If a velocity index rating is used

If a velocity index rating is used to derive mean-velocity and thus determine discharge (see 8.1), then the methods for deriving uncertainties in discharge estimates using velocity index ratings should be applied.If the mid-section or mean-section methods (see 8.2.2 and 8.2.3) are used to compute the discharge, the uncertainty for the depth of each slice should ideally be treated separately. However, in reality it is usually sufficient to consider the uncertainty in the overall depth measurement relative to mean bed level in the measuring cross-section. As modern water-level measuring equipment should be capable of measuring and recording the water level to within ±3 mm to ±5 mm (at two standard deviations), thelargest component of the overall depth uncertainty is usually the uncertainty in the determination of mean bed level.The above equation should be applied to at least two or three different flow conditions, e.g. low, medium and high, since the uncertainty varies with stage and velocity, and is also dependent on the number of operational flight paths.