ISO 748-2021 pdf free download – Hydrometry – Measurement of liquid flow in open channels – Velocity area methods using point velocity measurements (ISO 748:2021).
A measuring site shall be chosen conforming to the specified requirements.
The cross-sectional area shall be measured by a method specified in this document, appropriate to thedimensions.
Velocity observations shall be made by a method specified in this document.The discharge shall be calculated by a method specified in this document.5 Site selection
5.1 Selection of site
The site selected should conform to the following requirements.
a) The channel at the measuring site shall be straight and of uniform cross-section and slope in order to minimize abnormal velocity distribution. The straight length should be at least six times thewidth of the channel upstream, and at least three times the downstream width.
b) Flow directions for all points on any vertical across the width shall be parallel to one another and at right angles to the measurement section.
c)The bed and margins of the channels shall be stable and well defined at all stages of flow in order to facilitate accurate measurement of the cross-section and ensure uniformity of conditions duringand between discharge measurements.
d)The curves of the distribution of velocities shall be regular in the vertical and horizontal planes of measurement.
e]Conditions at the section and in its vicinity shall also be such as to preclude changes taking place in the velocity distribution during the period of measurement.
f)Sites displaying vortices, reverse flow or dead water shall be avoided.
g)The measurement section (including approach and exit) shall be clearly visible across its width and unobstructed by trees, aquatic growth or other obstacles.
h) When gauging from a bridge with divide piers, each section of the channel shall be measured separately.Particular care shall be taken in determining the velocity distribution when bridgeapertures are surcharged or obstructed.
i) The depth of water at the section shall be sufficient at all stages to ensure whichever device isdeployed it conforms to the manufactures minimum criteria for use.
j)If the site is to be established as a permanent station, it shall be easily accessible at all times with all necessary measurement equipment appropriate to the flow conditions.
k) The section shall be sited away from pumps,sluices and outfalls, if their operation during a measurement is likely to create unsteady flow conditions.
l)Sites where there is converging or diverging flow shall be avoided.
m) lf a suitable straight section includes a bridge, wading and boat measurements shall be made away from the effects of the bridge.
n) The measurement of flow under ice cover is dealt with in Annex E.For streams that are subject to formation of ice cover, the main part of this document shall be used when the stream is freeflowing.
o) It may, under certain conditions of river flow or level, prove necessary to carry out measurements on sections upstream or downstream of the original chosen location. This is quite acceptable if there are no substantial unmeasured losses or gains to the river in the intervening reach and so long as all flow measurements can be related to any stage value recorded at the principal reference section.ISO 748 pdf download.