Trees and brush may be aesthetically pleasing and provide other benefits; however, the growth of woody vegetation on and near dams, including the downstream toe area, can lead to serious problems.
Sudden uprooting of trees by strong winds can result in the movement of a relatively large amount of embankment material and create large voids in the embankment. This in turn can lower the crest of the dam, reduce the effective width of the dam, lead to instability of the embankment, and facilitate seepage. Falling trees can also cause structural damage to concrete, steel, stone, or timber structures.
The root systems of trees can be a potential hazard by allowing seepage pathways to develop through a dam. Trees eventually die and their roots decay and rot. The root cavity leaves a void within the dam through which water can enter and flow. This can ultimately lead to failure of the dam by piping (internal erosion). In general, a tree’s root system may extend to the edge of the tree canopy or tree drip line.
Brush and woody vegetation prevent the proper visual inspection of the dam surfaces. The observation of sinkholes, slides, animal burrows, seeps, and other irregularities can be obscured by trees and brush. Woody vegetation can also cause excessive shade which in turn can hinder the growth of sturdy, dense grass coverage. These affected areas are more prone to surface erosion.
Excessive vegetation can provide habitat for burrowing animals, which can create problems on a dam.
Erosion Control
Grass cover is a very effective and inexpensive means to prevent the erosion of embankment surfaces. The stems and root systems of grasses tend to trap fine particles of soil, thus inhibiting the migration of these particles. A good grass cover provides an excellent means against erosion due to runoff caused by rains, and may protect the embankment during limited overtopping. Extensive testing and well documented incidents have consistently shown that a good grass cover is highly effective in preventing erosion at dams.
Maintenance
Grass cover should be routinely cut to provide a surface that can be easily inspected. In general, the grass on a dam should be cut at least twice a year. Trees and brush should never be allowed to grow on or very near to a dam including the downstream toe area! Many older dams have very large trees growing on or near them. Trees at or less than 4 inches in trunk diameter should be cut flush with the ground as part of the dam maintenance program and monitored during routine inspections for any changed conditions around the stump. For very small dams, trees less than 4″ diameter may compromise the dam when removed. Removal of brush, and trees and roots larger than a 4 inch trunk diameter should be done under the direction of a qualified professional engineer knowledgeable about dam safety and maintenance, and may require a permit from the Department. When in doubt, consult with a licensed professional engineer.
For Questions or Comments Contact:
New York State Department of Environmental Conservation
Bureau of Flood Protection and Dam Safety, 4th Floor
625 Broadway
Albany, NY 12233-3504
References
For more information, please refer to the following resources:
- “Technical Manual for Dam Owners: Impacts of Plants on Earthen Dams”, FEMA 534
- NYSDEC publication entitled “Owners Guidance Manual for the Inspection and Maintenance of Dams in NYS”
