Received: 9 November 2020 Revised: 6 January 2021 Accepted: 7 January 2021 DOI: 10.1002/esp.5071 RESEARCH ARTICLE Late Holocene flood magnitudes in the Lower Rhine river valley and upper delta resolved by a two-dimensional hydraulic modelling approach Bas van der Meulen1 | Anouk Bomers2 | Kim M. Cohen1 | Hans Middelkoop1 1Faculty of Geosciences, Utrecht University, Princetonlaan 8a, Utrecht, CB, 3584, the Abstract Netherlands Palaeoflood hydraulic modelling is essential for quantifying ‘millennial flood’ events 2 Faculty of Engineering Technology, University not covered in the instrumental record. Palaeoflood modelling research has largely of Twente, Dienstweg 1, Enschede, ND, 7522, the Netherlands focused on one-dimensional analysis for geomorphologically stable fluvial settings because two-dimensional analysis for dynamic alluvial settings is time consuming and Correspondence Bas van der Meulen, Faculty of Geosciences, requires a detailed representation of the past landscape. In this study, we make the Utrecht University, Princetonlaan 8a, 3584. CB step to spatially continuous palaeoflood modelling for a large and dynamic lowland Utrecht, the Netherlands. Email:
[email protected] area. We applied advanced hydraulic model simulations (1D–2D coupled set-up in HEC-RAS with 950 channel sections and 108 × 103 floodplain grid cells) to quantify the extent and magnitude of past floods in the Lower Rhine river valley and upper delta. As input, we used a high-resolution terrain reconstruction (palaeo-DEM) of the area in early mediaeval times, complemented with hydraulic roughness values. After conducting a series of model runs with increasing discharge magnitudes at the upstream boundary, we compared the simulated flood water levels with an inventory of exceeded and non-exceeded elevations extracted from various geological, archae- ological and historical sources.