Broad Crested Weir Calculator
Find flow over a broad-crested weir.
A broad-crested weir is a flat-topped barrier used to measure open-channel flow.
How the Math Works
The Broad Crested Weir Calculator uses the formula Q = Cd·(2/3)^(3/2)·√g·b·H^(3/2) to compute flow rate. Here, Q represents the volumetric flow rate, Cd is the discharge coefficient (accounting for energy losses and weir geometry), g is gravitational acceleration (9.81 m/s²), b is the weir width, and H is the head (vertical height of water above the weir crest). The term (2/3)^(3/2) arises from integrating velocity profiles across the weir's height, while H^(3/2) reflects the relationship between head and discharge. This formula assumes steady, uniform flow and a rectangular weir cross-section, simplifying complex fluid dynamics into a practical engineering tool.
Practical Applications
Engineers apply this calculation to design and assess weir structures in channels, such as those in irrigation systems, flood control reservoirs, or water treatment plants. By inputting measured head (H) and known weir width (b), along with an appropriate Cd value (often derived from empirical studies), users can quickly determine flow rates to size weirs correctly or evaluate existing structures. Field measurements of H using staff gauges or pressure sensors, combined with precise geometric data for b, ensure accurate results. This helps optimize water management by balancing flow capacity with structural safety and efficiency.
Day-to-Day Use
This calculator aids in everyday water resource management, ensuring reliable water supply for cities, farms, and industries. Accurate flow measurements prevent flooding by optimizing drainage systems and help maintain reservoir levels for drinking water or irrigation. For example, farmers use it to regulate canal flows for crop growth, while municipalities rely on it to balance water distribution during droughts or storms. By simplifying complex calculations, the tool empowers professionals to make informed decisions quickly, safeguarding communities and ecosystems through precise water control.
Worked example
Cd 0.85, 3 m wide, 0.4 m head → about 1.16 m³/s.
FAQ
Why measure with a weir?
Flow depends only on the easily measured head over the crest.