Droplet Logo
Davenport Flow Technology Logo

Simple Flow Calculations


Open Channel Flow Gauges - V Notches

A V notch weir is a type of flow gauge.


Its purpose is:-

To ensure that the velocity flowing over the weir is only dependent on the upstream head.

To provide an accurate cross sectional area at the point of measurement.


The only variable is the head and so only a single measurement is required.



  Start Calculator

The cross sectional area of the flow is:-

A = Tan(0.5α) h2

Where:   α =

h =

The angle of the V

The head (depth)


The velocity of the flow is:-

V = k h0.5 Where:   k =

A constant taking into

account g and various


The basic flow equation (Q = V A) can be written:-  

Q = k Tan(0.5α) h2.5


“The rate of flow of a liquid moving in a pipe or an open channel is dependent on the velocity and the cross sectional area of the liquid”.  

The basic equation for most flow measurement applications is:-

Q = V A



   Q = Flow rate e.g. m3/s

   V = Velocity e.g. m/s

   A = Area e.g. m3/s

“An Open Channel is any pipe or conduit where the water surface is at atmospheric pressure”


The following flow calculations are available free of charge:-


Open channel Velocity Area Calculation

Open channel Slope Area Method (Manning Equation)

Open channel Flumes