Open channel flow is measured by the velocity-area method: survey the cross-section, read velocity at a series of verticals, then multiply velocity by depth by width and add it up. A handheld current meter such as the Swoffer 2100 or 3000 does the velocity part, and it is the right tool for streams, ditches and discharge channels you can wade or reach.
What does an open channel flow meter actually measure?
It measures how fast water moves, how deep it is, or both, and flow is calculated from those numbers. Search results for the phrase are dominated by installed systems: ultrasonic level sensors over a Parshall flume or weir, and submerged area-velocity probes in sewers. A handheld current meter is a different tool for a different job: a spot discharge measurement you carry to the channel.
The U.S. Geological Survey's Techniques and Methods 3-A8 is the standard reference for the manual method. It notes that current meters have traditionally supplied most discharge measurements, and that portable weirs, flumes, floats, volumetric tanks and tracers cover flows too small or too awkward for a meter. In Canada, the national hydrometric network run by Water Survey of Canada operates over 2,800 active gauges and publishes the data many consultants use to sanity-check a spot reading.
How does the velocity-area method work?
You split the channel width into partial sections, measure depth and velocity at each vertical, and add the partial discharges. Discharge for one section is velocity times depth times section width, and total flow is the sum.
How many verticals, and where?
USGS guidance is about 25 to 30 partial sections, spaced so that no section carries more than 10 percent of the total discharge, with about 5 percent as the ideal. Sections should be narrower where depth and velocity are greater, and equal widths are not recommended unless flow is evenly distributed. String the tape at right angles to the flow.
Where in the water column do you read velocity?
USGS describes two standard point methods. The two-point method averages readings at 0.2 and 0.8 of the depth and is preferred where depth allows. The 0.6-depth method uses a single reading at 0.6 of the depth below the surface as the vertical's mean velocity and is used in shallow water or when conditions prevent the two-point method. In USGS Table 6, a Price AA meter uses 0.6 depth between 1.5 and 2.5 ft and two-point at 2.5 ft and deeper, while a pygmy meter uses 0.6 depth between 0.3 and 1.5 ft. Keep the sensor clear of the surface and the bed; for a Price AA, USGS says at least 0.5 ft from each. Check your own sensor's clearance in its manual.
A worked example (ours, arithmetic only)
| Vertical | Depth (m) | Velocity (m/s) | Section width (m) | Partial flow (m³/s) |
|---|---|---|---|---|
| 1 | 0.20 | 0.15 | 1.0 | 0.030 |
| 2 | 0.35 | 0.40 | 1.0 | 0.140 |
| 3 | 0.50 | 0.65 | 1.0 | 0.325 |
| 4 | 0.55 | 0.70 | 1.0 | 0.385 |
| 5 | 0.40 | 0.45 | 1.0 | 0.180 |
| 6 | 0.25 | 0.20 | 1.0 | 0.050 |
The six partial flows add to 1.11 m³/s. This table is invented to show the arithmetic, and it also shows the pitfall: vertical 4 carries about 35 percent of the total, far above the 10 percent ceiling. Real measurements need many more, narrower sections in the fast centre of the channel.
Which Swoffer model fits: the 2100 or the 3000?
Pick the 2100 for spot readings you will record by hand, and the 3000 when you want the meter to store stations and compute discharge. Both pair a handheld indicator with the same type of sensor, and ERE lists the Swoffer 2100 and 3000 series as one product with several wand and wading-rod configurations. Read the Swoffer Model 2100 and 3000 product page for part numbers.
| Feature | Model 2100 | Model 3000 |
|---|---|---|
| Velocity range | 0.1 to 25 ft/s (0.03 to 7.5 m/s) | 0.1 to 25 ft/s (0.03 to 7.5 m/s) |
| Accuracy | Better than 1 percent | Better than 1 percent |
| Sensor | 2 in. fiber-optic propeller | 2 in. fiber-optic propeller |
| Controls | Dial, three preset display update times (90 s maximum) | Keypad, sampling interval 1 to 999 s |
| Data | Read and record by hand | Logs depth, width, velocity, flow angle, time and date; 1,000 stations in up to 100 cross-sections |
| Power | 9 V battery | Four AA batteries |
The 3000 is also designed to run with Price AA and pygmy meters, with or without optic sensors, which matters if your protocol specifies one of those. Rod choice depends on how you reach the water: telescoping wands from 2.5 ft to 19.5 ft, top-set wading rods, or a kit that adapts to your own USGS top-set rod. Browse the full flow meters collection.
When is a handheld current meter the wrong tool?
It is the wrong tool for continuous monitoring and for full pipes. A person must be at the channel for every reading, depth must be wadeable or reachable from a rod, and heavy debris, ice or very low velocity make point readings unreliable. For permanent monitoring, look at level-based systems over a flume or weir, and note that ERE does not list those. For very small flows, USGS points to volumetric, portable flume or weir plate methods instead.
Pair velocity work with level data. Our guides to water level meters and water quality meters cover the other measurements usually taken on the same site visit, and our instrument calibration services guide explains how often field instruments should be verified. The 2100 includes a self-calibration check for use in the field.
Need an open channel flow measurement instrument?
ERE Inc. has supplied environmental field equipment to Canadian consultants for 30+ years. Tell us the channel width, depth and velocity range and we will confirm the right Swoffer configuration.
→ Request a Quote | 1-888-287-EREC | Browse Flow Meters | sales@ereinc.com
Frequently Asked Questions
What is the difference between a current meter and an open channel flow meter?
A current meter measures point velocity and you compute flow from a surveyed cross-section. An installed open channel flow meter usually measures level (and sometimes velocity) continuously and computes flow itself, often over a flume or weir.
Can a Swoffer 2100 or 3000 measure flow in a pipe?
No. The Swoffer 2100 and 3000 are open-stream current velocity meters for streams, channels and ditches. Full pipes need a different class of flow meter.
How many verticals do I need for a stream discharge measurement?
USGS guidance uses about 25 to 30 partial sections, spaced so no section holds more than 10 percent of the total flow. Fewer can work in a smooth channel with an even velocity profile.
Does ERE sell ultrasonic or area-velocity open channel flow meters?
No. ERE lists the Swoffer 2100 and 3000 current velocity meters. For continuous flow monitoring over a flume or weir, ask for a quote and we will tell you plainly whether we can source a suitable instrument.
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Lire en français : Débitmètre à canal ouvert : la méthode vitesse-section avec un courantomètre Swoffer