Important Terms Defined for Media Coverage

Early news coverage in high impact situations can be misleading and confusing. To reduce this confusion, we recommend use of the following terms when describing the incidents associated with dams. These terms are taken from FEMA’s Federal Guidelines for Dam Safety. We have added commentary relevant to recent events and news coverage in italics after several of the definitions. We have also added terms at the end of the list that we believe will be useful.

Dam – An artificial barrier that has the ability to impound water, wastewater, or any liquid-borne material, for the purpose of storage or control of water.
With regard to public safety and dams, size matters, as does the location of the dam relative to downstream residents.

Spillway – A structure over or through which flow is discharged from a reservoir. If the rate of flow is controlled by mechanical means, such as gates, it is considered a controlled spillway. If the geometry of the spillway is the only control, it is considered an uncontrolled spillway.
Spillways come in a variety of forms and can be located away from, near, or across the associated dam. The spillway may be referred to as the Primary Spillway if the structure is used for most water releases past a dam.

Auxiliary Spillway / Emergency Spillway – Any secondary spillway that is designed to be operated infrequently, possibly in anticipation of some degree of structural damage or erosion to the spillway that would occur during operation.
Auxiliary spillways are typically constructed with a higher entrance so that they are not activated except when very high pool levels occur.

Outlet / Outlet Works – An opening through which water can be freely discharged from a reservoir to the river for a particular purpose. / A dam appurtenance that provides release of water (generally controlled) from a reservoir.

Dam Failure – Catastrophic type of failure characterized by the sudden, rapid, and uncontrolled release of impounded water or the likelihood of such an uncontrolled release It is recognized that there are lesser degrees of failure and that any malfunction or abnormality outside the design assumptions and parameters that adversely affect a dam's primary function of impounding water is properly considered a failure. These lesser degrees of failure can progressively lead to or heighten the risk of a catastrophic failure. They are, however, normally amenable to corrective action.
A dam failure is an event that poses the greatest threat to public safety because of the suddenness with which water can be released and the potential speed and energy associated with the flow from such a release. 

Emergency Action Plan (EAP) - A plan of action to be taken to reduce the potential for property damage and loss of life in an area affected by a dam failure or large flood.
EAPs are an extremely valuable tool for protecting the public.


In addition to the above definitions taken from Federal Guidelines for Dam Safety, we encourage the use of the following terms to more accurately describe dam and spillway incidents.

Spillway Activation – The initial passage of water across a spillway, typically by water passing over the top of the spillway entrance or by opening gates. Some news coverage have referred to activation of spillways or auxiliary spillways as dam overtopping, creating the false impression that a structure not designed for overtopping may be in danger.

Spillway Degradation – The partial loss of elements of a spillway. Degradation of a spillway is a serious condition but should not be described as a dam failure. Spillway degradation can lead to loss of the function of the spillway, erosion of the spillway elements that control flow, such as the spillway crest, or a dam failure depending on the location of the spillway.

Dam Overtopping – Flow of water across the top of the dam. Unless a dam is designed with overtopping protection, overtopping an earthen dam is typically a serious condition that can lead to rapid erosion of the dam and a dam failure.


For additional information on dam safety and dam basics please visit the Dams 101 section of the site.

Special thanks to our sustaining members