Large dam - Old definition (before 2011) For the purpose of inclusion in the World Register of Dams, a large dam is defined as any dam above 15 metres in height (measured from the lowest point of foundation to top of dam) or any dam between 10 and 15 metres in height which meets at least one of the following conditions: a) the crest length is not less than 500 metres; b) the capacity of the reservoir formed by the dam is not less than one million cubic metres; c) the maximum flood discharge dealt with by the dam is not less than 2000 cubic metres per second; d) the dam had specially difficult foundation problems; e) the dam is of unusual design.
Large dam - New definition (after 2011) A dam with a height of 15 metres or greater from lowest foundation to crest or a dam between 5 metres and 15 metres impounding more than 3 million cubic metres.

Composite dam - A dam consisting of at least two significant parts of different dam types.

Barrage or Gate-structure dam (Am.) - A dam where the gated portion occupies most of the river width and most of the height above the river bed. The term “movable dam” should be avoided.

Sill
a) A submerged structure across a river to control the water level upstream.
b) The crest of a spillway.
c) The horizontal gate seating, made of wood, stone, concrete or metal at the invert of any opening or gap in a structure. Hence the expressions; gate sill, stoplog sill.

Weir
a)    A low dam or wall across a stream to raise the upstream water level. Termed fixed-crest weir when uncontrolled.
b)   A structure built across a steam or channel for the purpose of measuring flow. Sometimes described as Measuring weir or Gauging weir. Types of weir include Broad-crested weir, Sharp-crested weir, Drowned weir or Submerged weir.

Embankment - Fill material, usually earth or rock, placed with sloping sides and with a length greater than its height. An “embankment” is generally high­er than a “dike”.

Dike or Dyke or Levee - A long, low embankment. The height is usually less than four to five metres and the length more than ten or fifteen times the maximum height. Usually applied to dams built to protect land from flooding and in this case sometimes referred to as Flood hank. In India and the Far East, the term Bund is used. If built of concrete or masonry, the structure is usually referred to as a Flood wall.
In the Mississipi basin, where the old French word “levee” has survived, this now applies to flood embankments whose height can average up to 10 to 15 metres.

No English equivalent - In French, “levee means flood embankment less than one metre in height. “Petite digue” is now in more common use. In French therefore levee has a narrower meaning than in English, specially in the Mississipi basin (see also levee).

Regulating dam - A dam impounding a reservoir from which water is released to regulate the flow in a river.

Afterbay dam or Re-regulating dam - A dam constructed to regulate the discharges from an upstream power plant.

[Generally in English the purpose of the reservoir is described rather than the purpose of the dam; “ Regulating dam ” is used but “ Water supply dam ” is not We therefore have to say “ Dam for water supply In French, Portuguese, Spanish, American and to a lesser extent in Italian and German the purpose is described for both reservoir and dam]

Tailings dam - An embankment dam, usually built in stages, to create storage for the disposal of waste products from industrial or mining operations. The waste products are conveyed as fine material suspended in water to the settling lagoon formed by the embankment. The embankment may be built of conventional materials but sometimes incorporates suitable waste products.

Cofferdam - A temporary structure enclosing all or part of the construction area so that construction can proceed in the dry. A diversion cofferdam diverts a river into a pipe, channel or tunnel.

Gravity dam (PG) - A dam constructed of concrete and/or masonry which relies on its weight for stability.

Buttress dam (CB) - A dam consisting of a watertight part supported at intervals on the downstream side by a series of buttresses. Buttress dam can take many forms.

Arch dam (VA) - A concrete or masonry dam which is curved in plan so as to transmit the major part of the water load to the abutments.

Embankment dam or Fill dam - Any dam constructed of excavated natural materials or of waste materials.

Masonry dam (PG) - Any dam constructed mainly of stone or concrete blocks jointed with mortar. A dam having only a masonry facing should not be referred to as masonry dam.

Rubble dam (PG) - A masonry dam in which the stones are unshaped or uncoursed.

Crib dam (PG) - A gravity dam built up of boxes, cribs, crossed timbers or gabions, filled with earth or rock.

Gabion dam (PG) - Special name given to a crib dam when built up of gabions.

Precast dam - A dam constructed mainly of large precast concrete blocks or sections.

