Uncertainty of design water levels due to combined bed form and vegetation roughness in the Dutch River Waal

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.