Broad Crested Weir

Not Reviewed
Equation / Last modified by KurtHeckman on 2018/06/01 13:23
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
  • (h1) This is the first head on the weir
  • (h2) This is the second head on the weir
  • (Cd) This is the discharge coefficient.

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.   Broad-crested weir.png

Water, Rain and Snow Calculators:

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 (h1 and h2).

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))`