Table of Contents
What is a Broad-Crested Weir?
A broad-crested weir is a hydraulic structure with a flat horizontal crest that is wide enough for the flow to become parallel to the crest (critical depth is established on the crest). Unlike sharp-crested weirs where the nappe springs free, flow over a broad-crested weir adheres to the crest surface, and the water depth on the crest approaches the critical depth for the given flow rate.
Broad-crested weirs are commonly used in open channels for flow measurement, flood control, and water level regulation. They are more robust than sharp-crested weirs and less susceptible to damage from debris. The broad crest also makes them easier to construct, and they are often used as spillways in dams and as flow measurement structures in irrigation canals and rivers.
Flow Equations
Where Q is the volumetric flow rate, Cd is the discharge coefficient (typically 0.544 for ideal conditions), b is the weir width, g is gravitational acceleration, and H is the total head above the weir crest measured upstream.
Design Considerations
| Parameter | Guideline |
|---|---|
| Crest Length/Head ratio | L/H ≥ 2 for broad-crested behavior |
| Upstream face | Rounded or chamfered to reduce contraction |
| Discharge coefficient | 0.54-0.58 depending on geometry |
| Approach velocity | Include velocity head if Va > 0.3 m/s |
Frequently Asked Questions
When is a weir "broad-crested" vs "sharp-crested"?
A weir is considered broad-crested when the ratio of crest length to upstream head (L/H) is greater than about 2 and less than about 12. When L/H is less than 2, the weir behaves as a sharp-crested weir with a free nappe. When L/H exceeds about 12, friction effects on the crest become significant and the weir acts more like an open channel. The broad-crested range ensures critical depth forms on the crest.
How accurate is the discharge coefficient?
The theoretical discharge coefficient for a broad-crested weir is 0.544, derived from critical flow theory. In practice, the coefficient varies from about 0.50 to 0.58 depending on the weir geometry, upstream face shape, approach conditions, and H/P ratio. Rounded upstream edges increase the coefficient, while sharp edges cause flow separation that reduces it. Field calibration improves accuracy to within 2-3%.
Can broad-crested weirs handle submerged conditions?
Yes, but with reduced accuracy. When the downstream water level rises above the crest, the weir becomes submerged and the standard equation must be multiplied by a submergence correction factor. As submergence increases, the flow rate decreases because the downstream backwater reduces the effective head. When submergence ratio exceeds about 0.85, the weir is no longer reliable for flow measurement.