Probable Causes and Possible Consequences
Trees should not be planted or allowed to grow on dams. Tree root systems create pathways for water and can damage dams if they are uprooted.
Seepage through dam damaged by tree roots. Trees should not be allowed to grow on dams. (Photo source: ASDSO)
Problems with Trees and Brush Near Dams
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.
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.
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.
Technical Manual for Dam Owners
“Problems with Trees and Brush near Dams. While trees and brush may be aesthetically pleasing, give off oxygen,and provide cooling shade, 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 displacement of a relatively large amount of embankment material. This in turn can lower the crest of the dam, reduce the effective width of the dam, and facilitate seepage. Falling trees can also cause structural damage to concrete, steel, stone, or timber structures…………………………”
Policy on Trees on Dams
Vegetation, erosion and dams
A dense cover of low-growing grassy vegetation is recommended because it will provide protection from surface erosion, but its root structure does not penetrate the embankment so deeply as to create a potential path for internal erosion.
Grass on dams requires regular mowing. Although woody vegetation such as trees and brush may protect against surface erosion, such growth can cause other, serious problems. These problems develop over years and may go undetected until it’s too late. In addition, trees or brush can hide an embankment surface, making inspection difficult.
Many times, dam failure is described as a sudden event when in fact conditions leading to the failure went undetected or ignored for years.
|Why is it recommended that trees not be allowed to grow on dams?|
|While trees on a dam may be aesthetically pleasing they are not good for the dam. As trees grow they tend to shade out grasses that prevent erosion. They also develop large root systems, and one of the leading causes of dam failure is “piping” along the root system of a tree that has died. As the roots rot, conduits are formed allowing the passage of water through the dam. Also, trees uprooted from high winds can cause the loss of a significant section of the dam.