Reconstructing floodplain topography and river bathymetry of the Lower Rhine for palaeoflood hydraulic studies

Sedimentary and documentary archives for the Lower Rhine valley (Germany and the Netherlands) show that floods in medieval times were among the largest in Late Holocene and historic times. A few events from the 7-8th cy and 14th cy carried significantly higher peak discharges than any measured flood. In our project we use these historic cases in geomorphological analyses and two-dimensional hydraulic modelling to assess the extents and magnitudes of these past extreme floods. Doing so requires a suitably-detailed reconstruction of floodplain topography and river geometry, which we produce using various geomorphological, geological, historical-geographical and archeological data. In order to create a palaeo-DEM of the floodplain, recent morphological elements were identified and removed from high-resolution LiDAR data of Nordrhein-Westfalen and the Netherlands. For this step we applied two different methodologies. First, by using OpenStreetMap to identify, buffer and remove anthropogenic features such as embankments and elevated highways. Second, by subtracting trend surfaces from the original DEM, converting outlying areas to shapes, and identifying and removing all shapes consisting of non-natural elements. The latter requires a morphologically-trained eye, but gives more complete results. The early historic channel geometry of the Rhine was reconstructed using existing geological data sets, providing locations where deposition took place in the past millennia and channel belt end ages. This information was combined with existing work from historians focusing on river courses in Roman and medieval times. These involve reconstructions based on descriptive and archaeological data, which often provide detailed information at a scale of one to a few meander bends. By integrating the sedimentary archive with these local studies, our reconstruction makes maximum use of all kinds of available information.