Broad-Crested Weir Calculator

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Metric uses metres and cubic metres per second. Imperial uses feet and cubic feet per second.
Choose which quantity to calculate. All other inputs are treated as known.
Depth of water above the weir crest measured upstream, at least 3-4 crest widths back from the weir face.
m
The span of the weir crest perpendicular to the flow direction.
m
Dimensionless coefficient accounting for flow losses. Typical range 1.4-1.8 for broad-crested weirs; 1.705 is the theoretical free-flow value derived from critical flow theory.
Discharge (Q)Supercritical - verify weir geometry
3.7755

Volumetric flow rate over the weir

Discharge unitm³/s
Critical depth (Hc)0.3333
Critical depth unitm
Flow velocity at crest (V)5.6632
Velocity unitm/s
Froude number (Fr)3.132
Discharge coefficient (Cd)1.705
Head over crest (H)0.5
Head unitm
Weir length (L)2
Length unitm
Flow regimeSupercritical
3.132
Submerged / drowned<0.67Near-critical, low0.67-0.85Critical flow (ideal)0.85-1.15Supercritical1.15-1.5Highly supercritical1.5+

Discharge is 3.7755 m³/s

  • Critical depth at the crest is 0.3333 m, which is two-thirds of the upstream head - this is the hallmark of broad-crested weir hydraulics.
  • Flow velocity across the crest is 5.663 m/s.
  • Froude number is 3.132, above 1.0 - verify that the weir crest is long enough (crest width b >= 2H) to establish critical flow.
  • The Cd falls within the normal range for broad-crested weirs (1.4-1.8).

Next stepFor irrigation, flood control, or gauging applications, confirm that the upstream head gauge is placed 3-4 crest widths upstream and that the tailwater does not exceed 67% of the head (S < 0.67) to maintain free-flow accuracy.

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