A venturi meter would normally measure in millimetres, whereas a venturi flume measures in metres.
2.
A venturi meter is more efficient, but usually more expensive and less accurate ( unless calibrated in a laboratory ) than an orifice plate.
3.
A venturi flume is similar to the Parshall flume, without the contoured base, but the cross section is usually rectangular, the inlet shorter, and there is a general taper on the outlet similar to the venturi meter.