Two stations are marked along the stream and the distance between them is measured along the stream centerline. The time it takes for the float to travel through the marked distance is recorded. Velocity is then calculated. Surface velocity is greater than average velocity. Manning's Equation most popular Need depth, width, stream profile and energy gradient estimates to apply.
Flumes are conduits built such as the sides constrict the flow.
Orifice They are used to control smaller flows than weirs.