers after the grasses on the levee were burned.
Tree toppling and taking levee with it. Large trees can topple during high winds and leverage a large volume of levee material out, leaving a hole. The cause is usually not clearly explained because the tree and that portion of the levee are washed away. However, the cause of the tree toppling has to do with more than high velocity wind. The levee material that supports a tree has likely lost shear strength due to saturation of the entire levee by flood water.
(Above) One concern about planting trees on levees is that the leverage caused from a falling tree could dislodge and remove levee material. Illustration by Zina Deretsky from the National Science Foundation.