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Palaeoflood hydraulic modelling is essential for quantifying 'millennial flood' events not covered in the instrumental record. Palaeoflood modelling research has largely focused on one‐dimensional analysis for geomorphologically stable fluvial settings because two‐dimensional analysis for dynamic alluvial settings is time consuming and requires a detailed representation of the past landscape. In this study, we make the step to spatially continuous palaeoflood modelling for a large and dynamic lowland 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, archaeological and historical sources. This comparison demonstrated a Lower Rhine millennial flood magnitude of approximately 14,000 m3/s for the Late Holocene period before late mediaeval times. This value exceeds the largest measured discharges in the instrumental record, but not the design discharges currently accounted for in flood risk management. [ABSTRACT FROM AUTHOR]

A Chézy-based hydraulic model was run to estimate the magnitude of extreme floods of Middle Holocene age in the Lower Rhine Valley (Germany). Input parameters were gathered from the field and literature, and used in ten scenarios to calculate a best guess estimate for the minimum size of extreme floods. These events have been registered as slackwater deposits on elevated terrace levels and in a palaeochannel fill. The modelled minimum discharge is 13,250 m³ sec-1 for a Middle Holocene flood with an estimated recurrence interval between 1,250 and 2,500 years. A sensitivity analysis on different input parameters enables evaluation of factors which cause the relatively large range in modelled discharges. Understanding the origin of uncertainties in modelled discharges is important for making geologically based calculations of palaeoflood magnitudes important in modern flood frequency analyses, which generally lack information on the magnitudes of rare events. [ABSTRACT FROM AUTHOR]

Climate change may well lead to an increased risk of river floods in the Netherlands. However, the impacts of changes in water management on river floods are larger, either enhancing or reducing flood risks. Therefore, the abilities of water-management authorities to learn that climate and river flows are changing, and to recognize and act upon the implications, are of crucial importance. At the same time, water-management authorities respond to other trends, such as the democratization of decision making, which alter their ability to react to climate change. These complex interactions are illustrated with changes in river flood risk management for the Rhine and the Meuse in the Netherlands over the last 50 years. A scenario study is used to seek insight into the question of whether current water-management institutions and their likely successors are capable of dealing with plausible future flood risks. The scenarios show that new and major infrastructure is needed to keep flood risks at their current level. Such a structural solution to future flood risks is feasible, but requires considerable political will and institutional reform, both for planning and implementation. It is unlikely that reform will be fast enough or the will strong enough. [ABSTRACT FROM AUTHOR]