MMaker Label Studio
Environmental, EHS & Energy

Broad-Crested Weir Flow Calculator

Estimate open-channel flow over a broad-crested weir using Q = C x L x H^(3/2), with optional approach velocity head correction.

Inputs

Use SI or US customary units consistently. Defaults are C = 1.6 for SI and C = 3.09 for US units.

Q = C x L x H3/2
Typical broad-crested weir starting value: 1.6 in SI units.
Adds velocity head, v²/(2g), to the measured head before applying the broad-crested weir equation.
Enter values to calculate flow and generate the rating table.

Results

Primary and secondary flow units are shown for the selected unit system.

Flow
—m³/s
Secondary flow
—L/s
Effective head
—m
Velocity head
—m
Selected units SI
Coefficient used —
Correction Off

Rating Table

The current head row is highlighted. Table values use the same coefficient and correction setting.

Head H Effective head Flow (m³/s) Flow (L/s)
No rating table yet.

Self-Tests

Runs golden cases against the core Q = C x L x H^(3/2) calculation.

Self-tests: not run
Cited category: Environmental, EHS & Energy. This calculator applies the broad-crested weir open-channel flow relationship Q = C x L x H^(3/2); verify coefficient selection and applicable local design or regulatory requirements before use.

About the Broad-Crested Weir Flow Calculator

Broad-crested weir flow calculations help water operators, civil engineers, and field technicians estimate open-channel discharge from measured head over a long crest. The tool applies a coefficient-based Q equals C x L x H to the 1.5 power relationship, builds a small rating table, and keeps unit conversions visible for defensible field notes.

How it works

  1. Measure upstream head at the specified approach location, not at the crest edge.
  2. Enter crest length, head, units, and the discharge coefficient for the weir geometry.
  3. Confirm that the flow is free and not submerged by downstream water level.
  4. Review discharge and use the rating table for nearby head values.

Frequently asked questions

When is a broad-crested weir formula valid?

It is most appropriate when the crest is long enough for pressure distribution over the crest to differ from a sharp-crested weir and the flow is modular or free-flowing. Poor approach conditions, submergence, or nonstandard geometry require correction or field calibration.

Where should head be measured for a broad-crested weir?

Head should be measured upstream far enough to avoid drawdown at the crest, using the reference datum specified for the weir. Measuring too close to the crest can understate the true upstream energy head.

What discharge coefficient should I use?

The coefficient depends on crest shape, approach velocity, weir proportions, and units. Use values from the applicable standard, design document, or a site-specific calibration rather than copying a generic coefficient.

Can this calculator be used for submerged flow?

Only as a rough screen unless a submergence correction is included. Downstream water backing up over the crest changes the discharge relationship.

Why does flow change so quickly with head?

The broad-crested weir relationship uses head raised to the 1.5 power, so small head measurement errors can create larger flow errors. Careful level measurement matters.

References