Civil Engineering June 2022 | Vol 30 No 5

Civil Engineering June 2022 27 The reliability of a single stage-discharge relationship is demon- strated by the hydraulic similarity that exists among the models commercialised by Endress+Hauser. Their standard model range is outlined in Table 1. The Delft Hydraulics Laboratory paper titled, “Calibration Khafagi Venturi – Discharge characteristics for free flow and submerged flow”, uses the ISO 4359 equation for the flow through a streamlined rectangular contraction. The equation is: Q = 0.6667 1.5 • g 0.5 • b • m • f • h 1.5 Q = Discharge (m 3 /s) g = Gravitational acceleration (9.81 m/s 2 ) b = Throat width (m) m = Cd • Cv Combined discharge coefficient Cd – Characteristic discharge coefficient Cv – Approach velocity coefficient f = Flow reduction factor for submerged flow (free flow f = 1) h = Measured head (m) DESCRIPTION AND CONDITIONAL CONSTRAINTS TO THE FLUME GEOMETRY Channel/flume width: B = 120 mmminimum Throat width: b = 0.4 * B Flume inlet radius: 1.82 * B Flume inlet length: L1 = B Discharge length: L2 = 2.4 * B Maximum head: h max <= 2 * b Monitoring position: Before flume entrance = 3 to 4 x h max CONCLUSION Khafagi-Venturi Channels are simple in construction and measure free discharge flow accurately. They have become popular in measuring wastewater flow as the smooth profile of the flume does not obstruct the main stream. The passing of detritus, sediment and other solids rarely cause blockages. With reference to the term Khafagi-Venturi Channel, 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 calculation according to Bernoulli’s energy equation. Therefore, the term Khafagi Flume is more appropriate.  Khafagi flume annotation L B B L1 L2 Flow b R AutoCAD ® World-Leading CAD Software WaterMate Water Reticulation Design SurfMate Survey & Digital Terrain Modelling AeroTURN Aircraft Movement & Space Analysis RebarMate Reinforced Concrete Detailing & Scheduling Autodesk Civil 3D ® Site Development & Transportation AutoTURN Vehicle Manoeuvre Simulation & Analysis DatumMate Datum Conversion of Drawings RoadMate Urban & Rural Road Design ParkCAD Automated Parking Area Layouts Architecture Engineering & Construction Collection PipeMate Sewer & Stormwater Design and Analysis