**Weir flow calculation**

## Measurement of liquid flowrate in open channel

1.
Introduction

2. Rectangular weirs : flow calculation

3. Triangular notch 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*h_{0})*h_{0}^{1.5}*(2*g)^{0.5}

With :

Q = volume flow rate (m^{3}/s)

L = crest length (m)

h_{0} = weir head (m)

g = gravity acceleration = 9.81 m.s^{-2}

In case h_{0} > L, a modification of the calculation
formula must be considered :

Q=0.386*L*h_{0}^{1.5}*(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

**Source**

[Perry] Perry's Chemical
Engineer's Handbook, Section 10 Transport and storage of fluids, page
10-24, McGraw-Hill, 2008