Evaluation of design hydrograph methods and probabilistic methods for estimating design water levels on the river Meuse

Design water levels are needed for the design of flood defences along the Dutch river Meuse. Current practise is that these water levels are determined by hydrodynamic simulation of a standard design hydrograph at the upstream gauging station Borgharen. The peak discharge of this synthetic hydrograph is based on a frequency analysis of the 100-year discharge dataset and its shape is determined by scaling and averaging all hydrographs in the dataset. Subsequently it is assumed that the simulated water levels along the river have the same return period as the peak discharge at Borgharen. This paper evaluates the accuracy of the current method and alternative methods to determine design water levels, using a dataset of 50,000 years of generated discharges at the station Borgharen based on extrapolation of rainfall events. The alternative methods include both design hydrograph methods and new methods that incorporate the hydrograph shape in a probabilistic way. The peak curvature is used as second stochastic variable in the probabilistic methods. The methods are evaluated by comparison with a reference set composed of 1D hydrodynamic simulations of all floods in the 50,000 year dataset. The results show that the currently used standard hydrograph method overestimates the design water levels up to 35 cm with respect to the reference. Alternative methods yield more accurate design water level estimates. The results obtained show no significant difference in accuracy between the probabilistic methods and the vertically averaged design hydrograph method