Unsteady flow discharge measurement over sharp-crested weirs: Insights from a comprehensive experimental dataset

Sharp-crested rating curves, traditionally derived from steady-flow conditions, have been used for discharge measurements in both steady and unsteady flow scenarios. Although numerous studies have employed the reservoir drawdown method to calibrate large weirs since the 1960s, data on flow measurements during the passage of hydrographs remain scarce. This article investigates the behavior of unsteady flow over rectangular and triangular sharp-crested weirs under various hydrograph shapes with differing intensities of unsteadiness. Two experimental setups were constructed to collect the data and to observe the performance of these weirs. The collected data were then used to evaluate the accuracy of applying steady-flow rating curves for discharge measurements under unsteady flow conditions. A new index, the Relative Loop Width Index (RLWI), is introduced to quantify the effects of unsteadiness on discharge measurements and the associated errors when utilizing conventional steady flow rating curves. The findings indicate that using steady-flow rating curves in unsteady conditions, which are dominant in field conditions, can result in errors in water volume estimation ranging from +10 % to −20 %. Furthermore, steady-flow rating curves applied to flow measurements during hydrograph passage tend to overestimate peak flows and predict earlier occurrence of peak times. Sensitivity analysis reveals that weir geometry and reservoir capacity significantly influence measurement errors.