BS EN ISO 748-2021 pdf free download – Hydrometry — Measurement of liquid flow in open channels — Velocity area methods using point velocity measurements.
5.2.1 A permanent station, or one likely to be used frequently for future measurement, shall beprovided with means for demarcation of the cross-section and for determination of stage.
5.2.2The position of each cross-section, normal to the mean direction of flow, shall be defined on thetwo banks by clearly visible and readily identifiable markers. Where a site is subject to considerablesnow cover, the section line-markers may be referenced to other natural objects and, if possible, theposition 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. lf thewater 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 adifference in the level of water surface between the two banks. The mean of the measurements takenfrom 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.
6Measurement of cross-sectional area6.1General
The cross-sectional profile of the open channel at the gauging-site shall be determined at a sufficientnumber of points to establish the shape of the bed and to minimize the uncertainty in the calculation ofthe 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 bymeasuring the horizontal distance from or to a fixed reference point which shall be in the same verticalplane as the cross-section at the measuring site.
6.3Measurement of depth
Measurement of depth shall be made at intervals close enough to define the cross-sectional profileaccurately. The number of points at which depth is to be measured shall be at each vertical wherevelocity 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 thecross-section obtained.
Where it is impracticable to take more than one reading of the depth, the uncertainty in measurementmay be increased (see Clause 9).
When measuring depths with a wire not normal to the surface, see Annex F.BS EN ISO 748 pdf download.