Extremely heavy rainfall may occur anywhere and affect anyone. Predictions as to when and where these events will happen exactly, however, remain rather unreliable. Berlin has seen a rise in extreme rainfall events causing considerable damage in recent years (July 2016, June 2017, July 2017, July 2018, August 2019, September 2021). Extreme weather events, particularly heavy rain and flooding are likely to increase in frequency and intensity, due to climate change. Water tends to gather more on impervious surfaces, resulting in floods. Roads temporarily transform into rivers. Small bodies of water and ditches swell significantly. Residential and commercial areas might experience flooding. The urban infrastructure might be damaged substantially.
It is impossible to completely prevent all negative effects of flooding caused by heavy rainfall. Damages may be reduced considerably, however, through effective heavy rain risk management or preventive measures. Data on heavy rain hazards and their analyses form the basis for the development of action plans to prevent or mitigate damage caused by heavy rain. These action plans raise awareness among stakeholders and those potentially affected by such events. They further assist in gauging the danger and risk associated with flooding induced by heavy rainfall. Additionally, measures may be assessed and prioritised in their planning.
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, combining topographic depression analysis with data on fire service operations. It displays areas potentially at risk of flooding based on low elevations and refers to heavy rainfall events that have caused damage in the past. Areas potentially at risk of heavy rainfall may therefore undergo an initial assessment. Based on this, the protection of buildings, infrastructure, and new construction projects may be optimised. In areas where heavy rainfall hazard maps are available (see below), they should be used to assess the risks of flooding caused by heavy rainfall. 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 floods, flood depths, and flow velocities for a variety of heavy rainfall scenarios. They are the cornerstone of the 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 heavy-rainfall-induced floods 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 system. 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 Federal Agency for Cartography and Geodesy is working with the federal states to develop a heavy rainfall information map that also includes the State of Berlin. The findings of the Federal Agency for Cartography and Geodesy’s heavy rainfall hazard map are planned to be integrated into the heavy rainfall information map mentioned above.
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, operators of critical infrastructure, entrepreneurs, homeowners, and property owners 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.