Doppler method
ultrasonic discharge measurement

An ultrasonic transducer, installed diagonally to the flow direction,
sends an acoustic signal, that is reflected by suspended particles
in the water. Afterwards the transducer receives the reflected signal.
Because the particles move with the flow, there will be a
Doppler frequency shift (sent and received signal).
The Doppler frequency shift is proportional to the flow velocity and
therefore directly related to the discharge of a known cross section.