The following glossary of selected hydrometric terms makes use of ISO Standards to which the reader is referred. All terms related to hydrometric operations, river hydraulics and stream sedimentation are defined under ISO Standards (ISO T72: 1988(E)). In addition a Glossary of Aquatic terms, including Hydrometric references is published by the Resources Inventory Committee.
Air line correction: The correction to the sounding line measurement corresponding to that part of the sounding line above the liquid surface.
Approach channel: The reach of the channel upstream of the gauging structure in which suitable flow conditions shall be established to ensure correct gauging.
Backwater: A rise in stage produced by an obstruction in the stream channel caused by ice, weeds, control structure, etc. It may be caused by channel storage for which the reservoir properties vary with the depth of flow at the given location. The difference between the observed stage for a certain discharge and the stage as indicated by the stage-discharge relation for the same discharge is reported as the backwater at the station.
Bank, right or left: The margin of a channel as viewed facing downstream. The expression "right" or "left" applies similarly to right or left abutments, cableway towers, etc.
Bench mark: A permanent, fixed reference point for which the elevation is known. It may when practicable, be related to GSC datum.
Broad-crested weir: A weir of such crest length in the direction of flow that critical flow occurs on the crest of the weir.
Control: The condition downstream from a gauging station that determines the stage discharge relation. It may be a stretch of rapids, a weir or other artificial structure. In the absence of such features, the control may be a less obvious condition such as a convergence of the channel or even simply the resistance to flow through a downstream reach. A shifting control exists where the stage-discharge relation tends to change because of impermanent bed or banks.
Crest stage gauge; A gauge, usually vertical, used to indicate a peak stage that has occurred since the previous setting.
Critical flow: The flow in which specific energy (depth of flow + velocity head) is a minimum for a given discharge; under this condition a small surface disturbance can not travel upstream. The ratio of inertia to gravity forces (Froude Number) is equal to unity.
Critical-depth flume: A Venturi flume in which the flow changes from sub-critical upstream to super-critical downstream in which the measurement of one water level, the upstream water level, allows a calculation of the discharge.
Cross section of a stream: A specified vertical plane through a stream bounded by the wetted perimeter and the free surface.
Discharge coefficient: A coefficient in the discharge equation, in general relating the actual discharge to a theoretical discharge.
Discharge, Q: The volume of liquid flowing through a cross section per unit of time. It is not synonymous with "flow".
Discharge measurement: The determination of the rate of discharge at a gauging station on a stream, including an observation of `no flow', which is classed as a discharge measurement.
Double-drum winch: A winch with two drums, one of which controls and measures the vertical displacement of hydrometric instruments and the other of which controls and measures the horizontal displacement of an unmanned cableway carriage.
Draw-down curve: The profile of the liquid surface when its surface slope exceeds the bed slope.
Drowned flow; submerged flow: The flow which is influenced by the water level downstream of the measuring structure.
Flat-V weir: A long-base weir with a triangular longitudinal profile. The height of the triangle increases linearly from the middle of the channel to the abutment of the weir.
Float gauge: A manual gauge consisting of a float that rides on the water surface, rising and falling with the surface. The float's movements are transmitted to an indicating device.
Flood mark: A trace of any kind left by a flood on the banks, obstacles or flood plain. It may be used to determine the highest level attained by the water surface during the flood.
Flow: The movement of water in a channel without reference to rate, depth, etc.
Flume: A specially shaped open channel flow section that may be installed in a channel to measure discharge. Depending on the shape of the section, flumes may be termed Parshall, Montana H-flumes, cut-throat, etc.
Free flow; modular flow: A flow which is not influenced by the level of water downstream of the measuring device.
Gauge correction: Any correction that must be applied to the gauge observation or gauge reading to obtain the correct gauge height.
Gauge datum: The elevation of the zero of the gauge (referenced to bench marks, or GSC datum) to which the level of the liquid surface is related.
Gauge height: The height of the water surface above the "Gauge datum"; it is used interchangeably with the terms "stage" and "water level".
Gauge observation; Gauge reading: An actual notation of the height of the water surface as indicated by a gauge, it is the same as a "gauge height" only when the 0.000 metre mark of the gauge is set at the "gauge datum".
Gauging section; measuring section: The cross section of an open channel in the plane of which measurements of depth and velocity are made.
Gauging station: The complete installation at a measuring site where systematic records of water level and/or discharge are obtained.
Head on (or over) the weir: Elevation of the water above the lowest point of the crest, measured at a point upstream. The distance upstream for the point of measurement depends on the type of weir used but is upstream of the transition zone from sub- to supercritical flow at full weir flow.
Hydraulic jump: The sudden passage of water in an open channel from super-critical depth to sub-critical depth, accompanied by energy dissipation.
Hydraulic mean depth; hydraulic radius: The quotient of the wetted cross sectional area and the wetted perimeter.
Ice, Anchor: One of three types of ice formed in streams. This ice forms on the streambed and results from loss of heat by radiation from the streambed to the atmosphere.
Ice, Frazil: One of three types of ice formed in streams. This consists of fine needles of ice which do not unite to form surface ice. Usually these needles float downstream and come together in a slushy mass.
Ice, Surface: One of three types of ice formed in streams. It is formed on the surface either as a fringe of shore ice or as a continuous ice cover from bank to bank.
Inclined gauge; ramp gauge: A gauge on a slope, generally graduated directly to indicate vertical gauge height.
Left [right] bank: The bank to the left [right) of an observer looking downstream.
Level check: The procedure followed to determine the movement of a gauge with respect to the gauge datum.
Manual gauge: A non-recording type of gauge from which observations of stage are obtained.
