A Division of the Georgia Department of Natural Resources

Safe Dams Glossary

Terminology frequently used when discussing dams.

  1. 100 year flood - A flood that has a one percent chance of being equaled or exceeded during any given year.
  2. Abutment - the bordering area of the dam site which functions as a support for the ends of the dam structure.
  3. Acre-Foot - The volume of water that would cover one acre to a depth of one foot. It is equal to 43,560 cubic feet or 325,851 gallons.
  4. Appurtenant works - Structures located either in or separate from the dam such as spillways, low level outlet works, access bridges, and other structures.
  5. Berm - A horizontal step in the slope of an embankment dam usually for the purpose of reducing erosion or to increase the thickness and stability of the embankment.
  6. Blanket Drain - A layer of pervious material placed to facilitate drainage of the foundation and/or embankment.
  7. Breach - An opening through a dam that allows a release of water from a reservoir.  These can be controlled (plans developed and implemented under the direction of an Engineer of Record) or uncontrolled (failure of the structure of the dam.)
  8. Crest - The top surface of the dam.
  9. Dam - Any artificial barrier that impounds or diverts water which is 25 feet or more in height or has a maximum impounding capacity of 100 acre-feet or more.
  10. EAP - “Emergency Action Plan,” plan of action to be taken to reduce the potential for loss of life and property damage in an area affected by a dam failure or large flood.
  11. EMA - Emergency Management Agency.
  12. Emergency or Auxiliary Spillway - A spillway designed to provide additional protection against overtopping of a dam intended for use under extreme conditions such as malfunction of the principal spillway or extreme rainfall.
  13. EOR - “Engineer of Record,” licensed engineer registered in the State of Georgia who has at least 7 years of demonstrated experience related to the investigation, design and/or construction of dams. The EOR designation is conferred by the SDP after an engineer has substantiated his/her qualifications by application prior to his/her engagement by an owner/operator of a Category I dam. An EOR must be involved with the design or modification of a Category I dam. While the EOR designation indicates demonstrated experience related to dams, it is not an endorsement by the State of any particular engineer.
  14. EPD - Environmental Protection Division.
  15. Flashboards - Structures installed at the top of a dam, gates, or spillways to raise the pool elevation.
  16. Flood of Record - The highest recorded flood in a particular river basin.
  17. Freeboard - The difference in elevation between the top of the dam and the maximum reservoir water surface that would result should the inflow design flood occur and should the outlet works function as planned.
  18. Head - The energy possessed by water due to its elevation, velocity, or being under pressure. Usually refers to the difference in elevation between water surfaces upstream and downstream of a dam.
  19. Height of Dam - Vertical distance measured from the bottom of the plunge pool at the downstream toe of the dam to the lowest elevation on the crest of the dam.
  20. Inundation map - A map showing areas that would be affected by flooding from releases from a dam’s reservoir.
  21. Normal Pool - The reservoir storage volume at normal storage elevation.
  22. Owner - Whoever owns or controls any portion of the dam or its appurtenant works.
  23. Piezometer - Device commonly used to measure the hydraulic pressures within an earthen dam.
  24. Piping - The progressive development of internal erosion of an embankment or foundation material by seepage.
  25. Plunge Pool - A natural or artificially created pool at the base of a dam that dissipates the energy of water flowing through the principal spillway.
  26. PMP - “Probable maximum precipitation” as determined by the United States Weather Service to be the greatest amount of rainfall of a set duration which would be expected for a given location.
  27. Principal Spillway - Spillway which conveys normal runoff from the reservoir over, through, or around the dam.
  28. Riprap - A layer of large stones, broken rock or other suitable material generally placed in random fashion on the upstream and/or downstream faces of embankment dams and around plunge pools to protect them from erosion.
  29. SDP - Safe Dams Program.
  30. Seepage - Internal movement of water that may take place through the dam, foundation, or abutments.
  31. Slough - A shallow slope failure. Movement of a soil mass downward along a slope because of a slope angle too great to support the soil, wetness reducing internal friction among particles of soil or seismic activity.
  32. Spillway - An outlet from a reservoir or section from a dam designed to release surplus water that is not discharged though a turbine or other outlet works.
  33. Stilling basin - A basin constructed to dissipate the energy of rapidly flowing water, e.g., from a spillway or outlet, and to protect the stream or riverbed from erosion.
  34. Tailwater - The water level immediately downstream from a dam. The water surface elevation varies due to fluctuations in the outflow from the structures of the dam and due to downstream influences of other dams or structures.
  35. TBS (To Be Studied) List - A list which contains any Category II dam which requires further study in order to determine if reclassification to Category I status is necessary.
  36. Toe - The junction of the slope or face of a dam with the ground surface; these are usually identified as the upstream toe and downstream toe.
  37. Toe drain(s) – A system of pipes and/or pervious materials along the downstream toe of a dam used to collect seepage from the foundation and embankment and convey it to a free outlet.
  38. Trashrack - A device located at the intake of a spillway to prevent floating or submerged debris from entering the intake.