Civil Engineering June 2022 | Vol 30 No 5

26 June 2022 Civil Engineering T he shape of Khafagi flumes aims to reduce the width of the original channel in order to assure a free-flow condition and that the critical depth occurs in the narrowest section. Khafagi flumes may be regarded as short, and thus cheaper, vari- ants of the long-throated flumes. The Khafagi flume is capable of passing fine and coarse mechanical inclusions, i.e. suspended load, sand, branches, logs and other debris. Flume hydraulics has been studied for over a hundred years and, even today, is of interest to the engineering and scientific community. This is reflected in the number of publications devoted to flumes with research carried out across the entire spectrum of experimental, numerical and theoretical science. Undoubtedly, this is due to the necessity to improve the methods of calculating the hydraulic characteristics of a flume – the main open channel water monitoring device. THE HISTORY OF THE KHAFAGI FLUME In the 18 th century, Giovanni Battista Venturi was the first to observe the effects of pressure and velocity distribution in a conduit due to local contractions. The flume defines a reduction of the channel width, and the contraction is obtained by reducing the channel sidewalls. Measurement channels having a flat bottom and a local width reduction were, in about 1917, named Venturi Channels. To obtain more accurate values of flow as a function of depth in a Venturi Channel, Dipl. Ing. Anwar Khafagi of the Swiss Federal Institute of Technology, ETH: Eidgennossische Technische Hochschule, in Zurich, Switzerland, developed a new form for a measuring channel. This form became known as the Khafagi-Venturi Channel and was widely accepted and applied. Endress+Hauser commercialised a range of the Khafagi- Venturi Channel models, characterised by different lengths of flume and channel widths. In European countries, the use of this flume, also called a Khafagi flume, became, and still is, widespread. RATING DESIGN OF THE KHAFAGI FLUME The brochure “Canal Khafagi-Venturi” issued by Endress+Hauser France and Germany for their range of Khafagi-Venturi flumes states the following regarding the head discharge relations: “The Khafagi-Venturi flumes were individually tested at the Institute for Hydraulic Engineering, University of Stuttgart and the stage-discharge relationship of a generic Endress+Hauser Khafagi-Venturi meter can be expressed by the following equation: Q = 0.01744 • b • h1.5+0.00091• h2.5 Q = Flow in litres/second b = Throat width (cm) h = Depth upstream of the flume (cm)” With reference to the Endress+Hauser publication, “Wastewater measurement and control, first edition, 1982”, the following state- ment was made: “The Khafagi Venturi was made more accurate by using some experimental determined constants in its design. The com- puter program for designing Khafagi Venturi measurement flumes, developed expressly for Endress+Hauser, is based on the design methods suggested by Khafagi and incorporates the constants he suggested. All expressions for the basic de- sign have been checked experimentally and are scalable. They obey similarity laws according to Froude and can therefore be modelled on a prototype.” Open channel flowmeasurement and monitoring: the Khafagi flume Peter van der Merwe Pr Tech Eng Consultant for flumes and weirs flumesandweirs@gmail.com Flumes are recognised as an effective means of determining the discharges of clean and suspended load-bearing open flows in natural watercourses, canals and in sewerage systems. Khafagi flumes are all characterised by the cross-sectional area having various degrees of convergence and subsequent divergence. A Khafagi flume Table 1 Endress+Hauser standard model range TYPE b B L1 L2 h QV302 10 ℓ/S 48 120 120 420 300 QV303 25 ℓ/S 120 300 300 1 050 300 QV304 50 ℓ/S 160 400 400 1 400 400 QV305 75 ℓ/S 200 500 500 1 750 450 QV306 100 ℓ/S 240 600 600 2 100 450 QV308 250 ℓ/S 320 800 800 2 800 670 QV310 500 ℓ/S 400 1 000 1 000 3 500 870 QV313 800 ℓ/S 520 1 300 1300 4 550 1 020 QV316 1500 ℓ/S 640 1 600 1600 5 600 1 320