Dealing with extreme floods in densely populated areas

Extreme floods are increasing in times of climate change. Flood prevention must consistently take this into account, especially in densely populated areas. The flood event in 2021 in western Germany and also extreme floods in many other regions of the world have shown how necessary this is.

In the densely populated Emscher region with 2.7 million inhabitants, between Dortmund and Duisburg, this problem must be addressed just as urgently. By building efficient flood protection systems, the Emschergenossenschaft has enabled one of Germany's largest metropolitan regions to emerge. In accordance with legal requirements, the flood protection systems in the Emscher region are designed up to an HQ200. Nevertheless, extreme precipitation events can lead to extreme floods that exceed the capacity of the flood protection systems. Here, more far-reaching measures must be initiated, which are brought together in the program "Roadmap Crisis Flood". It was set up in March 2022 with an investment volume of around €500 million over 15 years. To strengthen flood resilience in a changing climate, five action areas have been identified:

1. The creation of additional retention areas requires a different approach to undeveloped land than in the past. This applies in particular to the realization of easements to enable the flooding of emergency polders. Also, more space must be made available for the river in order to be able to slow down the flood wave through larger floodplain areas.

2. An essential field of action is adaptation. This applies to flood protection facilities in order to avoid total failure during extreme events with major damage. Bridges must also be adapted in terms of their performance in order to make backwater effects as manageable as possible. Last but not least, the buildings of the Emschergenossenschaft must be protected against heavy rain and flood damage in accordance with the floodlabel.

3. The increase of heavy rainfall events is causing flood events with faster flood waves. For this reason, flood forecasting must be more highly resolved in its temporal and spatial discretization. The flood forecasts currently available for the Emscher will be extended to more tributaries. The gauging network will have to be expanded and made more flood-resilient for this purpose. A high frequency of forecasts must be available, especially for small catchments.

4. Flood risk management is joint tasks with distributed responsibilities. Here, functioning communication and cooperation are essential. In order to support this, relevant information on the flood situation is made available via a portal for public agencies and the population. This information must be understood and lead to adequate actions. Dealing with uncertainties of the flood forecast is of particular importance here.

5. The question arises whether the preparation for extreme events above the design limit can be handled sufficiently well with the legal framework available today. Ideas for improving further legal and political frameworks at federal and state level are discussed.

It is obvious that the deliberate handling of extreme events leads to actions that need to go beyond the existing FRM. These are presented for discussion.