Extremely heavy rainfall may occur anywhere and affect anyone. Predictions as to when and where these events will happen exactly, however, remain rather unreliable. Heavy rainfall events have been documented throughout Berlin’s history, including those in April 1902, August 1959, and more recently in July 2016, June 2017, July 2017, July 2018, and August 2019. While the water’s velocity and destructive force are lower in the city than in the mountains, these events still cause considerable damage. With about 34 % of the city area being impervious, rainwater has limited ability to seep into the ground naturally. Berlin continues to grow, but its sewer system was not designed to handle extreme rainfall and can only be expanded with immense effort. As a result, the system is often pushed beyond its capacity. Meanwhile, climate change is making these intense rainfall events more common. Even in Berlin, roads turn into rivers, small streams swell, and areas far from any body of water face damage, impacting buildings, vehicles, and the city’s infrastructure. With valuable buildings, cultural landmarks and critical infrastructure at stake, the potential damage is especially serious.
It is impossible to completely prevent all negative impacts of flooding caused by heavy rainfall (urban flooding). Damage may be reduced considerably, however, through effective heavy rain risk management or preventive measures. Heavy rainfall hazard maps and the accompanying risk analyses provide a foundation for developing action plans to prevent or mitigate damage caused by heavy rain. These action plans raise awareness among decision-makers and those at risk, helping to assess the threat and risk of urban flooding while guiding the prioritisation and planning of preventive measures.
Berlin has a comprehensive heavy rainfall information map, which may serve as an initial guide. This heavy rainfall information map provides a straightforward hazard assessment, based on a combination of potentially expected water levels and flow velocities for two different rainfall scenarios, a topographic depression analysis and fire service operation data related to heavy rainfall. The Federal Agency for Cartography and Geodesy (BKG) collaborated with the federal states to develop expected water levels and flow velocities for extraordinary and extreme rainfall events in the Berlin-Brandenburg region, using a 2D rainfall-surface runoff analysis. This analysis, however, did not account for the sewer system or soil infiltration. Additionally, some terrain details, such as culverts beneath roads, could not always be fully captured. The heavy rainfall information highlights areas prone to urban flooding, indicating flood depths, flow velocities, and referencing heavy rain events that caused damage in the past. This allows for an initial assessment of areas at risk from heavy rainfall, aiding in the optimisation of protective measures for buildings, infrastructure, and new construction projects. In areas where heavy rainfall hazard maps are available (see below), these should be used to assess the risks of urban flooding, as the simplified model renders the heavy rainfall information map less reliable than the detailed heavy rainfall hazard map. It is essential, however, to compare these results with the actual conditions on site.
The heavy rainfall hazard map provides a detailed analysis of the spatial extent of urban flood events, flood depths, and flow velocities for a variety of heavy rainfall scenarios. These maps serve as the cornerstone of municipal heavy rainfall risk management.
The heavy rainfall hazard map illustrates the spatial extent of floods, flood depths (water level above ground), and flow velocities during urban flood events for different scenarios (rare, extraordinary, and extreme events). Surface runoff from heavy rainfall is calculated in two dimensions here, while also taking into account the sewer network (1D/2D coupled model). In 2021, initial steps were taken to develop a heavy rainfall hazard map for individual areas. Due to the size of Berlin, the city is divided into multiple catchment areas. The Berlin Waterworks (BWB) and the Senate Department responsible for water management are planning to jointly develop heavy rainfall hazard maps for additional areas in order to gradually expand the overall map for Berlin. The areas are prioritised according to their immediate need for heavy rainfall prevention and the planned rehabilitation measures for the city’s sewer network. Until Berlin’s heavy rainfall hazard map is complete, the heavy rainfall information map (see above) provides an overview of the potential dangers of flooding caused by heavy rain and the recorded events across the city.
The heavy rainfall information map and/ or the heavy rainfall hazard map are an important element of risk prevention and form the basis for risk-adapted planning and prevention. The heavy rainfall hazard map may support planners, managers of critical infrastructure, entrepreneurs and residents in identifying water-sensitive areas. From the map, risks associated with heavy rainfall may be deduced. Information on water depths, flow paths, areas of origin, and catchment areas may assist in the planning of targeted measures. The map plays a crucial role in preparing for rare, extraordinary, and extreme rainfall events, as well as in adapting to the risks of heavy rainfall caused by flood hazards.