Five years on from the 2021 floods: what we’ve learnt
In July 2021, Southern Limburg, the Netherlands, was hit by an exceptional amount of rainfall. Streams and rivers burst their banks, homes and businesses were flooded, and infrastructure was damaged. In Belgium and Germany, the consequences were even more severe and there were casualties. Now, five years on, we have a better understanding of how this event came about, how water systems responded, and what lessons we can learn from it.
Extreme weather with far-reaching consequences
The floods were caused by a system of exceptionally heavy rainfall for this region. In an area roughly half the size of the Netherlands, more than 110 millimetres of rain fell in less than three days. Locally, more than 200 millimetres fell in just two days.
It was precisely the combination of extreme rainfall and a large affected area that made the event so disruptive. Following the floods, Deltares investigated how the water systems in the catchment areas of the Geul, Geleenbeek and Roer each responded in their own way to these exceptional circumstances.
The 2021 floods demonstrate how important it is to consider the entire water system.
“The 2021 floods demonstrate just how important it is to consider the entire water system. Extreme rainfall affects not just a stream or river, but entire catchment areas. By better understanding how water moves through an area, we can take measures to limit the consequences of future floods,” says Nathalie Asselman, a water safety expert at Deltares.
The research also highlighted the importance of the landscape’s natural ‘sponge’ effect. Soils, grasslands, and forests can retain large quantities of water and release it gradually. It's important to preserve and expand the soil’s existing ‘sponge’ effect.
Measures such as providing more space for water storage, restoring stream valleys to their natural state and reducing hard surfaces also help to lower river flows and water levels. Furthermore, the analyses underline the importance of water and soil as a starting point for spatial planning. Not all flooding can be prevented, but by taking an area’s natural characteristics into account, risks can be reduced.
Not just a Limburg issue
Deltares also investigated what would happen if a similar rainfall pattern were to occur over other parts of the Netherlands, for example in South Holland. Supra-regional stress tests have identified where water accumulates, where drainage systems are inadequate and where cascades of problems could arise. Other regions in the Netherlands are also vulnerable to extreme rainfall.
In low-lying polder areas, water can remain standing for long periods because pumping stations cannot drain such volumes of water quickly enough. In areas with natural drainage, on the other hand, streams and small rivers can quickly burst their banks. The consequences can be severe. Roads, rail links, tunnels, and vital infrastructure may fail, whilst emergency services find it more difficult to access affected areas. In some scenarios, the damage amounts to more than one billion euros.
Water knows no boundaries
The floods of 2021 also served as a reminder that water pays no heed to municipal, provincial or national borders. What happens upstream can have consequences dozens of kilometres away.
This underlines the importance of considering the entire catchment area when taking measures, rather than just the specific area under the responsibility of a single manager or government body. It also highlights the importance of cooperation across national and regional borders.
International cooperation
The events of 2021 were one of the factors that led to the launch of the JCAR ATRACE cooperation programme. As part of this programme, Deltares is working with European research organisations and public authorities to develop a better shared understanding of flood and drought risks in cross-border regional river basins.
During an international workshop held in Belgium and Germany in April this year, scientists, policymakers and practitioners shared their experiences of post-flood recovery.
It became clear once again that recovery is not just about infrastructure and buildings, but above all about the people and communities who have to rebuild their lives.
It also became clear just how important (but also how complex) it is to use recovery measures to better prepare areas for future extremes: ‘build back better’.
Better prepared for the future
The past five years have led to new insights into extreme rainfall, improved forecasting systems, supra-regional stress tests and a better understanding of recovery processes. There has also been increased focus on extreme events with a low probability but potentially major societal consequences.
The key lesson remains that extreme rainfall cannot be entirely prevented, but with better warnings, smart spatial planning decisions, cooperation within and across river basins, and greater awareness amongst residents, businesses and public authorities, the consequences can be significantly mitigated.
