There are 75,900 dams in the United States, according to the National Inventory of Dams 2005 update. About one third of these pose a high or significant hazard to life and property if failure occurs. Dam failure can occur with little warning. Severe storms can produce a flood in a few hours or even minutes for upstream locations. Flash floods can occur within six hours of the beginning of heavy rainfall and dam failure may occur within hours of the first signs of breaching. Dam owners are responsible for the safety and the liability of the dam and for financing its maintenance, repair, and any necessary upgrades.
All regulated dams are placed into one of three hazard classifications based on the threat to downstream life and property should dam failure occur. The classification may change as residential development or other land use changes occur downstream.
Failure may cause loss of life, serious damage to homes, industrial or commercial buildings, important public utilities, main highways, or railroads. Dams constructed in existing or proposed residential, commercial, or industrial areas will be classified as high hazard dams unless the applicant presents clear and convincing evidence to the contrary.
Failure may cause significant damage to main roads, minor railroads, or cause interruption of use or service of relatively important public utilities.
Failure may cause damage to farm buildings (excluding residences), agricultural land, or county or minor roads.
A dam’s frequency of inspection should be commensurate with the danger posed to downstream lives and property if the dam should fail as well as be in compliance with state requirements. All inspections should be retained as should proof of any correction action arising from inspections. Frequency may vary…some dams need almost daily inspection, others may only need annual. Dams should be inspected after a major rainfall, especially if the emergency spillway activated or the dam overtopped during the event. Contact your State Dam Safety Official for scheduling.
In the U.S. 56.4% of all dams are privately owned and the majority of privately owned dams are embankment dams that are constructed of compacted earth. Embankment dams need to be maintained and monitored to prevent deterioration and dam failure. Vegetation is the primary source of dam failure according to FEMA (Federal Emergency Management Agency). A program that includes regular inspection, mowing, and repair of minor problems as they occur is the most economical approach to responsible dam maintenance. Postponing minor items only creates major problems and increases repair costs. Erosion of the embankment slopes is a major problem that in almost all cases can be prevented by a good vegetation cover. Essential to a good vegetation cover is a fertilization program that is best suited to the site specific conditions and the type grass being grown. Control of animals like beaver and muskrat and repair of damage caused by them should be done as soon as it is discovered.
Trees on a dam may be aesthetically pleasing but they are structurally detrimental. The shade that they produce often reduces grass cover and thus increases erosion. Their root systems are one of the leading causes of dam failure, specifically 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. Trees uprooted during high winds can also damage significant sections of a dam.
According to FEMA, dams with a high or significant hazard classification need an emergency action plan and prudent behavior would have EAPs for low hazard dams as well. Responsible organizations should know the necessary steps to minimize risks of human life and damage to property in the event of dam failure. Detailed information on developing a Dam Safety EAP can be found in the Federal Guidelines for Dam Safety: Emergency Action Planning for Dam Owners (FEMA 64) at the following website http://www.fema.gov/.