Rating Curves: Part 1 – Correction for Surface Slope

Rating curves are the means by which readings of surface elevation at gauging stations on rivers are converted to discharges. Their implicit assumption of a unique relationship between stage and discharge overlooks some of the hydraulics of the problem by ignoring the effects of surface slope. This paper describes a method for allowing for possible variable slope by incorporating time derivatives of the stage readings, providing an extension to the Jones method. In many cases the correction is indeed not necessary. To this end, formulae are provided for calculating when such corrections should be made. An even sounder procedure would be, however, always to measure the slope of the surface at stations and, rather than developing and using rating curves (stagedischarge curves), stage-conveyance curves would be generated and used so that the method used to calculate flows would be based on the actual hydraulics of each situation.