What Is the Broad-Crested Weir Discharge Calculator?
A broad-crested weir is an obstruction with a sufficiently long horizontal crest such that streamlines become parallel and flow passes through critical depth.
The crest length in the flow direction must typically satisfy 0.08 < H/L < 0.50 to qualify as broad-crested.
How Does the Broad-Crested Weir Discharge Calculator Work?
Because critical flow occurs over the crest, the discharge is strictly a function of upstream head H raised to the 3/2 power.
Tailwater submergence does not affect the flow rate until downstream water levels rise above roughly 70–80% of upstream head.
Broad-Crested Weir Discharge Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Critical flow discharge relationship across broad-crested weir crests.
How to Use the Broad-Crested Weir Discharge Calculator
- Input crest width, head over crest, weir height, and discharge coefficient Cd.
- Review flow rate in m³/s and L/s, and critical depth on the crest.
Step-by-Step Example Calculation
Irrigation Canal Metering Weir
Input Values:
Understanding Your Result
Directly converts water stage measurements from ultrasonic or staff gauges into volumetric flow rate.
Factors That Affect the Result
- Rounding the upstream edge of the crest prevents flow separation and increases Cd closer to 0.98.
When Should You Use This Calculator?
- Canal discharge monitoring, dam intake structures, and hydrological gauging stations.
Assumptions & Limitations
- Assumes free-discharge conditions without modular drowning or significant aeration cavitation.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Standard ISO 3846 and USBR Water Measurement Manual standard.