Stuttgart, Germany, October 21, 2026. Heavy rainfalls, flooded streets and overloaded drainage lines: more and more German cities suffer from the impact of heavy rain events. According to the Climate Risk Index (CRI) 2026 published by Germanwatch, an independent development, environmental, and human rights organization, Germany is among the thirty most affected countries by extreme weather events worldwidei. “Periods of heat and rain tend to increasingly occur in clusters and can basically affect any region in Germany. Heavy rain is especially treacherous: it often begins suddenly, with only a local area affected, and it is hardly predictable,“ warns Gregor Grassl, Associate Partner and an expert in the field of climate-friendly urban development at Drees & Sommer. The Stuttgart-headquartered company provides project consulting services with a focus on construction, real estate and infrastructure. Gregor Grassl is sure that “the task of climate change adaptation will get even more challenging for cities and municipalities in the coming decades.”
Already in 2021, research conducted by the international scientific initiative World Weather Attribution (WWA) found that climate change made heavy rainfall between 1.2 and 9 times as likely to happen in Western Europe.ii At the same time, the intensity of extreme rainfall is increasing considerably. As the devastating flood disaster in Germany’s Ahr Valley in July 2021 demonstrated, the consequences of extreme weather events can be dramatic. And the world saw massive floods hit Austria, the Czech Republik and Northern Italy, due to heavy precipitation, in the years thereafter.
“There is a growing risk of heavy rain, in particular after long periods of heat and drought, as we have experienced in Europe over the last years,” the expert points out. The reason: warm air can hold substantially more water vapor than cold air. As it cools down, the moisture condenses and falls as rain. The longer a heatwave lasts and the more intense it is, the heavier the subsequent rainfall
Philipp Alber, expert for urban development with integrated water management at Drees & Sommer explains why cities are particularly vulnerable to flooding during heavy rain events: “Our current urban systems are unable to cope with these quantities of water. Some cities are lacking basic facilities such as green spaces for water absorption, separate rain and wastewater systems, or adequate emergency drainage outlets.“
Drees & Sommer’s experts are convinced that cities and municipalities can adapt to the risks of climate change and become more resilient, if they have the right know-how. The following measures can help to deal with heavy rainfall.
Identifying and Minimizing Risks
Deficiencies in flood protection often do not become apparent until heavy rainfall floods streets and paralyzes infrastructure. Gregor Gassl explains: “Overloaded drainage lines are one of the most common causes of urban flooding. The ability of the sewer system to cope with the current flows which drain to it and unimpaired bodies of water are essential for effective flood protection.” This is why it is important that cities and municipalities regularly review existing flood protection measures and take additional precautions where necessary. Access to vital infrastructure, such as fire stations, hospitals and emergency shelters, must be possible at all times. A helpful tool is the national advisory map for heavy rainfall hazards. Some federal states – North Rhine-Westfalia, Lower Saxony and Baden-Wuerttemberg – have additional heavy rain hazard maps, based on local circumstances and requirements.
Turning from Gray Surfaces to Green Cities
It is not only areas near rivers that are at risk during heavy rainfall. After long periods of heat and drought, water absorption in urban areas is severely limited. Instead of seeping away, it collects on the surface or flows down streets, paths and slopes. Gregor Gassl comments: “In order to prepare for extreme weather scenarios, such as heavy rain, cities and municipalities will have to considerably develop their blue-green infrastructure. It combines green spaces, water management and modern technology, and it makes cities more resilient.“ The sponge city model plays a key role here: parks, greenways and urban open spaces act as natural retention areas that absorb excess water during heavy rainfall, and so mitigate the impact of the flooding effectively. Greenery on buildings, parks and wildlife crossings improve the urban climate, protect the soil from drying out and boost biodiversity.
Depaving Reduces Flooding in Times of Heavy Rain
Similar to soil suffering from drought conditions, asphalt, concrete and other sealed areas prevent rainwater from penetrating into the ground. Water flows unchecked into the drainage system, which quickly becomes overstretched during heavy rainfall. “As a preventive measure, existing large impervious surfaces should be removed and replaced with water-permeable solutions,” Philipp Alber adds. Grass pavers, porous covering materials or green access routes, for example, are recommended as alternatives, because they enable natural infiltration and allow rainwater to be absorbed where it falls. Not only do they reduce pressure on the drainage system during periods of heavy rainfall, but they also support the natural water cycle and allow for recharge of the groundwater levels. Desealed surfaces also help cool urban spaces. “Even in areas where depaving is not possible, rainwater retention should be considered. Green roofs can be helpful, too. They act like spoons, absorb rainwater and temporarily store it,” the expert continues.
In order to benefit from synergies and to reduce costs, heavy rain prevention measures must be an integral part of the planning of conversion and construction projects at an early stage. Rainwater can be collected in smart cisterns and help reduce the consumption of potable water. The urban development project Potsdamer Platz in Berlin is a great example in this respect. The consulting firm Drees & Sommer oversaw the project as general construction manager. Four cisterns with a total volume of 2,550 cubic meters gather rainwater for use and retention.iii
The collected rainwater is used for toilet flushing, green areas and fire extinguisher systems. So, the drinking water saving corresponds to the annual potable water consumption of about 150 to 200 people. This involves protection against heavy rain also serving as protection against drought.
Giving Room to Water
To further reduce the strain on the public sewer network, municipalities can also build retention areas and rainwater retention basins. For example, the rainwater overflow basin under Schwanenplatz in Stuttgart can take in the equivalent of 665 bathtubs of water per second and has a capacity of 18 million liters.iv Landscaping measures (as for example trough-trench systems) that closely resemble nature, such as those that store water in a cavity and slowly release it into the soil, can also reduce the effects of heavy rain.
“Removing rainwater as quickly as possible from where it falls can be a counter-productive measure,” says Gregor Grassl. As an example, rigid dams have long been regarded as the standard method for flood protection in residential areas near bodies of water. It is now clear that dams alone are not enough and, in some cases, they provide a false sense of security. Because they restrict the natural flow of water, they can increase the flow velocity at local level and tend to facilitate high water levels. “Instead, the water should be retained and stored in a controlled way. To do this, the planning process needs to give greater consideration to natural landscape structures and retention areas,” the expert explains.
The climate adaptation plans of the district of Cochem-Zell and the cities of Berlin and Potsdam show how effective climate mitigation measures can be implemented in practice. Drees & Sommer’s teams of experts provide advice on this subject matter to the cities and municipalities[MC1] .
Rainwater Management at 24 Schools in Berlin – the Local Roads and Parks Department of Berlin’s Lichtenberg district commissioned a report on the potential of decentralized rainwater management at 24 schools in Lichtenberg. The expert opinion is the basis for further measures.
Climate adaptation in the district of Cochem-Zell – the district of Cochem-Zell, located in the federal state of Rhineland-Palatinate, is currently developing a comprehensive climate adaptation plan which is due to be completed by the end of October 2026.
Climate action plan for Potdam’s Telegrafenberg – due to the heat and drought, up to 78 percent of the trees on Potsdam’s Telegrafenberg hill are considered damaged. With a comprehensive landscape plan, the long-established center of science will get ready for the future.
Climate adaptation plans of Düren and Dormagen – the district of Düren in the federal state of North Rhine-Westphalia and the city of Dormagen are developing individual climate adaptation plans on the basis of impact analyses in order to be prepared for climate change.
Master plan for Hamburg’s university campus – Helmut Schmidt University (HSU) plans a new, climate-neutral campus in Hamburg’s Jenfeld district. The master plan includes a comprehensive water management system, which is based on the principle of a sponge city.