Dam Safety - Glossary of Terms


ABUTMENT - The part of a valley side (wall) against which a dam is constructed. An artificial abutment is sometimes constructed as a concrete gravity section, to take the thrust of an arch dam where there is no suitable natural abutment. Right and left abutments are those on respective sides of an observer looking downstream.

AIR VENT - A pipe designed to provide air to the outlet conduit to reduce turbulence and prevent negative pressures during release of water. Extra air is usually necessary downstream of constrictions.

ANTI-SEEP COLLAR - A projecting collar of steel or concrete built around the outside of a pipe, tunnel, or conduit under or through an embankment dam to lengthen the seep path along the outer surface of the conduit. This method of design is considered outdated and largely ineffective.

APPURTENANT STRUCTURES - Auxiliary features of a dam such as outlet, spillway, powerhouse, tunnel, etc.

ARCH DAM - A concrete or masonry dam that is curved to transmit a major part of the thrust due to water pressure to the abutments.

AUXILIARY SPILLWAY - See "Spillway .

AXIS OF DAM - The horizontal centerline of a dam in the longitudinal direction.

BERM - A horizontal step or bench in the upstream or downstream face of an embankment dam.

BLANKET -

Drainage Blanket - A drainage layer placed directly over the foundation material

Grout Blanket - See Consolidation Grouting.

Upstream Blanket - An impervious layer placed on the reservoir floor upstream of a dam. In the case of an embankment dam, the blanket may be connected to the impermeable element in the dam.

BUTTRESS - See "Berm".

BUTTRESS DAM - A dam consisting of a watertight upstream face supported at intervals on the downstream side by a series of buttresses.

CHIMNEY DRAIN - A vertical structure of free-draining material (usually coarse sand or gravel) within an embankment placed to intercept and control seep water. See also, "Filter".

COFFER DAM - A temporary structure enclosing all or part of a construction area so the construction can proceed in a dry area. A "diversion coffer dam" diverts a river into a pipe, channel, or tunnel.

CONDUIT - A closed channel for conveying discharge through, under or around a dam.

CONSOLIDATION GROUTING (Blanket Grouting) - The injection of grout to consolidate a layer of the foundation, resulting in greater impermeability and/or strength.

CONSTRUCTION JOINT - The interface between two successive placing or pours of concrete where a bond, not permanent separation, is intended.

CORE WALL - A wall built of impervious material, usually steel, concrete or asphaltic concrete, in the body of an embankment dam to prevent leakage.

CREST LENGTH - The length of the top of a dam from left abutment to right abutment, including the length of spillway, powerhouse, navigation lock, fish pass, etc. where these appurtenances form a structural part of the dam. If detached from the dam, these structures are not included.

CREST OF DAM - The elevation of the uppermost surface of a dam excluding any parapet wall, railings, etc. In international usage, it refers to the crown of an overflow section of a dam.

CREST WIDTH - The width or thickness of a dam at the level of the crest of the dam. In general, the term "thickness" is used for gravity and arch dams and "width" is used for other dams.

CRIB DAM - A gravity dam built up of boxes, cribs, crossed timbers, or gabions and filled with earth or rock.

CUTOFF - An impervious construction or material that reduces seep rate or prevents water from passing through foundation material.

CUTOFF TRENCH - An excavation later to be filled with impervious material to form a cutoff. Sometimes used incorrectly to describe the cutoff itself.

CUTOFF WALL - A wall of impervious material (e.g., concrete, asphalt concrete, steel sheet, piling, etc.) built into the foundation to reduce the seep rate under the dam.

DAM - A barrier built, usually across a watercourse, for impounding or diverting the flow of water.

Embankment Dam (Fill Dam) - Any dam constructed of excavated natural materials or industrial waste materials.

Rolled Earth Dam (Earth Fill or Earth Dam) - An embankment dam in which more than 50% of the total volume is formed of compacted fine-grained material obtained from a borrow area.