Mean velocity at a cross section: The velocity at a given cross section of a stream, obtained by dividing the discharge by the cross sectional area of the stream at that section.
Mean velocity depth: The depth below the surface at which the mean velocity on a vertical occurs.
Modular limit; point of incipient submergence: The condition of flow where a rising downstream level just begins to affect the discharge.
Painting: This refers to the wide ink trace on water level recorder analogue charts that is caused by short term water level fluctuations or by a malfunction in a recorder having a gas purge system.
Panel: The area at a vertical defined by the depth at that vertical multiplied by one-half of the distance between the preceding and the succeeding verticals.
Peak stage: The maximum instantaneous stage during a given period.
Point method (one-; two-; three-; five-; six-): Method of measuring the velocity in a vertical by placing a current-meter at a number of designated points in the vertical.
Open channel: The longitudinal boundary surface consisting of the bed and banks or sides within which the liquid flows with a free surface. The term "channel" generally means the deep part of a river or other waterway, and its meaning is normally made clear by a descriptive term, either stated or implied, such as "low water" channel, "main" channel, "artificial" channel.
Reach: A length of open channel between two defined cross sections.
Reference current-meter: A current-meter which is immersed at a fixed position in the cross section during the carrying out of a discharge measurement. For slight changes in discharge during the gauging operation, it is assumed that the change in velocity indicated by the reference current-meter is proportional to the change in discharge.
Reference point: A point of known elevation from which measurements may be made to a water surface. It is also known as a "measuring point".
Sensitivity (of the stage-discharge relation): A measure of the change in stage at a gauging station due to a change in discharge. When a small increase in discharge produces a relatively large increase in stage, the relation is said to be sensitive. When a large increase in discharge produces a relatively small increase in stage, the relation is said to be insensitive.
Shift: A change in the stream control which alters the stage-discharge relationship. This change can be either temporary or permanent.
Slope-area measurement: A method of computing peak flow at a gauging station by determining the water surface profiles and channel dimensions over a short reach of a stream.
Slope-area method: An indirect method of discharge estimation in a reach based on the surface slope, the reach roughness, the wetted perimeter and flow areas of the various cross sections in the reach.
Sounding: The operation of measuring the depth from the free surface to the bed.
Stable [unstable] channel: A channel in which the bed and the sides remain stable unstable] over a substantial period of time and in which scour and deposition during the rising and falling stages are negligible [appreciable].
Staff gauge: A manual gauge consisting of a graduated plate or rod that is set vertically in streambed or attached to a solid structure.
Stage; gauge height; water level: The elevation of the free surface of a stream, lake or reservoir relative to a gauge-datum.
Stage: A general term used to describe the height of a water surface and, in a particular application, may be either a gauge height or a water elevation.
Stage-discharge relation: A curve, equation or table which expresses the relation between the stage and the discharge in an open channel at a given stream cross section.
Steady [unsteady] flow: Condition in which the discharge does not change [changes] in magnitude with respect to time.
Stilling basin: A pool downstream of a structure in which the velocity and the energy of the flow are reduced.
Stilling well: A well [tube] connected with the stream in such a way as to permit the measurement of the stage in relatively still conditions (natural surging dampened).
Stilling-well lag: During conditions of rising and falling stage in a channel, the difference at a given time between the channel stage and the stilling-well stage.
Stream: The generic term for water flowing in an open channel, e.g., including creeks and rivers.
Stream gauging; All of the operations necessary for measuring discharge.
Sub-critical flow: The flow in which the Froude number is less than unity and surface disturbances can travel upstream.
Submergence ratio: The ratio of the downstream measured head to the upstream total head over a weir, the crest being taken as the datum.
Sub-surface float: A float with its greatest drag below the surface for measuring sub-surface velocities.
Super-critical flow: The flow in which the Froude number is greater than unity and small surface disturbances can not travel upstream.
Surface draw-down: The local lowering of the water surface in an approach channel caused by the acceleration of the flow passing over an obstacle or through a control.
Surface float: A float with its greatest drag near the surface for measuring surface velocities.
Throat: The minimum cross sectional area within a flume. The throat may be rectangular, trapezoidal, U-shaped or of another specially designed shape.
Triangular-profile weir: A long-base weir having a triangular longitudinal profile.
Uniform flow: Flow in which the depth and velocity remain constant with respect to distance along the channel. Uniform flow is possible only in a channel of constant cross section.
Velocity-area method: Method of discharge determination deduced from the area of the cross section, bounded by the wetted perimeter and the free surface, and the integration of the component velocities in the cross section.
Velocity of approach; approach velocity: The mean velocity in an open channel at a known distance upstream of a measuring section.
Vertical: The vertical line in which velocity measurements or depth measurements are made.
Vertical velocity coefficient: The coefficient applied to a single, or an equivalent single, velocity determination at any depth in a vertical to infer the mean velocity on that vertical.
Wading rod: A light, hand-held, graduated, rigid rod, for sounding the depth and positioning the current meter in order to measure the velocity in shallow streams suitable for wading. It may also be used from boats or ice cover, at shallow depths.
Water level recorder: An instrument that records water levels in an analogue or digital form. The recorder may be actuated by a float or by any one of several other sensor types.
Weir: An overflow structure built across an open channel to measure the discharge in the channel. Depending on the shape of the opening, weirs may be termed rectangular, trapezoidal, triangular, etc.
Wet line correction: The correction to the sounding line measurement corresponding to that part of the sounding line below the liquid surface.
Wetted perimeter, P: The wetted boundary of an open channel at a specified section.
Wire-weight gauge: A gauge consisting essentially of a graduated wire or chain, weighted and lowered to make contact with the surface of the water. Contact with the water surface is determined visually.