Roughness uncertainty analysis in river flooding using HEC-RAS model

Although flood maps based on the deterministic approach play an important role in minimizing flood losses, there is considerable uncertainty in calculating the level of water inundation. Roughness is a key parameter in water surface elevation. Since roughness is not easily measurable and is estimated based on experimental and laboratory methods, it introduces a significant degree of uncertainty into the model. Therefore, in this paper, a probabilistic framework based on the Monte Carlo simulation approach is used to analyze the uncertainty of the roughness coefficient using the HEC-RAS model. In this framework, a computational module in Visual Basic language was introduced to communicate with the HECRASController to automatically run 84,000 Monte Carlo simulations within 7.5 km of the Karde River. In each performance, 1000 Monte Carlo simulations were performed in 84 separate cross-sections based on the Manning roughness probability distribution and the results of roughness uncertainty at water elevation were presented at 99%, 95%, 50%, 5%, and 1% confidence levels. The results indicate a significant effect of roughness on the water surface elevation uncertainty at 90% confidence level so that changes in water surface elevation in many cross-sections reach more than 1 meter. Comparing the cross-sections with the minimum and maximum uncertainty bands, show that the cross-sections with higher lateral gradients have more uncertainty and the V-shaped cross-sections with the lower lateral gradients have fewer uncertainty bands. Also, the results of sensitivity analysis to select the optimal number of simulations indicate that the bandwidth of uncertainty is not affected by the number of simulations of more than 1000 runs. The presented results based on the deterministic approach and increasing the reliability and improving the performance of the model based on the uncertainty of roughness in the Karde River can be used in the design of rainwater catchment.