Weir flow calculation
Measurement of liquid flowrate in open channel
1. Introduction
2. Rectangular weirs : flow calculation
1. Introduction
Calculation of flow rate through weirs is particularly interesting in water systems engineering, for the calculation of flow of rivers, dams, sewage system...etc... that have an overflow system that takes the form of a weir.
Different weir designs are available :
- Rectangular weir
- Triangular notch weir
The equations below [Perry] are valid for water flow. With a different fluid, a correction due to the viscosity is required.
2. Rectangular weir : flow calculation
The flow capacity through a rectangular weir is given by the following formula, which applies to sharp edged rectangular weirs :
Q=0.415*(L-0.2*h0)*h01.5*(2*g)0.5
With :
Q = volume flow rate (m3/s)
L = crest length (m)
h0 = weir head (m)
g = gravity acceleration = 9.81 m.s-2
In case h0 > L, a modification of the calculation formula must be considered :
Q=0.386*L*h01.5*(2*g)0.5
3. Triangular notch weir : flow calculation
The flowrate that can be achieved through a triangular notch weir (also called V notch weir), sharp edged, is calculated with the following equation :
Q=(0.31*h02.5*(2*g)0.5)/tan Φ
