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Flow rate using weirs and flumes

Flumes and weirs are designed to change a flow from sub-critical to super-critical. Many flumes cause this transition with a narrowing of the throat and a drop in the channel bottom. This transition causes flow to pass the critical depth. At the critical depth, there is a direct relationship between water depth, velocity and flow-rate


Weirs give us a simple method of measuring the rate of fluid flow. Since the geometry of the top of the weir is known, and all water flows over the weir, the depth of water flowing over the weir indicates the amount of flow. Weir can be a simple metal plate with a V notch cut into it or it may be a concrete and steel structure across the bed of a river. A v-notch weir will give a more accurate indication of low flow rates. Flow rate (F) over a weir is equal to the height (H) to an exponent (e) times a constant(k).

F = kHe

Types of weirs:


A venturi flume has a special shaped open channel flow section which may be installed in a ditch ,canal, or lateral to measure the flow rate. The Parshall flume is a particular form of venturi flume.

Types of flumes:

Open Channel

The Open Channel method of flow rate measurement uses the height or head of the liquid as it passes over an obstruction in the channel.

Other flow rate techniques involve square root extraction

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