Two‐dimensional hydrodynamic models are frequently used for flood protection studies to compute inundation patterns and to estimate potential economic damage. However, the modelling of river processes involves numerous uncertainties. Knowledge of the type and magnitude of uncertainties is crucial for a meaningful interpretation of the model outcomes and its usefulness in decision making. The objective of this study was to quantify the uncertainty in the design water levels due to uncertain bed form and vegetation roughness for the Dutch River Waal. We quantified and combined these sources of uncertainty by means of a Monte Carlo simulation. The results showed that the 95% confidence interval of the design water levels is 0.49 m, 0.34 m and 0.12 m for bed form, vegetation classification and vegetation roughness parameterisation, respectively, and approximately 0.68 m for the combined roughness. These uncertainties are significant in view of Dutch river management practice.
Structures integrated in a grass-covered dike may increase erosion development. Currently, safety assessment methods for flood defences are only applicable for a conventional grass-covered dike and the effects of structures on dike cover erosion are poorly understood. Since many dikes have a road on top, it is important to study the effect of such a road structure on erosion onset during wave overtopping. To investigate this effect, a coupled hydrodynamic-erosion model was developed. The erosion onset caused by overtopping waves was predicted by combining the time-varying bed shear stresses from the hydrodynamic model with a depth-dependent erosion model. The results show that roads on top of a dike increase the erosion of the neighbouring grass cover. This increase in erosion may have a negative impact on dike stability. Therefore, we recommend considering effects of constructions on top of dike profiles during safety assessments. Explicitly, consideration of the roughness transitions in the safety assessments of dikes is recommended.Electronic supplementary material The online version of this article (https://doi
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