Heavy Rain and Water in the City: Prevention for Municipalities

In short. Heavy rain prevention in the city means knowing where water flows during extreme rainfall, identifying the critical points, and slowing runoff through both structural and planning measures. The path leads from the heavy rain hazard map through structured risk management to concrete measures such as desealing, retention and water-sensitive urban development. Sewer networks alone are not enough: they are usually designed for rain events that recur every one to five years, not for the events that flood basements.

Why this matters for your municipality. Heavy rain can affect any municipality, even without a river nearby. The German Weather Service (Deutscher Wetterdienst) observes that short, very intense rainfall events are increasing. Damage occurs where water collects and has nowhere to go: in low-lying areas, underpasses, at the foot of slopes. A single event such as the one in the Ahr valley (2021) caused around 8.75 billion euros in insured damage alone. For your administration, this means: prevention is cheaper than reconstruction, and it can be planned if you establish the right foundations.

Where does heavy rain prevention concretely start?

It starts with knowledge of how water flows during extreme rainfall. A heavy rain hazard map shows, for defined scenarios (such as a hundred-year or an exceptional event), where water will stand at what depth and how it flows. This is the foundation on which everything else is built. Without this map, you plan blind; with it, you can identify which streets become flow paths and which buildings lie within the hazard zone.

How does the map become an action plan?

The map alone protects no one. You need a structured process that brings together hazard, affected assets and possible measures, and prioritises them. This is exactly what municipal heavy rain risk management provides: it links the hazard map with the vulnerability of assets (hospitals, daycare centres, critical infrastructure) and results in a concept of measures. The DWA code of practice M 119 (Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall, German Association for Water, Wastewater and Waste) describes this approach as the recognised standard.

Which structural approaches reduce the risk?

Three principles work together. First: retain water instead of draining it away quickly, the sponge city principle makes surfaces absorbent. Second: open up sealed surfaces so water can infiltrate, desealing, which also reduces heat. Third: align the entire planning process to be water-sensitive, so that new construction and redevelopment do not worsen runoff. No single approach is sufficient; the combination is what makes it effective.

How can this be financed?

For capital investment measures, the DAS funding guideline (Deutsche Anpassungsstrategie, German Adaptation Strategy) of the Federal Ministry for the Environment, Climate Action, Nature Conservation and Nuclear Safety (BMUKN, project sponsor ZUG) and the federal programme "Adaptation of Urban and Rural Areas to Climate Change" run by the Federal Ministry for Housing, Urban Development and Building (BMWSB) and the Federal Institute for Research on Building, Urban Affairs and Spatial Development (BBSR) are the key federal routes, supplemented by state programmes. Both work with time-limited calls, check the current application window before planning (last reviewed: August 2026). Larger, integrated projects are addressed by the federal programme "Adaptation of Urban and Rural Areas to Climate Change", though its funds are significantly reduced for 2026 to 2028, and demand clearly exceeds supply (last reviewed: August 2026).

The honest limit. Prevention reduces risks, it does not eliminate them. No municipality is fully protected against an extreme event that statistically occurs only every few hundred years. And hazard maps, prioritisation and funding applications are technically demanding and tie up staff that are scarce in many departments. Anyone who underestimates this loses time between insight and implementation.

When KomAdapt is worthwhile. If you do not want to run the individual steps, risk analysis, prioritisation of measures, funding roadmap, impact monitoring, in separate spreadsheets, KomAdapt links them in one continuous process. This is especially worthwhile when several specialist departments are involved and funding is at stake.

_What is the difference between flooding and heavy rain?_ River flooding occurs when a body of water overflows its banks, tied to known watercourses. Heavy rain falls locally and suddenly, often far from any body of water; the water runs off at the surface and collects in low-lying areas. This is why municipalities without a river are also affected. _Is the municipality liable for heavy rain damage?_ There is no general obligation to compensate. Operating the sewer system is a sovereign task; official liability under section 839 of the German Civil Code (BGB) in conjunction with Article 34 of the Basic Law (GG) may apply. The benchmark is the recognised state of the art: for a hundred-year heavy rain event, case law tends to assume force majeure. Only a legal assessment of the individual case can provide a reliable answer. _How long does it take to build up heavy rain prevention?_ From the hazard map to an adopted concept, realistically one to two years. Initial immediate measures can be implemented earlier, in parallel. _Isn't expanding the sewer system enough?_ No. Sewers are economically designed for frequent rain events. A heavy rain event delivers a multiple of that in a short time. This is why modern prevention relies on surface retention, emergency water routes and water-sensitive surfaces in addition to the sewer. _Who in the administration should be responsible?_ Usually the civil engineering department or municipal drainage authority takes the lead, closely coordinated with urban planning, the environmental department and climate adaptation management. A clear lead with a firm mandate is important.

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