Open channel flow occurs with a free surface open to the atmosphere, thus meters using change in pressure cannot be used as in pipe flow measurement. For open channel flow measurement, a change in depth of flow at some point is typically measured and correlated with water flow rate. The most common methods of measuring open channel flow rate are with a weir or a flume. The sharp crested weir (v notch weir and rectangular weir), broad crested weir, and Parshall flume are introduced in the following sections.
A weir is basically an obstruction in the flow path in an open channel. The weir will cause an increase in the water depth as the water flows over the weir. In general, the greater the flow rate, the greater will be the increase in depth of flow, The height of water above the top of the weir is the measurement usually used to correlate with flow rate.
The two major types of weir are sharp crested weir and broad crested weir. The crest is the term used for the top of the weir, where the water flows over it. The two diagrams here show a sharp crested weir and a broad crested weir. As you can see by the diagrams, the names are very descriptive. The sharp crested weir has a sharp surface at the crest or top, where the water flows over it, and the broad crested weir has a broad flat surface at the top.
The height of the water above the crest of the weir is called the head over the weir and is shown as H in both of the diagrams. It is the parameter that is measured and used to determine the flowrate. There are equations available to calculate flow rate, Q, over the weir for given head over the weir, H. Also for a given weir, Q can be experimentally correlated with H. See the article, The Broad Crested Weir for Open Channel Flow Measurement for more detailed information on the broad crested weir.
Types of Sharp Crested Weir
Two common shapes for a sharp crested weir are the v notch weir and the
rectangular weir. A rectangular weir (horizontal across the top) may go all the way across the channel (called a suppressed rectangular weir). If it is inset at the sides as shown in the diagram, then it is called a contracted rectangular weir. Three common types of sharp crested weir are shown in the diagram at the right. Another type of contracted, sharp crested weir is the cipolletti weir. It has a trapezoidal opening for the water to go through,
The Flume For Flow Rate Measurement
Instead of putting in a vertical obstruction, as with a weir, a flume consists of a constriction in the channel width. This constriction causes
the water level to change, and that water level in the constriction can be correlated with flow rate. The diagram at the right shows a top view of a venturi flume and a top view and side view of a Parshall Flume. A venturi flume has no bumps or dips in the floor of the channel. The Parshall Flume has the specific shape shown for its floor. The Parshall Flume has very specific dimensions for any channel width. There are equations and tables available, giving flow rate through the flume as a function of head (water height) above the bottom of the flume at a specific location. Parshall Flumes are widely used to measure wastewater flow rates in wastewater treatment plants. See the article, The Parshall Flume for Open Channel Flow Measurement for more detailed information about the Parshall flume.