To measure the flow of an open channel we have to make the flow pass through obstructions. The flow is measured from this obstruction like Weirs and Flumes. A special method used to measure the flow is using Current meters other than pitot tube and measurement using float.
Weir is an artificial structure made along the flow channel to measure the flow of the fluid. Cost of the construction increase as the width of the flow channel increase.
Weirs have a defined cross-section area and hence computation of discharge is simple. Weir used to maintain an upstream level or for measuring discharge.
Weir should be build rigid, water tight normal to the flow direction. The site chosen for weir should have less chance of sediment growth. And we must avoid conditions for sedimenting sludges, weeds in the channel.
A weir should withstand the high flow in case of emergency,
Flume is another flow measuring structure made along the flow channel. Flume makes flow constriction in a channel.
Just like venturi, the flume is narrow constriction. So a unique stage-discharge relationship exists independent of the downstream condition.
The measuring instrument is placed above the structure.
The measuring instruments are commonly pitot tube, ultrasonic level measuring technique, current meters etc,
Current meters measure the flow of channel of the cross-section, accurate measurement can be read using the current meter.
It consists of a rotating element which rotates due to the reaction of the stream. A current is generated according to the velocity of rotating element which is in relation to the velocity of the flowing stream.
The circuitry attached to the current meter counts the number of rotations of the rotating element during a time interval, the velocity of the flow is determined.
The current meter is hanged from a structure to the weir or flume. Measuring devices such as ultrasonic meters are done so.