🌊 Broad Crested Weir Calculator
Enter the weir width, head of water above the crest, and discharge coefficient to calculate flow rate over a broad crested weir.
What is a broad crested weir?
A broad crested weir is a flat-topped obstruction placed across an open channel, wide enough (relative to the flow depth) that the flow passes over it at critical depth. It’s a common structure for measuring or controlling flow in rivers, canals, and drainage systems.
The formula
For a rectangular broad crested weir, flow rate is calculated as Q = Cd × b × (2/3)1.5 × √g × H1.5, where b is the weir width, H is the head (water depth) above the crest, g is gravitational acceleration, and Cd is an empirical discharge coefficient, typically around 0.85 for a well-constructed weir.
The discharge coefficient depends on weir geometry, approach conditions, and crest roughness, so site-specific calibration improves accuracy for engineering applications. This calculator is for general hydraulic education and preliminary estimates.