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Tree and woody vegetation growth on earthen dams and in close proximity to other dams such as concrete dams is undesirable and at a minimum has some level of detrimental impact upon operation, inspection, performance, and safety of dams. Tree roots cause serious structural damage to earth embankment and appurtenant dam features such as gate wells, spillway walls and other components.
It is recommended that earth embankment dams be maintained with a healthy uniform cover of desirable vegetation such as an appropriate variety of grasses. Dam embankment grass should be mowed periodically to promote healthy cover and prevent infestation of undesirable woody growth and weeds.
Trees and woody growth can make it difficult to conduct inspections of dams. Tree roots can cause leaks, damage concrete joints and overturn during high wind events causing large voids due to pull out of root balls and cause many other problems that will be very costly to repair. Trees and woody growth located in spillways will dramatically reduce spillway flow capacity. Trees are known to accelerate deterioration of dams and can lead to dam failure.
It is recommended that the area at least 20 feet downstream from the entire downstream toe of earth embankment dams be maintained free of trees and woody growth. This is necessary to prevent root systems from growing into the dam embankment causing damage to this area of the dam.
For concrete dams and appurtenant features of all dams it is recommended that tree growth not be allowed to occur within 20 feet of such features. In some cases it may be necessary to maintain a greater distance to ensure roots do not adversely impact dam components. Do not allow tree growth in areas located above buried conduits/pipes.
Prior to removal of existing trees and woody growth from dams, part A of a Chapter 253 Dam Safety Permit Application must be submitted to the Office of Dam Safety. Permit applications should be prepared by a qualified dam engineer for larger projects involving removal of trees in excess of 4 inches and where there is planned excavation of roots. The Office of Dam Safety will review applications and determine if the planned work requires a permit. If the project involves removal of brush and trees 4 inches and less in diameter the Office of Dam Safety may find a permit is not necessary to conduct the work. In general routine maintenance activity does not require a permit.
Dam Owner’s Guide to Plant Impact on Earthen Dams, FEMA Publication L-263, September 2005This brochure is intended to help dam owners nationwide identify and mitigate problem vegetation before adverse effects occur.
Technical Manual for Dam Owners, Impacts of Plants on Earthen Dams, FEMA Publication 534, September 2005Damage to earthen dams and dam safety issues associated with tree and woody vegetation penetrations of earthen dams is all too often believed to be routine maintenance situations by many dam owners and engineers.
Contrary to this belief, tree and woody vegetation penetrations of earthen dams and their appurtenances have been demonstrated to be causes of serious structural deterioration and distress that can result in failure of earthen dams.