ENGINEERING – HYDRAULIC & OPEN CHANNEL FLOW CALCULATOR
Weir Sharp Crested
A precise tool.
What is the Weir Sharp Crested & How does it work?
A sharp‑crested (Francis) weir is a simple, widely used structure for measuring flow in open channels. Water flows over the narrow crest, and the discharge is governed by the head of water above the crest and the geometry of the opening. The flow accelerates as it passes over the crest, creating a vena contracta downstream. Because the crest is thin, the energy loss is minimal, allowing the theoretical discharge equation to be applied with a relatively small empirical coefficient. The discharge equation for a rectangular sharp‑crested weir is expressed as:
Q = C_{d},b,sqrt{2g},H^{3/2}
Q = discharge (m³/s)
C_{d} = discharge coefficient (dimensionless)
b = width of the weir (m)
g = acceleration due to gravity (9.81 m/s²)
H = head over the weir (m)
C_{d} = discharge coefficient (dimensionless)
b = width of the weir (m)
g = acceleration due to gravity (9.81 m/s²)
H = head over the weir (m)
Parameters
Result
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Frequently Asked Questions
What is a sharp-crested weir?
A sharp-crested weir is a simple structure used to measure flow in open channels, where water flows over a narrow crest.
How does the discharge of a sharp-crested weir depend?
The discharge depends on the head of water above the crest and the geometry of the opening.
What is vena contracta in the context of a sharp-crested weir?
Vena contracta refers to the narrowest point downstream of the weir where the flow velocity is maximum.
Why is the energy loss minimal in a sharp-crested weir?
The energy loss is minimal because the crest is thin, allowing the theoretical discharge equation to be applied with a small empirical coefficient.
How do you calculate the discharge over a sharp-crested weir?
Discharge can be calculated using the head of water above the crest and the geometry of the opening, applying the appropriate discharge formula for sharp-crested weirs.
What is the advantage of using a sharp-crested weir in flow measurement?
The advantage is its simplicity and minimal energy loss due to the thin crest, which allows for accurate discharge calculations with a small empirical coefficient.
Can a sharp-crested weir be used in any type of water channel?
Yes, it can be used in various types of open channels where flow measurement is required.
Results are for informational purposes only and do not constitute professional advice.