Open Channel Flow Measurement

I need to measure the flow in an open channel 15ft wide, 13 ft deep, several hundred ft long. (Chlorine Contact Tank, water is pretty clean)

The channel has a weir, but is not straight, more of a "L" shape, therefore using a regular ultrasonic level meter poses the problem of establishing proportionality between level and flow.

I am looking into the Area Flowmeters and Velocity Profiling with pulse doppler technology;

anybody out there who has experience with these different technologies?