The effect of hydraulic roughness on design water levels in river models

Accurate estimates of design water levels are essential, because they determine the required dimensions of the flood defences. Hydrodynamic models are used for the prediction of flood water levels to support flood safety and are often applied in a deterministic way. However, the modelling of river processes involves numerous uncertainties. Literature has shown that the hydraulic roughness is one of the main sources of uncertainty in hydrodynamic computations. Knowledge of the type and magnitude of uncertainties is crucial for a meaningful interpretation of the model outcomes and their usefulness in decision making. We show that the uncertainty of a complex model factor, such as the hydraulic roughness, can be quantified explicitly. The hydraulic roughness has been unravelled in separate components, which have been quantified separately and then combined and propagated through the model. Expert opinions revealed that the uncertainty due to bed form roughness in the main channel and vegetation roughness in the floodplains were shown to be the major contributors. Quantifica-tion of these sources and propagation through the WAQUA model using Monte Carlo analyses showed that this resulted in a 95% confidence interval around the Design Water Levels (DWL) of 68 cm for the river Waal. This uncertainty range consisted of the uncertainty due to bed forms with an uncertainty of 49 cm on the DWL, and an uncertainty of 34 cm due to vegetation roughness uncertainty. The main source of uncertainty was shown to be the variability of the roughness values depending on the various possible roughness models.