Broad-Crested Weir Calculator

About the Broad-Crested Weir Calculator

The **Broad-Crested Weir Calculator** computes the discharge (flow rate) over a broad-crested weir using the standard hydraulic equation Q = Cd × B × (2/3)^(3/2) × √g × H^(3/2). It also calculates the critical depth on the crest, crest velocity, Froude number, approach velocity correction, H/L validity, and specific energy.

Broad-crested weirs are widely used in irrigation canals, dam spillways, laboratory flumes, and flow-measurement structures. Unlike sharp-crested weirs, the crest is long enough in the flow direction for the flow to become parallel, reaching critical depth. This makes them reliable flow-measurement devices when designed within the valid H/L range (0.08 to 0.5).

Use the presets for typical configurations, explore the head-discharge table, and refer to the Cd reference for different weir types. The extra checks on approach velocity and H/L range make it easier to see when the standard broad-crested assumption is appropriate and when the setup is drifting outside its reliable window.

When This Page Helps

Broad-crested weirs are simple in concept, but field calculations still depend on head measurement, coefficient choice, and staying within the conditions where critical flow forms on the crest. This calculator combines discharge, critical-depth checks, and validity indicators so hydraulic design and field verification can start from the same set of assumptions.

How to Use the Inputs

1. Select a preset or enter the weir width (perpendicular to flow).
2. Enter the head above the crest (H) — the water level minus crest elevation.
3. Set the crest length (in the flow direction) and weir height above the channel bed.
4. Adjust the discharge coefficient Cd (default 0.848 for ideal broad-crested).
5. Enter the upstream channel width for approach velocity correction.
6. Read discharge, critical depth, velocities, and validity indicators.

Example Calculation

Tips & Best Practices

• Measure head H at least 3–4× the maximum head upstream of the weir to avoid drawdown effects.
• A rounded upstream edge (r/H ≥ 0.1) increases Cd by reducing separation losses.
• For very low H/L (<0.08), the weir behaves as a rough flat surface with friction effects.
• For very high H/L (>0.5), it transitions toward sharp-crested behaviour.
• Use the corrected Q (with approach velocity) for narrow channels.

Check Whether The Geometry Matches The Theory

The standard broad-crested weir equation assumes the crest is long enough for the flow to become nearly parallel and reach critical depth. That is why the H/L ratio matters so much. If the geometry falls outside the usual range, the structure may behave more like a sharp-crested or submerged control and the simple discharge relation becomes less reliable.

Measure Head Carefully

Head should be measured far enough upstream that drawdown over the crest does not distort the reading. In practice, poor head measurement often creates larger error than the equation itself. Use a stable reference point, verify crest elevation, and keep the measuring section far enough upstream to avoid local acceleration effects.

Treat Coefficients As Real Design Inputs

The discharge coefficient is not just a cosmetic adjustment. Crest shape, surface roughness, edge rounding, and installation quality all influence Cd. Use the calculator to test reasonable coefficient ranges and see how sensitive the discharge is before treating a single value as exact.

Frequently Asked Questions

• What is a broad-crested weir?

  A raised barrier across an open channel with a wide enough crest for the flow to reach critical depth. Used for flow measurement and control.

• What is the H/L validity range?

  For reliable broad-crested behaviour, the ratio of head to crest length should be between 0.08 and 0.5. Outside this range, the weir may behave as sharp-crested or submerged.

• What is Cd?

  The discharge coefficient accounts for energy losses not captured by the ideal equation. It is determined experimentally and typically ranges from 0.80 to 0.87.

• How does this differ from a sharp-crested weir?

  A sharp-crested weir has a thin plate edge; flow separates cleanly. A broad-crested weir has a long flat top where flow reattaches and becomes critical.

• Why correct for approach velocity?

  If the approach channel is not much larger than the weir, kinetic energy in the approaching flow adds to the effective head and increases discharge. Ignoring that correction can understate flow, especially in narrower approach sections.

• Can I use this for submerged conditions?

  No — this calculator assumes free (unsubmerged) flow. Submerged weirs require a submergence reduction factor.