A dynamic rating curve approach to indirect discharge measurement

摘要

The operational measurement of discharge in medium and large rivers ismostly based on indirect approaches by converting water stages intodischarge on the basis of steady-flow rating curves. Unfortunately, underunsteady flow conditions, this approach does not guarantee accurateestimation of the discharge due, on the one hand, to the underlying steadystate assumptions and, on the other hand, to the required extrapolation ofthe rating curve beyond the range of actual measurements used for itsderivation.Historically, several formulae were proposed to correct the steady-statedischarge value and to approximate the unsteady-flow stage-dischargerelationship. In the majority of these methods, the correction is made onthe basis of water level measurements taken at a single cross section wherea steady state rating curve is available, while other methods explicitlyaccount for the water surface slope using stage measurements in tworeference sections. However, most of the formulae available in literatureare either over-simplified or based on approximations that prevent theirgeneralisation. Moreover they have been rarely tested on cases where theiruse becomes essential, namely under unsteady-flow conditions characterisedby wide loop rating curves.In the present work, an original approach, based on simultaneous stagemeasurements at two adjacent cross sections, is introduced and compared tothe approaches described in the literature. The most relevant feature isthat the proposed procedure allows for the application of the full dynamicflow equations without restrictive hypotheses. The comparison has beencarried out on channels with constant or spatially variable geometry under awide range of flood wave and river bed slope conditions. The results clearlyshow the improvement in the discharge estimation and the reduction ofestimation errors obtainable using the proposed approach.