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# Broad Crested Weir

EmilyB.Broad Crested Weir

The **Flow Rate for a Broad-crested Weir** calculator computes the flow rate of water (or similar liquids) over a weir that is characterized as broad (flat and wider than deep).

**Instructions:** Choose units and enter the following:

- (
**b**) This is the width of the weir - (
**h**) This is the first head on the weir_{1} - (
**h**) This is the second head on the weir_{2} - (
**C**) This is the discharge coefficient._{d}

**Flow Rate (q)**: The calculator returns the flow rate in cubic meters per second. However, this can be automatically converted into other volumetric units (e.g. gallons per minute) via the pull-down menu.

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### General Information

A broad-crested weir is a flat-crested structure, with a long crest compared to the flow thickness. When the crest is "broad", the streamlines become parallel to the crest invert and the pressure distribution above the crest is hydrostatic. The net effect is a double drop resulting in two head measurements (h_{1} and h_{2}).

The hydraulic characteristics of broad-crested weirs were studied during the 19th and 20th centuries. Practical experience showed that the weir overflow is affected by the upstream flow conditions and the weir.

The following formula is used to calculate the Flow Rate for a Broad Crested Weir:

`q = Cd⋅h2⋅b⋅sqrt(2g⋅(h1−h2))`

where:

- q is the discharge rate of a broad crested weir
- Cd is the discharge coefficient
- b is the width (breadth) of the weir
- h1 is the first head drop
- h2 is the second head drop
- g is the acceleration due to gravity

## References

**Broad Crested Weir**, references 1 equation/constant.

**Equations and Constants **