ISO 748:2021 pdf download – Hydrometry — Measurement of liquid flow in open channels — Velocity area methods using point velocity measurements
5.2 Demarcation of site 5.2.1 A permanent station, or one likely to be used frequently for future measurement, shall be provided with means for demarcation of the cross-section and for determination of stage. 5.2.2 The position of each cross-section, normal to the mean direction of flow, shall be defined on the two banks by clearly visible and readily identifiable markers. Where a site is subject to considerable snow cover, the section line-markers may be referenced to other natural objects and, if possible, the position noted using a global navigation satellite system (GNSS). 5.2.3 The stage shall be read from a gauge at the start and end of the measurement period. If the water level changes rapidly, a level measurement is recommended to be taken at least every 30 min. 5.2.4 An auxiliary gauge on the opposite bank shall be installed where there is likelihood of a difference in the level of water surface between the two banks. The mean of the measurements taken from the two gauges shall be used as the mean level of the water surface and as a base for the cross- sectional profile of the stream. 6 Measurement of cross-sectional area 6.1 General The cross-sectional profile of the open channel at the gauging-site shall be determined at a sufficient number of points to establish the shape of the bed and to minimize the uncertainty in the calculation of the cross-sectional area. 6.2 Measurement of width Measurement of the width of the channel and the width of the individual segments shall be obtained by measuring the horizontal distance from or to a fixed reference point which shall be in the same vertical plane as the cross-section at the measuring site.
6.3 Measurement of depth Measurement of depth shall be made at intervals close enough to define the cross-sectional profile accurately. The number of points at which depth is to be measured shall be at each vertical where velocity is measured. The number of sampling verticals depends on the variability of the water depth in the cross-section. This number is adequate when the number of points does not significantly change the value of the cross-section obtained. Where it is impracticable to take more than one reading of the depth, the uncertainty in measurement may be increased (see Clause 9). When measuring depths with a wire not normal to the surface, see Annex F.