Late Holocene flood magnitudes in the Lower Rhine river valley and upper delta resolved by a two‐dimensional hydraulic modelling approach

Bas Van Der Meulen,Anouk Bomers,Kim M Cohen,Hans Middelkoop

doi:10.1002/esp.5071

Bas Van Der Meulen, Anouk Bomers + Show 2 more

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https://doi.org/10.1002/esp.5071

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Abstract

AbstractPalaeoflood 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.

Highlights

Palaeoflood analysis informs current and future flood risk (St. George et al, 2020; Wilhelm et al, 2019)
In addition to calculating a Late Holocene millennial flood magnitude for the Lower Rhine river, this study demonstrates for the first time the potential and the difficulties of spatially continuous palaeoflood modelling in large lowland areas
Our results further indicate that geological–geomorphological, archaeological and historical inferences on past flood levels show a large range in values and may contain significant errors (Figure 13), and cannot individually constrain palaeoflood magnitudes in alluvial settings

Summary

Introduction

Palaeoflood analysis informs current and future flood risk (St. George et al, 2020; Wilhelm et al, 2019). Palaeoflood analysis informs current and future flood risk Identifying past flooding patterns helps to determine areas at risk under different management scenarios (Alkema & Middelkoop, 2005; Remo et al, 2009), and quantifying past flood magnitudes helps to assess future discharge extremes (Benito & Thorndycraft, 2005; Schendel & Thongwichian, 2017). Many studies on past floods aim to quantify the peak discharge reached during specific events (Herget & Meurs, 2010; Hu et al, 2016; Toonen et al, 2013) or a non-exceeded discharge over a prolonged time period (Enzel et al, 1993; England et al, 2010). Palaeoflood modelling studies have been conducted in geomorphologically stable fluvial settings, such as bedrock canyons, Earth Surf.

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