ISO 4359:2013 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. The water level downstream of the structure is low enough to have no influence upon its performance.
ISO 4359:2013 is applicable to three commonly used types of flumes, covering a wide range of applications, namely rectangular-throated, trapezoidal-throated and U-throated.
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.