These are the trade terms from HyFlow's series on measuring canal flow with a fixed Doppler meter, the Lodestar. Each is explained in everyday words.
- Acoustic Doppler current profiler (ADCP)
- In this series, a boat-carried instrument that uses sound to measure water speed at many depths. A specialist crosses the channel with it to measure the whole flow, as a snapshot.
- Acoustic Doppler velocity meter (ADVM)
- A fixed sensor that uses sound to keep recording water speed in one part of a channel. Turning that speed into flow also takes a water-level record and ratings.
- Backwater
- Water backed up from downstream, for example by a gate that holds the canal level up. Where it varies, the same water level can go with high or low flow.
- Beam
- The narrow path of sound a meter sends into the water, like a flashlight beam; the Lodestar has one. A Doppler beam senses only motion along its own line, toward or away from the meter.
- Calibration measurement
- One of the independent flow measurements used to build a site's index rating. Despite the name, it builds the site's chart and does not adjust the sensor.
- Cell
- One short segment of a meter's sound beam, with its own water-speed reading. Echoes from farther away return later, so timing them splits the beam into cells.
- Check measurement (validation measurement)
- A fresh, independent flow measurement that tests whether the site's rating still holds. It is made from time to time, and after anything that may have changed the site.
- Discharge (flow)
- The volume of water passing a fixed line across the channel each second. It equals the water's average speed times the water-filled area of a slice across the channel (its cross-section).
- Doppler shift
- A change in pitch caused by motion, like a siren passing you. A Doppler meter reads it in echoes from particles carried by the water to get water speed along the beam.
- Gaging consultant
- A flow-measurement specialist or firm whose staff, called hydrographers, measure channel flow in the field and build and check ratings. An irrigation district normally hires one to develop its ratings.
- Index mark
- A permanent mark on a sensor's mount that shows whether the sensor has moved or turned. Despite the name, it has nothing to do with index velocity.
- Index rating
- The site's chart that turns index velocity into the average water speed across the whole channel. The canal owner's gaging consultant normally builds it and tests it with periodic check measurements.
- Index velocity
- Water speed a fixed meter reads in one part of the channel; on a single-beam meter, the speed along the beam. An index rating converts it to the channel's average speed.
- Main thread of flow
- The fastest-moving part of the channel, usually over the deepest water. Gates, bends and weeds can push it toward one side.
- Mean channel velocity
- The water's average speed over the whole cross-section, equal to flow divided by water-filled area. No fixed Doppler meter samples the whole cross-section, so an index rating works it out from index velocity.
- Measurement volume
- The stretch of beam whose cells are averaged into the index velocity, set by a rule that stays fixed once a rating uses it. It leaves out water the sensor disturbs and cells spoiled by bed or surface echoes.
- Rating
- A site's chart, not a sensor adjustment, built from field measurements to turn sensor readings into flow. An index rating takes a program of independent flow measurements across the full range of flow, including reverse flow where it occurs, plus periodic check measurements.
- Reverse flow
- Water running backward along a channel, reported by a Doppler meter as a negative speed. A correct flow figure also needs a rating built with measurements made during reverse flow.
- Side lobe
- Weaker sound leaving the transducer off the main beam, like the faint glow around a flashlight's bright center. Its echoes off the bed or water surface can drown out the water's fainter echoes in some cells.
- Side-looking meter
- A fixed meter mounted on a bank or wall. It sends its sound roughly level across the channel.
- Signal strength
- How loud the returning echoes are in each cell, normally fading smoothly with distance. A change from the site's usual pattern can point to silt, weed, a badly shifted mount, or clearer or muddier water.
- Stage (water level)
- The height of the water surface above a fixed reference level (the site's datum), not the water's depth. The Lodestar does not measure it, so a station needs a separate water-level sensor.
- Stage-area rating
- The chart that turns water level into the water-filled area of the channel's cross-section. It comes from a survey of the channel's shape, tied to the same reference level (datum) as the water-level sensor.
- Stage-discharge rating
- A chart that turns water level straight into flow, which works where one level always means one flow. Where backwater varies or the flow reverses, it fails, and water speed must be measured too.
- Standard cross section
- A marked line across the channel, at a right angle to the flow and near the instrument, where the channel's shape is surveyed. Marking it lets the same line be surveyed again later.
- Tilt
- The angle at which a sensor sits, which the Lodestar's built-in 3D attitude sensor records. A slight change in tilt is offset by fine-tuning the angle settings; left alone, it would shift a single-beam meter's speed readings though the water had not changed.
- Transducer
- The part of a sensor that sends sound pulses and hears the echoes, a combined speaker and microphone. Its face must stay under water and clear of silt and growth.
- Up-looking meter
- A fixed meter on or near the channel bed that sends its sound slanting up. A meter can also be mounted at the surface looking down, from a fixed structure or a float.
- Velocity profile
- A picture of how water speed varies along a line, such as cell by cell along a meter's beam. For a channel, it usually means how speed changes from bed to surface.
- Vertical
- A line from the water surface to the bed where a hydrographer measures velocity. A discharge measurement samples many verticals across the channel and adds up the slices around them.