This document specifies methods for the measurement of flow in rivers and artificial channels under steady or slowly varying flow conditions, using certain types of critical-depth flumes (also known as “standing-wave flumes”). A wide variety of flumes has been developed, but only those critical-depth flumes which have received general acceptance after adequate research and field testing, and which therefore do not require in situ calibration, are considered herein.

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 document is applicable to three commonly used types of flumes, covering a wide range of applications, namely rectangular-throated, trapezoidal-throated and U-throated. The hydraulic theory behind this document was presented in Reference [7].

This document 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？The Venturi form of flume 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 formula.