Uncertainty in design water levels due to uncertain bed form roughness in the river Rhine

Hydrodynamic river models are applied to design and evaluate measures for purposes such as safety against flooding. The modeling of river processes involves numerous uncertainties, resulting in uncertain model results. Knowledge of the type and magnitude of these uncertainties is crucial for a meaningful interpretation of the model results. The aim of this study is to quantify the uncertainty due to the hydraulic roughness predictor for the river bed in the main channel and assess its effect on the uncertainty in the modelled water levels under design conditions. The uncertainty in the bed roughness consists of the uncertainty between the different roughness predictors and the uncertainty within the roughness predictor itself. We compared five roughness predictors, which determine the hydraulic resistance of the river bed in the Dutch river Rhine, based on measurements of bedform characteristics and water levels for different discharges. Generalized Extreme Value distributions have been fitted through the predicted roughness values and extrapolated to the design return period. The results show that the 95% confidence interval for the Generalized Extreme Value distributions the predicted Nikuradse roughness values under design conditions ranges from 0.32 m to 1.03 m. This uncertainty is propagated through the model by means of a Monte Carlo Simulation. The Monte Carlo analysis shows that the uncertainty in the hydraulic roughness results in a 95% confidence interval for the water depth that range from 11.7 m to 12.4 m.