Could a one-size-fits-all approach apply to the extension of stage-discharge relationships at flow-gauging weirs?

At a flow-gauging weir, the stage or flow depth is normally measured continuously and converted into discharge using a stage-discharge (SD) rating curve (RC). During flood events, the observed water levels often exceed the flow-gauging weir's designed measuring capacity or structural limit. Subsequently, the standard calibration of the flow-gauging weir becomes irrelevant and the extension of the SD RC for above-structure-limit flow conditions is required. This paper attempts to identify a one-size-fits-all approach for the extension of SD RCs by assessing seven indirect hydraulic extension methods and a one-dimensional HEC-RAS modelling approach against direct SD measurements or extension methods at selected flow-gauging sites in South Africa. In considering a ranking-based selection procedure and associated goodness-of-fit (GOF) criteria, the stepped backwater analysis, slope-area method, and 1-D HEC-RAS steady flow modelling proved to be the most appropriate. The other indirect extension methods resulted in larger statistical differences between the at-site benchmark and modelled values. Given that the extension of RCs is significantly more affected by the site and river reach geometry, initial hydraulic conditions, flow regimes, and level of submergence at high discharges than the actual extension method used, it was confirmed that a one-size-fits-all approach cannot be applied for the extension of SD RCs in South Africa.