Civil Engineering October 2022 | Vol 30 No 9

Civil Engineering October 2022 25 from the relevant flow equations, rating charts and stage-discharge curves. PLACEMENT OF CALIBRATED GAUGES FOR FLUMES AND WEIRS It is recommended that for every flume or weir, i.e. the primary measuring device, installation includes a calibrated staff gauge. The calibrated staff gauge must be solidly and accurately fitted and be placed at the proper head-measuring location for the primary device. The calibrated staff gauge zero should be precisely aligned with the primary device zero level. This will aid in the zero adjustment of the secondary device. Q Q The measuring point within the f lume: Q Q ISO 9826: ISO Parshall Q Q ASTM 1941-91: ASTM Parshall Q Q ASTM D5390-93: Palmer-Bowlus Q Q ISO 4359: U-throat and trapezoidal Q Q USDA-NRCS: H-flume (hybrid) Q Q The measuring point before the flume (H max is the maximum rating of the flume): Q Q ISO 4359: Rectangular long-throat <M = 3-4 H max > Q Q Khafagi <M = 3-4 H max > Note that a calibration certificate should be issued with the relevant gauge verifying the following details: Q Q The specified primary device (f lume or weir) Q Q The standard referenced Q Q Materials of construction Q Q Annotation application Q Q Rating units Q Q The reliability of the trend (R 2 ) and of the forecast of calculating the calibration. The reliability can be determined by the coefficient of determination (R 2 ) and average and maximum deviations in the data set (mm). R 2 is a measure of the goodness of fit of a model. In regression, the R 2 coefficient of determination is a statistical measure of how well the regression predictions approximate the real data points. An R 2 of 1 indicates that the regression predictions perfectly fit the data. CONCLUSION In conclusion, the following sections of ISO 4373:2008 (Hydrometry: Water Level Measuring Devices) should be noted: Q Q A.1.1.2 Materials A staff or ramp gauge is constructed of durable material, able to cope with alternating wet and dry conditions. It resists the accretion of both vegetable and mineral matter. The markings should be resistant to wear or fading. Q Q A.1.1.3 Strengths A staff or ramp gauge is an inexpensive, simple, robust and absolute method of determining water level. It can be uti- lised by relatively unskilled staff. A ramp gauge provides, in addition, the opportu- nity to achieve a higher resolution. Q Q A.1.1.4 Weaknesses A staff gauge can only be used for spot measurements. It is difficult to obtain readings in the field with a true resolution higher than +5 mm. Most staff gauge locations are such that the gauges require regular cleaning. Ramp gauges amplify surges and ripples. While a stilling box may reduce this, it may also introduce a bias due to flow across the gauge.  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