Although extensive researches have been conducted in the analytical and numerical analysis of flow through rockfill structures, one of the main issues in these analyses which still needs attention is uncertainty quantification. In this research, first, it was investigated and shown that the nature and source of uncertainty in rockfill hydraulic parameters can be different indicating random and fuzzy parameters. Then, uncertainty analysis of gradually varied flow computations in rockfill structures was conducted using proposed hybrid and transformation methods in order to quantify the uncertainty in computed water surface profiles resulting from uncertainty in the model hydraulic parameters. Although the proposed methods can be applied to any equation that quantifies the head loss in flow through rockfill structures, two commonly used equations, i.e., Stephenson and Wilkins equations, were used in this study. The methods were applied to a laboratory-scale physical model, and the numerical results were compared with the corresponding measured water surface levels. Three main conclusions drawn from this study were: (1) the results of the two different uncertainty analysis methods did not show significant differences, but the transformation method was rather simple and computationally efficient in comparison with the hybrid method, (2) for both Wilkins and Stephenson equations, the experimental data were within the 90% confidence intervals resulting from both uncertainty analysis methods and (3) the proposed approach and methodologies in this study can be used by hydraulic engineers in analysis and design of real-life rockfill structures.

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