Homogeneous Rolled Earth Dam - An embankment dam constructed of similar earth material throughout, except for possible inclusion of internal drains or drainage blankets; distinguished from a zoned rolled earth dam.

Hydraulic Fill Dam - An embankment dam constructed of material, often dredged, which is conveyed and placed by suspension in flowing water.

Rock Fill Dam - An embankment dam in which more than 50% fo the total volume comprises compacted or dumped pervious natural or crushed rock.

Zoned Embankment Dam - An embankment dam, which is composed of zones of selected materials having different degrees of porosity, permeability, and density.

DESIGN FLOOD -The largest flood that a given project is designed to safely accommodate. The reservoir inflow hydrograph used to estimate the spillway discharge capacity requirements and corresponding maximum surcharge elevation in the reservoir.

DRAINAGE AREA - An area that drains naturally to a particular point on a stream.

DRAIN LAYER - A layer of permeable material in a dam to relieve pore pressure or to facilitate draining fill. See also, "Blanket"

DRAIN - A vertical well or borehole, usually downstream of impervious cores, grout curtains, or cutoffs, designed to collect and direct seepate through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a drainage curtain. May also refer to a small valve and conduit that is used to evacuate an outlet pipe, stilling basin, or reservoir pool. See also, "Relief Well".

DRAWDOWN - The lowering of a reservoir's water surface elevation due to release of water.

EARTH DAM OR EARTHFILL DAM - See "Dam".

EMBANKMENT - Artificial hill or ridge constructed of fill material, usually earth or rock, placed with sloping sides and usually with a length greater than its height.

EMERGENCY ACTION PLAN - A predetermined plan of action to be taken to reduce the potential for property damage and loss of lives in an area affected by a dam break or excessive spillway discharges.

EMERGENCY SPILLWAY - See "Spillway".

FACE - The external surface of a structure, such as the surface of a dam or appurtenance.

FILTER (Filter Zone) - A band or zone of granular material that is incorporated into a dam and is graded (either naturally or by selection) so as to allow seep water to flow through the filter to a drain layer without allowing the migration of fine material from the protected zone to the drain layer.

FLASHBOARDS - A length of timber, concrete, or steel placed on the crest of a spillway to raise the retention water level but which may be removed in the event of a flood by manual retrieval, a tripping device or by deliberately designed failure of the flashboard or its supports.

FLOOD PLAIN - An area adjoining a body of water or natural stream that has been or may be covered by floodwater.

FLOOD ROUTING - The determination of the elevation of a given flood at discrete points along a watercourse taking into account obstructions and the attenuating effect of storage on the flood as it passes through a valley, channel or reservoir. Usually expressed as a series of hydrographs.

FOUNDATION OF DAM - The material on which the dam structure is placed. Often modified to provide more favorable hydraulic characteristics. See "Cutoff".

FREEBOARD - The vertical distance between a stated reservoir elevation and the crest of the dam. "Net freeboard", "dry freeboard", "flood freeboard", or "residual freeboard" are expressions for the vertical distance between the estimated maximum water elevation and the crest of the dam. "Gross freeboard" or "total freeboard" expressions for the vertical distance between the maximum normal water elevation and the crest of the dam.

GALLERY - (a) A passageway within the body of a dam or abutment; hence the terms "grouting gallery", "inspection gallery", and "drainage gallery". (b) A long and rather narrow hall; hence the following terms for a power plant: "valve gallery", "transformer gallery", and "busbar gallery

GATE - A device in which a leaf or member is moved across the waterway from an external position to control or stop flow.

Bulkhead Gate - A gate used either to temporary close a channel or conduit in order to empty it for inspection or for closure against flowing water when the head difference is small, e.g., a diversion tunnel closure. Although a bulkhead gate is usually opened and closed under nearly balanced pressures, it nevertheless may be capable of withstanding a large pressure differential when in the closed position.

Crest Gate (Spillway Gate) - A gate on the crest of a spillway to control overflow or reservoir water level.

Emergency Gate - A standby or reserve gate used only when the normal means of water control is not available.

