Mir, I fully agree with you. One observation/addition with regard to the use of open channel flowmeters:  Some companies offer non contact down looking radars to measure surface velocity in open channels. In order to convert the measured velocity into a profiled, (average), velocity, they use an universal factor of 0.95. A dynamic factor should be used, due to the constantly varying ratio between the wetted cross section and the wetted perimeter in the stream, (especially in sewers). The other option is to measure velocity with a contact Doppler sensor, which supposedly, sensed the echoes of al particles and bubbles dispersed in the liquid. Some flowing near the walls and others flowing closer to the center of the stream. In addition, the location point of the sensor in a sewer is extremely important as well in order to get a reliable velocity measurement. Down looking radars do not let many choices since they have to be installed on top of the stream, in the manhole pit, just where the stream is mostly affected by the connection between the pipe and the manhole's concrete.

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Mir, I fully agree with you.

One observation/addition with regard to the use of open channel flowmeters: 

Some companies offer non contact down looking radars to measure surface velocity in open channels. In order to convert the measured velocity into a profiled, (average), velocity, they use an universal factor of 0.95. A dynamic factor should be used, due to the constantly varying ratio between the wetted cross section and the wetted perimeter in the stream, (especially in sewers).

The other option is to measure velocity with a contact Doppler sensor, which supposedly, sensed the echoes of al particles and bubbles dispersed in the liquid. Some flowing near the walls and others flowing closer to the center of the stream.

In addition, the location point of the sensor in a sewer is extremely important as well in order to get a reliable velocity measurement. Down looking radars do not let many choices since they have to be installed on top of the stream, in the manhole pit, just where the stream is mostly affected by the connection between the pipe and the manhole's concrete.