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Pocket Safety Guide for Dams and Impoundments

Useful Terms

Abutment—That part of the valley side against which the 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. The left and right abutments of dams are defined with the observer viewing the dam looking in the downstream direction, unless otherwise indicated.

Appurtenant structure—Ancillary features of a dam, such as outlets, spillways, powerplants, tunnels, etc.

Berm—A nearly horizontal step in the sloping profile of an embankment dam. Also a step in a rock or earth cut.

Breach—An opening through a dam that allows the uncontrolled draining of a reservoir. A controlled breach is a constructed opening. An uncontrolled breach is an unintentional opening caused by discharge from the reservoir. A breach is generally associated with the partial or total failure of the dam.

Channel—A general term for any natural or artificial facility for conveying water.

Conduit—A closed channel to convey water through, around, or under a dam.

Core wall—A wall built of relatively impervious material, usually of concrete or asphaltic concrete, in the body of an embankment dam to prevent seepage.

Crest length—The measured length of the dam along the crest or top of dam.

Crest of dam—See "Top of dam."

Cross section—An elevation view of a dam formed by passing a plane through the dam perpendicular to the axis.

Cutoff wall—A wall of impervious material usually of concrete, asphaltic concrete, or steel sheet piling constructed in the foundation and abutments to reduce seepage beneath and adjacent to the dam.

Dam—Any artificial barrier, including appurtenant works, that impounds or diverts water either temporarily or long term.

Dam failure—Catastrophic type of failure characterized by the sudden, rapid, and uncontrolled release of impounded water or the likelihood of 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.

Drain, toe—A system of pipe and/or pervious material along the downstream toe of a dam used to collect seepage from the foundation and embankment and convey it to a free outlet.

Drainage area—The area that drains to a particular point on a river or stream.

Embankment dam—Any dam constructed of excavated natural materials, such as both earthfill and rockfill dams, or of industrial waste materials, such as a tailings dam.

Face—The external surface of a structure (e.g., the surface of a wall of a dam).

Filter (filter zone)—One or more layers of granular material graded (either naturally or by selection) to allow seepage through or within the layers while preventing the migration of material from adjacent zones.

Flashboards—Structural members of timber, concrete, or steel placed in channels or on the crest of a spillway to raise the reservoir water level but intended to be quickly removed, tripped, or fail in the event of a flood.

Flood—A temporary rise in water surface elevation resulting in inundation of areas not normally covered by water. Hypothetical floods may be expressed in terms of average probability of exceedance per year, such as one-percent-chance-flood, or expressed as a fraction of the probable maximum flood or other reference flood.

Foundation—The portion of the valley floor that underlies and supports the dam structure.

Freeboard—Vertical distance between a specified stillwater (or other) reservoir surface elevation and the top of dam, without camber.

Gate—A movable water barrier for the control of water.

Crest gate (spillway gate)—A gate on the crest of a spillway to control the discharge or reservoir water level.

Outlet gate—A gate controlling the flow of water through a reservoir outlet.

Height, hydraulic—The vertical difference between the maximum design water level and the lowest point in the original streambed.

Height, structural—The vertical distance between the lowest point of the excavated foundation to the top of the dam.

Intake—Placed at the beginning of an outlet-works waterway (power conduit, water supply conduit), the intake establishes the ultimate drawdown level of the reservoir by the position and size of its opening(s) to the outlet works. The intake may be vertical or inclined towers; drop inlets; or submerged, box-shaped structures. Intake elevations are determined by the head needed for discharge capacity, storage reservation to allow for siltation, the required amount and rate of withdrawal, and the desired extreme drawdown level.

Leakage—Uncontrolled loss of water by flow through a hole or crack.

Length of dam—The length along the top of the dam. This also includes the spillway, powerplant, 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.

Outlet—An opening through which water can be freely discharged from a reservoir to the river for a particular purpose.

Phreatic surface—The free surface of water seeping at atmospheric pressure through soil or rock.

Piping—The progressive development of internal erosion by seepage.

Reservoir—A body of water impounded by a dam and in which water can be stored.

Reservoir surface area—The area covered by a reservoir when filled to a specified level.

Riprap—A layer of large uncoursed stone, precast blocks, bags of cement, or other suitable material, generally placed on the slope of an embankment or along a watercourse as protection against wave action, erosion, or scour. Riprap is usually placed by dumping or other mechanical methods, and in some cases is hand placed. It consists of pieces of relatively large size, as distinguished from a gravel blanket.

Seepage—The internal movement of water that may take place through the dam, the foundation, or the abutments.

Slope—Inclination from the horizontal. Sometimes referred to as batter when measured from vertical.

Slope protection—The protection of a slope against wave action or erosion. See "Riprap."

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.

Spillway channel—An open channel or closed conduit conveying water from the spillway inlet downstream.

Spillway chute—A steeply sloping spillway channel that conveys discharges at super-critical velocities.

Spillway crest—The lowest level at which water can flow over or through the spillway.

Storage—The retention of water or delay of runoff either by planned operation, as in a reservoir, or by temporary filling of overflow areas, as in the progression of a flood wave through a natural stream channel.

Toe of the dam—The junction of the downstream slope or face of a dam with the ground surface; also referred to as the downstream toe. The junction of the upstream slope with ground surface is called the heal or the upstream toe.

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

Trashrack—A device located at an intake to prevent floating or submerged debris from entering the intake.

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 the flow.