Fixed Wheel Gate (Fixed Roller Gate, Fixed Axle Gate) - A gate having wheels or rollers mounted on the end posts of the gate. The wheels bear against rails fixed in side grooves or gate guides.

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 due to their shape in cross section.

Flood Gate - A gate to control flood release from a reservoir.

Outlet Gate - A gate controlling the outflow of water from a reservoir.

Pneumatic Gate (Rubber Gate or Bladder Gate) - A tubular flexible gate anchored to a foundation or inside a pipe that is operated by filling with air under pressure. Often used in conjuction with a Flap Gate as a Crest Gate or Flood Gate.

Radial Gate (Tainter Gate) - A gate with a curved upstream plate and radial arms hinged to piers or other supporting structures on which the gate pivots.

Regulating Gate (Regulating Valve) - A gate or valve that operates under full pressure and flow conditions to throttle and vary the rate of discharge.

Slide Gate (Sluice Gate) - A gate that can be opened or closed by sliding it in supporting guides.

GRAVITY DAM - A dam constructed of concrete and/or masonry that relies on its mass (weight) for stability.

GROUT CURTAIN (Grout Cutoff) - A barrier produced by injecting grout into a vertical zone in the foundation parallel to the dam centerline to reduce seep rates under a dam.

HEEL OF DAM - The junction of the upstream face of a dam with the ground surface. See also, "Toe of Dam".

HYDROGRAPH - A graphic representation of discharge, stage or other hydraulic property with respect to time at a discrete point on a stream.

INCLINOMETER - An instrument, usually consisting of a metal or plastic tube, inserted in a borehole and a sensitized monitor either lowered into the tube or fixed within the tube. This measures at different points the inclination of the tube from the vertical. The lateral position at different levels of the tube may be found by integration relative to an assumed fixed point, usually the top or bottom of the tube. The system may be used to measure settlement.

INTERNAL EROSION - See Piping.

INUNDATION AREA - Area downstream from a dam that would be subject to flooding in the event of a dam failure or large spillway release. See also, "Flood Plain".

INUNDATION MAP - A map delineating the area of inundation.

LEAK - Uncontrolled loss of water by flow through a hole or crack.

LEFT ABUTMENT - See Abutment.

LEVEE (Dike) - A long, low embankment usually built along a watercourse to protect land from flooding. If built of concrete or masonry the structure is usually referred to as a floodwall. The term "dike" is commonly used to describe embankments that block an area on a reservoir rim that are lower than the top of the main dam. See also, "Secondary Embankment".

LINING - With reference to a canal, tunnel, shaft or reservoir, a coating of asphaltic concrete, reinforced or unreinforced concrete, shotcrete, rubber or plastic to provide watertightness, prevent erosion, reduce friction, or support the periphery of the structure. May also refer to the lining, such as steel or concrete, of an outlet pipe or conduit.

LOW-LEVEL OUTLET (Drain, Bottom Outlet or Sluiceway) - An opening at a low level from a reservoir generally used for emptying or for scouring sediment and sometimes for irrigation releases.

MASONRY - Stone, brick or concrete blocks that may or may not be joined with mortar. A dam having only a masonry facing should not be referred to as a masonry dam.

NORMAL WATER LEVEL (Normal Pool Level) - For a reservoir with a fixed overflow, the lowest crest level of that overflow. For a reservoir whose outflow is controlled wholly or partially by movable gates, siphons or other means, it is the maximum level to which water may rise under normal operating conditions, exclusive of any provision for flood surcharge.

OUTLET - An opening through which water can be freely discharged from a reservoir.

PARAPET WALL - A solid wall built along the top of a dam for ornament, for the safety of vehicles and pedestrians, or to prevent overtopping.

PERVIOUS ZONE - An area within an embankment dam comprising material of high permeability.

PHREATIC SURFACE - The free surface of ground water at atmospheric pressure.

PIEZOMETER - An instrument for measuring pore water pressure within soil, rock or concrete.

