# 26.2 Flow measurement in open channels

Measuring the flow rate of liquid through an open channel is not unlike measuring the flow rate of a liquid through a closed pipe: one of the more common methods for doing so is to place a restriction in the path of the liquid flow and then measure the “pressure” dropped across that restriction. The easiest way to do this is to install a low “dam” in the middle of the channel, then measure the height of the liquid upstream of the dam as a way to infer flow rate. This dam is technically referred to as a weir, and three styles of weir are commonly used:

Another type of open-channel restriction used to measure liquid flow is called a flume. An illustration of a Parshall flume is shown here:

Weirs and flumes may be thought of being somewhat like “orifice plates” and “venturi tubes,” respectively, for open-channel liquid flow. Like an orifice plate, a weir or a flume generates a differential pressure that varies with the flow rate through it. However, this is where the similarities end. Exposing the fluid stream to atmospheric pressure means the differential pressure caused by the flow rate manifests itself as a difference in liquid height at different points in the channel. Thus, weirs and flumes allow the indirect measurement of liquid flow by sensing liquid height. An interesting feature of weirs and flumes is that although they are nonlinear primary sensing elements, their nonlinearity is quite different from that of an orifice.

Note the following transfer functions for different weirs and flumes, relating the rate of liquid flow through the device (Q) to the level of liquid rise upstream of the device (called “head”, or H):

Where,

= Volumetric flow rate (cubic feet per second – CFS)

= Width of notch crest or throat width (feet)

θ = V-notch angle (degrees)