Flumes and weirs are specially designed channel shapes that characterize the flow of water. Common types are Rectangular Weirs, V-Notch Weirs, Parshall flumes and Palmer Bowlus flumes. The choice of flume or weir type depends on the application: flow rate, channel shape and solids content of the water.
Traditional Methods : Flumes and weirs are now a days considered as traditional method to measure flow of open channel .
Modern Methods :
Area Velocity Flow Measurement Methods
There are two types of Area Velocity (AV) methods for measuring open channel flow. Both types use either Transit time or Doppler ultrasonic technologies. The area velocity method calculates flow rate by multiplying the area of the flow by its average velocity. This is often referred to as the continuity equation, Q=AxV, where Q is flow, Ax is the wetted area of flow times V, the average velocity of the flow stream.
Doppler Ultrasonic Flow Measurement Techniques
Doppler Flow Measurement for open channel flow is designed to record instantaneous velocity components at a single point with a relatively high frequency.Flow measurement is performed by measuring the velocity of particles in a remote sampling volume based upon the Doppler shift effect. The velocity components using this technique are combined with level measurement and an open channel flow computer using the manning equation to calculate true open channel flow measurement.
The Doppler flow meter is placed at the bottom of the channel which often times gets dirty thereby causing inaccurate flow measurement. Since Doppler techniques measure the speed of particles coming towards the sensor, it only measures what it sees. Particles in a stream move at different speeds and it is highly unlikely that it can measure all the particles in the stream, hence not representing the average flow in the stream and causing uncertainty in overall open channel flow measurement.
ImageSource : OpenChannelFlow
Article Source : Smart Measurement ;
Animation Credits : greyline