Reconstruction of the 1374 Rhine river flood event around Cologne region using 1D-2D coupled hydraulic modelling approach

Reconstructions of the most severe historic flood events contribute to improved quantification of design discharges corresponding to large return periods. Reducing the uncertainty of design discharges has a great significance in constructing proper flood defences to protect the hinterland from future flooding. However, reconstructions of the peak discharges of such historic flood events are generally associated with large uncertainties, which arise from the accuracy of the historic topography, hydraulic roughness of the river channels and floodplains, and the historic hydrograph shape. This study sets up a one dimensional-two dimensional (1D-2D) coupled hydraulic model, stretching from the upstream of Bonn at Remagen to downstream of Düsseldorf, Germany, with the length of 113 km to reconstruct the maximum discharge of the 1374 flood event (Qmax,1374), which is considered to be the largest flood of the last millennium in the Lower Rhine catchment. An uncertainty analysis was performed by adopting different river bed levels and roughness values in order to estimate the influence of these uncertainties on the reconstructed peak discharge. The upstream discharge wave was varied corresponding to a wide range of peak discharges from 12,000 to 24,000 m3/s. The resulting Qmax,1374 was determined of between 14,400 and 18,500 m3/s, were then used in a flood frequency analysis to determine the design discharges corresponding to different return periods. Compared to the design discharge computed with previous estimations of the 1374 peak discharge, we found a significant reduction of 2,000 m3/s in the design discharge corresponding to a 100,000 year return period, which is the maximum safety standard adopted in the Dutch water policy for some downstream dike sections.