ENGINEERING – HYDRAULIC & OPEN CHANNEL FLOW CALCULATOR
Weir Broad Crested
A precise tool.
What is the Weir Broad Crested & How does it work?
A broad‑crested weir is a rectangular obstruction that allows water to flow over its top. Unlike sharp‑crested weirs, the flow remains attached to the crest, producing a more gradual acceleration and a discharge that depends on both the head above the weir and the length of the crest. The governing relationship for a broad‑crested weir is derived from the energy equation and the continuity of flow. Assuming a uniform velocity profile across the width and neglecting downstream effects, the discharge can be expressed as a product of the weir geometry, the hydraulic head, and a dimensionless coefficient that accounts for viscosity and surface tension. Engineers use this formula to size weirs for spillways, overflow structures, and flow measurement devices. By selecting an appropriate discharge coefficient (typically between 0.6 and 0.8) and measuring the head, the volumetric flow rate can be estimated quickly and with reasonable accuracy.
Q = C_{d}\,L\,H\,\sqrt{2gH}
Q = discharge (m³/s)
C_{d} = discharge coefficient (dimensionless)
L = weir length (m)
H = head over weir (m)
g = acceleration due to gravity (9.81 m/s²)
C_{d} = discharge coefficient (dimensionless)
L = weir length (m)
H = head over weir (m)
g = acceleration due to gravity (9.81 m/s²)
Parameters
Result
—
Frequently Asked Questions
What is a broad-crested weir?
A broad-crested weir is a rectangular obstruction that allows water to flow over its top, with the flow remaining attached to the crest.
How does a broad-crested weir differ from a sharp-crested weir?
Unlike sharp-crested weirs, the flow over a broad-crested weir remains attached to the crest, leading to a more gradual acceleration and discharge that depends on both the head above the weir and the length of the crest.
What is the governing relationship for a broad-crested weir?
The governing relationship is derived from the energy equation and continuity of flow, assuming a uniform velocity profile across the width and neglecting downstream effects.
How do you calculate discharge over a broad-crested weir?
Discharge is calculated using the head above the weir and the length of the crest, with the specific formula depending on the flow conditions and the type of weir.
What factors affect the discharge over a broad-crested weir?
The discharge depends on the head above the weir, the length of the crest, and can also be influenced by the width of the weir and the approach flow conditions.
Results are for informational purposes only and do not constitute professional advice.