Civil Engineering nOVEMBER 2021 | Vol 29 No 10

14 November 2021 Civil Engineering F low in an open channel is measured by inserting a hydraulic structure into the channel, which changes the level of liquid in or near the structure. By selecting the shape and dimensions of the hydraulic structure, the rate of flow through or over the restriction will be related to the liquid level in a known manner. Thus, the flow rate through the open channel can be derived from a single measurement of the liquid level. The hydraulic structures used in measuring flow in open channels are known as primary measuring devices, with the level being measured using a secondary device which converts the level to a flow measurement. The relationship between level and flow has been investigated both theoretically and experimentally and the results well documented in the ISO standards. THE HISTORY OF THE LONG-THROATED FLUME The hydraulic theory for predicting discharge through long-throated flumes has resulted from over a century of development. The initial laboratory and theoretical studies on critical depth flumes were made in 1849 and later in 1896. These studies were extended into the early part of the 20 th century and by the 1950s the theory and dimensional requirements for these flumes were well known. However, calibration still required an empirical discharge coefficient. Theoretical predictions of flow were investigated in 1963 and further refined in 1975 by the stage-discharge theory, or calibration model. In 1978 and 1981 a procedure for determining the required head loss across these flumes was developed. The BS 3680-4A:1981 standard (BS, Venturi etc.) was one of the first standards for the long-throated flume. This was superseded by BS ISO 4359:1983 and subsequently ISO 4359:2013. The flume is now known as the RLT flume, a type of standing-wave or critical-depth flume and the ISO 4359:2013 standard clearly states that the expression “Venturi” is not applicable to open channel flumes. A Venturi meter is a closed conduit system with the head measurement at two locations and flow calculated with Bernoulli’s energy equation. The design of this flume does not require in situ calibration as, after years of research and testing in the field, the procedure for its design has been accepted and standardised. As the flume design is based on critical flow, the calculation is based on fundamental hydraulic theory, without the need for hydraulic laboratory testing to derive the formulae as for other forms of empirical flow meas- urement structures such as the Parshall flume. DESCRIPTION OF THE LONG-THROATED FLUME Long-throated flumes are generally composed from five primary structural components: 1. An approach channel for the development of uniform and symmetric flow conditions and the establishment of a stable water surface whose elevation can be determined accurately 2. A converging transition section in which the subcritical ap- proach flow accelerates smoothly toward the throat with no discontinuities or flow separation 3. A throat, or control section, in which the flow passes through critical depth 4. A diverging transition in which the velocity of the supercritical flow exiting the throat section is reduced and energy is dis- sipated or partially recovered. If energy recovery is not required, a truncated transition can be used Peter van der Merwe Pr Tech Eng Consultant for flumes and weirs flumesandweirs@gmail.com Open channel flowmeasurement and monitoring: the rectangular long-throat flume The rectangular long-throat (RLT) flume, also known as a “critical depth” flume, is often the most economical structure for accurately measuring open channel flows. GRP long-throated flume SS long-throated flume