River flood reconstruction in the Lower Rhine valley and delta : Water levels and discharges in past landscape contexts

Abstract

This thesis examines the largest floods of the Lower Rhine river over the past centuries to millennia (Late Holocene to historic time frame). Its central objective is to quantify these events. First, the thesis determines maximum water levels in the past based on sedimentological interpretation of archaeological stratigraphy and geographical survey of epigraphic marks. Next, it provides detailed reconstructions of the alluvial terrain of the Lower Rhine valley and delta, covering the early modern channel bathymetry restored from historic maps and the medieval floodplain topography restored from geomorphological analysis and interpolation. The past landscape context then serves to set up a palaeoflood hydraulic model that resolves the discharges of the largest floods by linking the simulation results to reconstructed water levels. The insights gained in this thesis may ultimately contribute to the design of future flood protection measures.

The overarching findings demonstrate that reliable quantification of past floods in lowland river settings necessitates a two-dimensional approach, when it comes to the required flood level and landscape reconstructions and the discharge calculations. Furthermore, the findings show that palaeoflood research benefits from a multidisciplinary approach, with the primary methodological focus depending on the selected casus and targeted time period. The results include successfully reconstructed pre-instrumental flood levels across the Lower Rhine valley and delta by integrating archaeological, geological, and historical sources (Chapters 2, 3, and 6). Additionally, the results encompass successful reconstructions of the landscape for two key periods: before the onset of river normalisation in the mid-nineteenth century and before the onset of embankment in medieval times (Chapters 4 and 5). Finally, the thesis provides an estimate for the ‘millennial’ discharge in the Late Holocene by numerical flood simulations in the pre-embanked landscape context, resulting in a value of 14,000 m3/s (Chapter 6). Further output of the Floods of the Past, Design for the Future project shows that the landscape context prior to normalisation is crucial to constraining discharge magnitudes of individual historic flood events.