In mid-July 2021, a persistent low-pressure system caused extreme precipitation in parts of the Belgian, German and Dutch catchments of the Meuse and Rhine river. This led to record-breaking water levels and flooding at many locations. Comparable heavy precipitation events in this area have never been registered in most of the affected areas before [1].
The flood did not hit the three affected countries with the same magnitude; nevertheless, some key aspects can be considered common room for improvement. Almost a year later, it is crucial to investigate the lessons to be learned from an event that demonstrates the need to prepare for the unimaginable [5].
Communication before and during the event
On the occasion of the hazardous event, the early warning chains failed, due to numerous reasons and with heavier consequences in Germany. One major factor is represented by the dependency of all systems on electricity, digital broadcasting, mobile phone and radio stations, etc. Moreover, in the aftermath, especially in the most exposed areas, people expressed frustration about not having received warnings or not understanding that heavy rain warnings in the news meant they needed to evacuate their houses. This reveals the demand for knowledge exchange, its management and documentation, even between direct neighbours, both in peacetime and during emergency management. The European flood directive helps research and local communities by pushing the European countries to produce flood hazard and risk maps and accordingly, management plans. But in some cases, as it is in Germany, certain data are still not openly accessible due to the ongoing conflict of autonomy between the 16 Bundesländer and the National State [3].
Infrastructures
Early warning systems are part of the critical infrastructures. They are just as dependent on power and roads to access them for repair as many other infrastructural systems. Key features of resilience such as redundancies must be kept or created.
Another key aspect concerning land use and infrastructure is represented by the significant changes that have occurred over decades. For example, the built infrastructure has narrowed the riverbeds and soil sealing has reduced infiltration rates [3]. As the importance of the hydrologic and ecological functions of floodplains becomes better understood, there are increasing calls to restore the connectivity of floodplains (such as the case of the Dutch “Room for the River” Programme), also considering that structural measures tend to be rigid and not easily adapted to increased flooding regimes resulting from environmental change [2].
Risk awareness
When the flood surprised the people living in flood-prone areas along the Rhine and the Meuse rivers, many of them had never received information about individual disaster preparedness, private precaution, and stockpiling for outages of critical infrastructures. All four priorities of the Sendai Framework were relevant, also in this disaster: there still is a major demand for knowing more about risk, especially amongst the breadth of society unfamiliar with such situations, despite all media coverage of similar disasters worldwide on an almost weekly scale [3].
The consciousness of flood risks and through this also personal precautions are strengthened through information, training and the raising of awareness [4]. Possibly, risk awareness and what to do in a crisis should be taught in schools [5].
Flood forecasting
Flood modelling is one of the key elements to take into account when it comes to decision-making. Nevertheless, event forecasting is largely based on historical data, which is not a good guide to future events [5]. Then, merely focusing on flood extent and depth, as output from the models, is not sufficient to estimate the impacts of extreme flood events on infrastructure. In particular, in Germany and Belgium, it became evident that the high flow velocities are a decisive factor in explaining the degree of destruction. Many of the observed failures such as bridge scour, road embankment instabilities, and erosion of aggregate foundations could likely better be explained by flow velocity rather than flood depth. Future flood impact studies should aim to account for flow velocity in their impact modelling. Furthermore, local-scale studies are essential to better understand the real impacts (and are also better able to do so). This is true for both the consequences to infrastructure assets and the services and the impacts on lives and livelihoods [1].
Conclusions
In the field of flood risk management, many efforts have already been made and are being made, as in the case of the ICPR-Programme “Rhine 2040”, whose overarching objective is to reduce the flood risks on the Rhine and its major tributaries by at least 15% by 2040 compared to 2020. To do so, for instance, the awareness of the population of the flood risk has, among others, been increased by the full-coverage publication of flood hazard and flood risk maps, such as the ICPR Rhine Atlas. Moreover, the adverse consequences during a flood are to be reduced by raising awareness of the need for risk-appropriate behaviour in the event of an incident, improving flood announcement systems and flood forecasting [4].
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[1] Koks, E., Van Ginkel, K., Van Marle, M., and Lemnitzer, A. - Brief Communication: Critical Infrastructure impacts of the 2021 mid-July western European flood event, Nat. Hazards Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/nhess-2021-394, in review, 2021.
[2] Serra-Llobet A, Jähnig SC, Geist J, Kondolf GM, Damm C, Scholz M, Lund J, Opperman JJ, Yarnell SM, Pawley A, Shader E, Cain J, Zingraff-Hamed A, Grantham TE, Eisenstein W and Schmitt R (2022) -Restoring Rivers and Floodplains for Habitat and Flood Risk Reduction: Experiences in Multi-Benefit Floodplain Management From California and Germany. Front. Environ. Sci. 9:778568. doi: 10.3389/fenvs.2021.778568
[3] Fekete, A.; Sandholz, S. - Here Comes the Flood, but Not Failure? Lessons to Learn after the Heavy Rain and Pluvial Floods in Germany 2021. Water 2021, 13, 3016. - https://doi.org/10.3390/w13213016
[4] International flood risk management plan for the International River Basin District “Rhine” (Part A) (catchment areas > 2.500 km²)
[5] https://www.climatechangenews.com/2021/08/20/lesson-german-floods-prepare-unimaginable/