What a broad-crested weir does
A broad-crested weir creates a known hydraulic control so flow can be related to water level. As water passes over the crest, the depth over the crest, called head, becomes the key measurement. With a suitable coefficient and geometry, head can be converted into discharge.
The method is useful because water level is easier to measure continuously than velocity across a channel. However, the accuracy depends on installation details: crest width, approach conditions, submergence, sediment, vegetation, side contractions, and whether flow is truly free over the control.
- Irrigation districts use weirs for turnout and canal measurement.
- Stormwater teams use them for monitoring basins and channels.
- Environmental staff use rating tables for periodic flow estimates.
How to calculate weir flow
A common simplified equation is: Q = C x L x H^1.5. Q is discharge, C is a coefficient that must match the units and weir geometry, L is crest length, and H is upstream head over the crest. Some standards express the equation with discharge coefficients and gravity terms instead of a single combined C.
Example in SI-style units: use C = 1.70 for the chosen installation, crest length L = 2.0 m, and head H = 0.30 m. H^1.5 = 0.30^1.5 = about 0.164. Q = 1.70 x 2.0 x 0.164 = 0.56 m3/s. A calibrated coefficient or site rating table should replace a generic value for reporting-grade work.
How to measure head correctly
Head should be measured upstream far enough from the crest to avoid drawdown at the control, but close enough to represent the approach water level. Measure from the crest elevation to the water surface. A staff gauge, stilling well, pressure transducer, or ultrasonic sensor can be used if the reference elevation is stable.
Do not measure head on the crest or downstream of the crest. Those locations are affected by acceleration, turbulence, and tailwater. If downstream water rises high enough to submerge the control, the free-flow equation may overstate discharge unless a submergence correction or separate rating is used.
- Survey the crest elevation before using the rating.
- Keep the crest clean and level.
- Record units and coefficient source with each calculation.
Common field mistakes
A coefficient copied from another site can be the largest source of error. Broad-crested weirs are sensitive to geometry and approach conditions, so a coefficient that works for one structure may not be valid for another. Field verification with measured flow improves confidence.
Another mistake is building a rating table beyond the range of observed or validated heads. Extrapolated high-flow values can be misleading, especially when the channel overtops, debris accumulates, or tailwater changes the flow regime.
Frequently asked questions
What is head over a weir?
Head is the vertical distance from the weir crest elevation to the upstream water surface used for the rating calculation.
Can I use the same coefficient for every broad-crested weir?
No. The coefficient depends on units, crest geometry, approach flow, contraction, and calibration. Use a site-appropriate source when accuracy matters.
What happens if the weir is submerged?
Tailwater can reduce the free discharge over the crest. A free-flow equation may need a correction or a different rating curve.
Why make a rating table?
A rating table lets field staff convert observed head to flow quickly and consistently without repeating the full calculation each time.