This International Standard specifies methods for the measurement of flow in rivers and artificial channels under steady or slowly varying flow conditions, using certain types of standing-wave, or critical-depth, flumes. A wide variety of flumes has been developed, but only those which have received general acceptance after adequate research and field testing, and which therefore do not require in situ calibration, are considered. The flow conditions considered are uniquely dependent on the upstream head, i.e. subcritical flow must exist upstream of the flume, after which the flow accelerates through the contraction and passes through its critical depth (see Figure 1). The water level downstream of the structure is low enough to have no influence upon its performance. This International Standard is applicable to three commonly used types of flumes, covering a wide range of applications, namely rectangular-throated, trapezoidal-throated and U-throated. Typical field installations are shown in Figure 2. Site conditions are important and Figure 3 shows acceptable velocity profiles in the approach channel. Detailed illustrations of the three types of flumes covered by this International Standard are given as follows: a) rectangular-throated (see Figure 4); b) trapezoidal-throated (see Figure 5); c) U-throated, i.e. round-bottomed (see Figure 6). It is not applicable to a form of flume referred to in the literature — sometimes called a “Venturi” flume — in which the flow remains subcritical throughout. NOTE This form is based on the same principle as a Venturi meter used within a closed conduit system and relies upon gauging the head at two locations and the application of Bernoulli’s energy equation.