The Sponge City Principle: Rainwater as a Resource

In short. The sponge city principle ("Schwammstadt") reverses classic drainage: instead of channelling rainwater into the sewer as quickly as possible, the city retains it, stores it, and releases it with a delay or lets it evaporate. This relieves the sewer during heavy rain and provides cooling through evaporation during heat. Components include desealed surfaces, swales, infiltration trenches, green roofs, tree trenches, and multifunctional areas that are allowed to flood in an emergency.

Why this matters for your municipality. The classic maxim of "get rid of water as fast as possible" runs into two limits: during heavy rain, the sewer is overloaded, and during heat periods, water is lacking for cooling and greenery. The sponge city principle addresses both problems with the same measures. For your planning, this means investments in retention pay off twice: flood prevention and heat prevention at the same time.

How does a sponge city work in practice?

The principle works with a chain of retention and infiltration. Rain falls on a permeable or greened surface, infiltrates there, or is temporarily stored in a swale, infiltration trench or cistern. From there it slowly infiltrates into the groundwater, evaporates, or is released into the sewer at a controlled rate. The delay is decisive: not all the rain hits the sewer network at once. In addition, multifunctional areas, a lowered sports field, a neighbourhood square, take on the role of a temporary retention basin in extreme cases.

Which components belong to it?

Common elements include:

• green roofs, which store rain and release it with a delay
• tree trenches, which supply street trees with water while buffering runoff
• infiltration swales and trenches in green spaces
• permeable surfacing on paths and car parks
• and desealing of sealed surfaces as a basic prerequisite. No single element works alone, the sponge city emerges from the sum of many small retention points distributed across the whole neighbourhood

Where is this most feasible to implement?

Measures are most effective where construction is happening anyway: street renovations, redesign of squares, new development areas. Here, retention can be integrated with little additional effort. In existing areas, retrofitting is more expensive and runs into pipes, existing trees and competing uses. It is therefore advisable to anchor sponge city elements as a fixed standard in land-use planning and renovation routines rather than treating them as individual projects.

The honest limit. A sponge city buffers, it is not a cure-all. Its storage capacity is limited: during an extreme, prolonged rain event, swales and trenches eventually saturate, and water then runs off at the surface as before. Infiltration also requires permeable subsoil and sufficient distance to groundwater, on clay or with a high groundwater table it only works to a limited extent. And the elements require maintenance: a silted-up swale no longer infiltrates.

When KomAdapt is worthwhile. Sponge city elements unfold their effect through the interplay of many locations. The challenge is finding the areas with the greatest benefit, prioritising measures and demonstrating impact over time. KomAdapt combines exposure analysis, measure planning and impact monitoring, so you can see where retention delivers the most benefit.

_What does sponge city mean, explained simply?_ A sponge city absorbs rainwater like a sponge: it stores it in permeable surfaces, swales and greenery instead of draining it away immediately. During heavy rain, the sewer is relieved; during heat, evaporation cools. _Does a sponge city also help against heat?_ Yes. Stored water evaporates during heat periods and cools the air; at the same time, the retained water supplies street trees and green spaces. This makes it economically attractive compared to purely technical solutions. _What does implementation cost?_ It depends on the element and location. When integrated into ongoing construction projects, the additional effort is often moderate; retrofitting existing areas is more expensive. Plan maintenance costs in from the start. _Is there funding available?_ Yes. Investment-related nature-based measures are fundable through the DAS funding guideline of the BMUKN and the federal programme "Adaptation of Urban and Rural Areas", with a higher funding rate for financially weak municipalities. State programmes are available in addition (last reviewed: August 2026). _Does the principle work on any soil?_ No. Infiltration requires permeable subsoil and distance to groundwater. On clay or with a high groundwater table, the focus shifts to storage, evaporation and controlled release.

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