Portable Water Flow Measuring

Well, when I was young, I used a thing called an ultrasonic flowmeter; these things just clamp onto the outside of the pipe and use the Doppler Effect on the stuff inside it.

Things may have moved on since then.

Nope, they're still in use.

Transit time clamp-on ultrasonic flow meters are used for clean water (fluid) and have a high degree of accuracy, if the pipe is not rusted/corroded.

Doppler clamp-on ultrasonic flow metering is used for waste water carrying suspended solids or highly aerated with bubbles. My experience is that Doppler is a relative reading ( more or less than a previous flow rate), but not useable for absolute flow rates unless there's some other means of 'calibrating' the flow, which if there was, doppler clamp-on would not be under consideration.