Reconstruction of the 1374 Rhine river flood event using a 1D-2D hydraulic modelling approach

Reconstruction of the most severe historic flood events contributes to a better quantification of design discharges corresponding to large return periods. This has a great significance in constructing flood defences to
protect the hinterland from future flooding. However, the reconstructed peak discharge values corresponding
to these historic flood events often have a large uncertainty related to the accuracy of the historic topography
and hydraulic river and floodplain roughness. The 1374 flood event is considered the largest flood of the last
millennium in the Rhine river. This study sets up a 1D-2D coupled hydraulic model for a study area stretching from Andernach to Haus Burgel (Germany) to reconstruct the maximum discharge of the 1374 flood event.
The historic topography of the main river and floodplains corresponding to the year 1374 was extracted from
a high-resolution Paleo-DEM reconstructed for the Lower Rhine catchment for 800 AD (van der Meulen et al.,
2020). The hydraulic roughness for land cover classes corresponding to the Paleo situation for 800 AD was also
used as the input data for this model. We performed an uncertainty analysis with different river bed levels
and roughness values to estimate the influence of these uncertainties on the reconstructed peak discharge. The
upstream discharge wave was also varied. The simulated flood water levels were compared with the 1374 flood
marks. Based on this comparison, the discharge magnitude was determined to be between 12,800-21,400 m3
/s,
with a best estimate between 14,000-18,300 m3
/s. These best estimate values were used in a flood frequency
analysis to determine the design discharges corresponding to different return periods (Bomers et al., 2019). A
significant reduction of 2000 m3
/s in the design discharge was found corresponding to a 100,000 year return
period.