Prestressed dam - A dam the stability of which depends in part on the tension in steel wires, cables or lods that pass through the dam and are anchored into the foundation rock.

Overflow dam or Overtoppable dam or Spill dam - A dam designed to be overtopped.

Non-overflow dam or Non-spill dam - A dam or section of dam which is not designed to be overtopped.

Name of dam - The official name given to the dam structure. If the name of a dam has been changed or if another name is in current use, the original name or this another name should be included in parenthesis or stated in a foot note.

Year of completion - In the World Register of Dams, the year in which construction of the dam was completed and ready for use.

Location of dam - The situation of a dam should be described by the name of the river, stream or tributary across which the dam is built; by the name of the nearest town or city which can be located on a general map of the country; and by the state, country, or department defined on a general map. In the absence of these details, the geographical position of the dam within the country should be given, e.g. N, S. NW, SE, etc.

Height above lowest foundation of dam (Fig. 1, Fig. 1a) - The maximum height from the lowest point of the general foundation to the top of dam.

Height above ground level - The maximum height from natural ground surface to top of dam.

Top of dam or Crest of dam - The elevation of the uppermost surface of a dam, usually a road or walkway excluding any parapet wall, railings, etc.

Crest length (Fig. 2) - The developed length of the top of the dam. This includes the length of spillway, powerhouse, navigation lock, fish pass, etc. where these form part of the length of the dam. If detached from the dam these structures should not be included.

Thickness (*) or Width (** ) - The thickness or width of a dam as measured horizontally between the upstream and downstream faces and normal to the axis of the dam.
** In general, for gravity or arch dams.
** In general, for other dams.

Heel of dam - The junction of the upstream face of a gravity or an arch dam with the ground surface. In the case of an embankment dam, the junction is referred to as the Upstream toe of dam.

Toe of dam (Fig. 1) - The junction of the downstream face of a dam with the ground surface. Also referred to as Downstream toe.

Base thickness (*) or Base width (**)
- The maximum thickness or width of the dam measured horizontally between upstream and downstream faces and normal to the axis of the dam, but excluding projections for outlets, etc.
* In general, for gravity or arch dams.
** In general, for other dams.

Top thickness (*) or Top width (**) - The thickness or width of a dam at the level of the top of dam.
* In general, for gravity or arch dams.
** In general, for other dams.

Cross-section at crown - Cross-section at crown of an arch dam which generally corresponds with the point where the height of dam is a maximum.

Maximum cross-section of dam Cross-section of a dam at point where the height of dam is a maximum.

Slope
a)    Side of a hill or a mountain. Where the idea of orientation is intended, the word versant has to be used (geographic term).
b)   The inclined face of a cutting or canal or embankment.
c)    Inclination from the horizontal. Measured as the ratio of the number of units of the vertical distance to the number of corresponding units of the horizontal distance. Used in English for any inclination: Expressed in percent when the slope is gentle; in this case also termed “gradient ’.

Batter - Inclination from the vertical. Not in common use in English; the concept “slope” is preferred.

Volume of dam - The total space occupied by the materials forming the dam structure, computed between abutments and from top to bottom of dam. No deduction is made for small openings such as galleries, adits, tunnels and operating chambers within the dam structure. Portions of powerhouses, locks, spillways, etc., may be included only if they are necessary for the structural stability of the dam.

Drainage gallery - A gallery in a dam, generally parallel to the upstream face, that intercepts leakage from the water face or seepage from drain holes drilled in the foundation.
A drainage gallery may also be located in the foundation or abutment of a dam.

Training wall or Guide wall - A wall built to confine or guide the flow of water over the downstream face of an overflow dam or in a channel.

Axis of dam (Fig. 2) - The plane or curved surface, arbitrarily chosen by a designer, appearing as a line, in plan or in cross-section, to which the horizontal dimensions of the dam can be referred.

Roadway of dam - That part of the top of dam available for use as a road for vehicles.

Guard rail - A railing for the protection of persons.

Parapet wall - A solid wall built along the upstream or downstream edge of the top of dam for ornament or for the safety of vehicles and pedestrians.

Wave wall (Fig. 1) - A solid wall built along the upstream side at the top of a dam and designed to reflect waves.