Stage-Discharge Rating Curves

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Stage-Discharge Rating Curves

Building Better Stage-Discharge Rating Curves

Dr. Amartya Kumar Bhattacharya BCE (Hons.) ( Jadavpur ), MTech ( Civil ) ( IIT Kharagpur ), PhD ( Civil ) ( IIT Kharagpur ), Cert.MTERM ( AIT Bangkok ), CEng(I), FIE, FACCE(I), FISH, FIWRS, FIPHE, FIAH, FAE, MIGS, MIGS – Kolkata Chapter, MIGS – Chennai Chapter, MISTE, MAHI, MISCA, MIAHS, MISTAM, MNSFMFP, MIIBE, MICI, MIEES, MCITP, MISRS, MISRMTT, MAGGS, MCSI, MMBSI Chairman and Managing Director, MultiSpectra Consultants, 23, Biplabi Ambika Chakraborty Sarani, Kolkata – 700029, West Bengal, INDIA. E-mail: dramartyakumar@gmail.com

Stream discharge is, arguably, the single most valuable environmental variable required for the effective management of food supply, energy generation, industrial production, transportation, health, and for the protection of global ecosystems. It is also one of the most difficult variables to measure and monitor on a continuous basis in natural streams and rivers.

The derivation of an empirical relation between stage (i.e. water level) and discharge (i.e. streamflow) is fundamental to the production of almost all information about fresh water quantity. This relation can be explained from first principles. Civil Engineers have progressively simplified the theory into equations: first to explain the relevant variables (e.g. the Bernoulli equation), then to collapse them into terms that explain the majority of the variance, assuming the physical properties of freshwater are constants (e.g. the Manning equation) and finally to reduce the equation to a univariate form (the stage-discharge equation) valid for steady, uniform flow conditions.

Civil Engineers are skilled at unveiling the truth from scatter plots of stage and discharge measurements. Armed with a set of working hypotheses that explain not only the underlying form, but also all deviations from that basic shape, Civil Engineers are able to build better rating curves. They are able to efficiently perform, explain and defend their work.

A true curve will hold its shape as the density of rating measurements increases and will predict accurately in extrapolated zones such that new data outside the previously calibrated range is likely to also agree with the curve. Any residuals will make intuitive sense: for example, if a rating measurement is affected by backwater it will plot left of the curve while a measurement during rapidly rising stage will plot to the right. Informative residuals are essential for accurate modelling of the dynamic processes governing flow in a natural channel.

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