A multi-disciplinary analysis of the exceptional flood event of July 2021 in central Europe – Part 1: Event description and analysis

Abstract

Abstract. The July 2021 flood in central Europe was one of the five costliest disasters in Europe in the last half century, with an estimated total damage of EUR 32 billion. The aim of this study is to analyze and assess the flood within an interdisciplinary approach along its entire process chain: the synoptic setting of the atmospheric pressure fields, the processes causing the high rainfall totals, the extraordinary streamflows and water levels in the affected catchments, the hydro-morphological effects, and the impacts on infrastructure and society. In addition, we address the question of what measures are possible to generate added value to early response management in the immediate aftermath of a disaster. The superposition of several factors resulted in widespread extreme precipitation totals and water levels well beyond a 100-year event: slow propagation of the low pressure system Bernd, convection embedded in a mesoscale precipitation field, unusually moist air masses associated with a significant positive anomaly in sea surface temperature over the Baltic Sea, wet soils, and steep terrain in the affected catchments. Various hydro-morphodynamic processes as well as changes in valley morphology observed during the event exacerbated the impact of the flood. Relevant effects included, among many others, the occurrence of extreme landscape erosion, rapidly evolving erosion and scour processes in the channel network and urban space, recruitment of debris from the natural and urban landscape, and deposition and clogging of bottlenecks in the channel network with eventual collapse. The estimation of inundation areas as well as the derived damage assessments were carried out during or directly after the flood and show the potential of near-real-time forensic disaster analyses for crisis management, emergency personnel on-site, and the provision of relief supplies. This study is part one of a two-paper series. The second part (Ludwig et al., 2022) puts the July 2021 flood into a historical context and into the context of climate change.

Brief communication: On the extremeness of the July 2021 precipitation event in western Germany

“…Similar observations were made for the July 2021 event (e.g. Mohr et al, 2023). Given the exceptional xWEI values of both the August 2002 and the July 2021 events, it might be worthy to discuss, in the future, whether heavy precipitation events that are extreme across scales could be framed as hydrometeorological compound events.…”

“…Numerous studies have been published since the event, e.g. related to meteorological, hydrological and impact-related aspects (Mohr et al, 2023); hydro-geomorphological processes (Dietze et al, 2022); or early warning (Fekete and Sandholz, 2021;Thieken et al, 2022a). The temporal development of the event (Fig.…”

“…With regard to the extremeness of the event's precipitation, Germany's National Meteorological Service (Deutscher Wetterdienst; DWD hereafter) estimated return periods of well over 100 years for large parts of North Rhine-Westphalia and Rhineland-Palatinate (Junghänel et al, 2021). Mohr et al (2023) reported return periods of more than 700 years over large areas. Junghänel et al (2021) and Dietze et al (2022) highlighted that an important feature of the July 2021 precipitation event was not only high rainfall accumulation at a large spatial extent but also the occurrence of high return periods at various durations as well as a remarkable heterogeneity in space.…”

The risk may not be limited to the flooding – Polluted high flood sediments pose a health threat to the unaware public

“…While research [88, [94][95][96] and reports [97] focus on the shocking amounts of debris transported by the rivers, ultimately exacerbating the event by clogging bridges that failed later on, little attention has been paid to the ne fraction of the ood sediments. However, as the transport capacity for ne materials is particularly high, they are spread over larger areas.…”

The climatology and nature of warm-season convective cells in cold-frontal environments over Germany

“…Taszarek et al (2019) showed based on European Severe Weather Database reports (ESWD; Dotzek et al, 2009) that much of Germany reports 8 or more days per year with heavy convective precipitation (their Figure 7). Recent events such as the exceptional floods of Central Europe in July 2021 (Mohr et al, 2023), which was exacerbated by embedded convection, highlight the need for further information on the underlying processes that lead to heavy convective precipitation.…”