Reconstructing peak discharges for historic flood levels in the city of Cologne, Germany

Abstract

Historic settlements which have developed in close proximity to rivers often contain numerous descriptions and/or epigraphic markings depicting flood levels relative to historic buildings, most of which are dated and have been inherited from long before the first river gauges were installed. Such evidence of historic flood water levels is regularly used to determine periods of increased flood frequency but, because of several methodological challenges, they are rarely used to calculate palaeodischarge. Due to missing information regarding the historic topography and hydraulic roughness of the floodplain and the river channel, the major problem tends to be the reconstruction of the river channel cross-section. The city of Cologne, located on the Lower Rhine in western Germany, has flood descriptions dating back to around 800 AD. Since the extreme flood event of the year 1342, these descriptions are sufficiently detailed to permit the estimation of water levels. Based on historic etchings, paintings and descriptions it is possible to reconstruct the characteristics of the river channel and floodplain to estimate cross-section areas during flood events. The reconstructions make use of all available data, including estimates of channel incision as well as anthropogenic modification of the river and its floodplain. The mean flow velocity at the time of the historic flood events is estimated by the Manning equation, based on the reconstructed river channel and floodplain. The slope of the water level is assumed to be comparable to recent values. The estimation of hydraulic roughness is challenging since no studies of the hydraulic roughness of developed floodplains have been carried out. Sensitivity studies with different n values within a reliable range of values have been made to estimate the influence of this uncertainty. Finally, the reconstructed data are tested by estimating the peak discharges of recent floods by the application of the described method and comparing the results with measured discharge data from the gauging station at Cologne. This approach offers considerable potential to investigate the relationships between flood dynamics, climate change and land use history over extended timescales.