Early news coverage in high impact situations can be misleading and confusing. To reduce this confusion, we recommend the 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.
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.
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.
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.
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.
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:
Abutment
The part of the valley side against which the dam is constructed. May also refer to an artificial abutment sometimes constructed as a concrete wall. Right and left abutments are those on respective sides as an observer when viewed looking downstream.
Dam Component Failure
While concerning, a dam component failure is not a catastrophic failure of the entire structure, but a failure of an important piece of the dam. Examples of component failures could include; a broken drain valve; a damaged trash rack; a broken or buckled concrete slab or wall in the spillway system; a corroded or collapsed spillway pipe; a failed gate motor; a broken spillway gate; a crack or slide area in an earthen embankment; and a broken or plugged toe drain or filter drain. Should a component failure not be corrected, it may lead to the failure of other components and possibly a complete dam failure.
Dam Incident
An incident is defined as an event, which takes place, or a condition, which is slowly developing, that is not normally encountered in the routine operation of the dam and reservoir, or necessitates a variation from Standard Operating Procedures. Such events are more common than emergency conditions and often offer time to conduct preplanned responses to the slowly developing situation. If addressed in a timely manner, such events can often be prevented from progressing into a worse event such as a component failure, or a catastrophic failure.
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.
Drawdown
The resultant lowering of water surface level due to release of water from the reservoir.
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, component failure, incident, or large flood. These plans may also be known as Incident and Emergency Action Plans (IEAPs). EAPs are an extremely valuable tool for protecting the public. EAPs are normally developed by the dam owner and their dam safety engineer in collaboration with local emergency response officials, and first responder agencies. The EAP is often first used to categorize the severity of the event: a slowly developing event; a rapidly developing emergency; or an imminent emergency. The EAP then typically contains dam failure flood inundation mapping, notification and contact call trees, and predetermined responses for the three (3) different event levels.
Flashboards
Lengths of timber, concrete, or steel placed on the crest of a spillway to raise the operating water level but that may be quickly removed in the event of a flood either by tripping a supporting device or by designing the flashboard supports to fail under specified conditions.
Gate
In general, a device in which a leaf or member is moved across the waterway from an external position to control or stop the flow.
- Crest Gate (Spillway Gate): A gate on the crest of a spillway that controls overflow or reservoir water level.
- Flap Gate: A gate hinged along one edge, usually either the top or bottom edge. Examples of bottom-hinged flap gates are tilting gates and fish belly gates so called from their shape in cross section.
- Outlet Gate: A gate controlling the outflow of water from a reservoir.
- Radial Gate (Tainter Gate): A gate with a curved upstream plate and radial arms hinged to piers or other supporting structures.
- Slide gate (sluice gate): A gate that can be opened or closed by sliding in supporting guides.
Imminent Dam Safety Emergency
A dam safety emergency event that may lead to a catastrophic dam failure in a very short time frame. Such an event would cause flash flooding downstream of the dam.
Low level outlet (bottom outlet)
An opening at a low level from the reservoir generally used for emptying the impoundment.
Operational Release or 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 designed and built to allow water to safely pass over or through the dam. Some news coverage have mistakenly 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.
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.
Rapidly Developing Dam Safety Emergency
A dam safety emergency event that is rapidly developing and could quickly lead to dam failure and flash flooding downstream of the dam.
Slowly Developing Dam Safety Incident
An unusual, slowly developing situation that may over time get worse and endanger the structural integrity of the dam.
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.
Stoplogs
Large logs, timbers or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit so as to provide a cheaper or more easily handled means of temporary closure than a bulkhead gate.