PIPING - The progressive development of internal erosion by seep water, appearing downstream as a hole or seam discharging water that contains soil particles.

PORE PRESSURE - The interstitial pressure of water within a mass of soil, rock or concrete.

PRESSURE RELIEF PIPES - Pipes used to relieve uplift or pore pressure in the dam structure such as from beneath a large expanse of concrete.

RELIEF WELL - See "Drain".

RESERVOIR AREA - The surface area of a reservoir when filled to normal water level.

RESERVOIR SURFACE - The surface of a reservoir at any level.

RIGHT ABUTMENT - See "Abutment".

RIP RAP - A layer of stones, broken rock or precast blocks placed in random fashion on the upstream slope of an embankment dam, on a reservoir shore or on the sides of a channel as a protection against waves, ice action and flowing water. Very large rip rap is sometimes referred to as armoring.

ROCKFILL DAM - See "Embankment Dam".

ROLLED EARTH DAM - See "Embankment Dam".

SECONDARY EMBANKMENT - Embankment that blocks an area on a reservoiur rim that is lower than the top of the main dam. See also, "Levee".

SEEP - The interstitial movement of water that may take place through a dam, its foundation, or its abutments.

SLOPE - The side of a hill or mountain, the inclined face of a cutting, canal or embankment or other inclination from the horizontal. In the United States, it is measured as the ratio of the number of units of horizontal distance to the number of corresponding units of vertical distance. The term is expressed as a percent when the slope is gentle, in which case the term "gradient" is also used.

SLOPE PROTECTION - The protection of a slope against wave action or erosion. See also, "Rip Rap".

SLUICEWAY - See "Low Level Outlet".

SPILLWAY - A structure over or through which flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway; if the elevation of the spillway crest is the only control, it is an uncontrolled spillway. * Auxiliary Spillway (Emergency Spillway) - A secondary spillway designed to operate only during exceptionally large floods. * Fuse Plug Spillway - An auxiliary or emergency spillway comprising a low embankment or a natural saddle designed to be overtopped and eroded away during flood flows. * Primary Spillway (Principal Spillway) - The principal or first-used spillway during normal inflow and flood flows. * Shaft Spillway(Morning glory Spillway) - A vertical or inclined shaft into which flood water spills and then is conducted through, under, or around a dam by means of a conduit or tunnel. If the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a "bellmouth" or "morning glory" spillway. * Side Channel Spillway - A spillway whose crest is roughly parallel to the channel immediately downstream of the spillway. * Siphon Spillway - A spillway with one or more siphons built at or near the crest of the dam. This type of spillway is sometimes used for providing automatic surface-level regulation within narrow limits or when considerable discharge capacity is necessary within a short period of time.

STILLING BASIN - A basin constructed to dissipate the energy of fast-flowing water, e.g., from a spillway or outlet, to prevent undercutting of the dam and to protect the streambed from erosion.

STOPLOGS - Large logs or timber or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit providing a temporary closure versus a permanent bulkhead gate.

TOE OF DAM - The junction of the downstream face with the ground surface. See also, "Heel of Dam".

TOP OF DAM - See "Crest of Dam".

TOP THICKNESS - See "Crest Width".

TRASH RACK - A screen located at an intake to prevent the ingress of debris.

UPLIFT - The upward pressure in the pores of material (interstitial pressure) or on the base of a structure.

UPSTREAM BLANKET - See "Blanket".

VALVE - A device fitted to a pipeline or orifice in which the closure member is either rotated or moved transversely or longitudinally in the waterway to control or stop flow.

WATERSTOP - A strip of metal, rubber or other material used to prevent leakage through joints between adjacent sections of concrete.

WEIR - 1. A low dam or wall built across a stream to raise the upstream water level, termed fixed-crest weir when uncontrolled. 2. A structure built across a stream or channel for the purpose of measuring flow, sometimes called a measuring weir or gauging weir. Types of weirs include broad-crested, sharp-crested, drowned, and submerged.