Extreme precipitation and flooding in Berlin under climate change and effects of selected grey and blue-green measures

Abstract. This paper aims to quantify potential changes in extreme precipitation under climate change scenarios in the city of Berlin, Germany, and their resulting impacts on urban flooding in a selected flood-prone area of the city. Furthermore, it investigates the effectiveness of the existing drainage system, infiltration from unsealed surfaces, and retention roofs during extreme rainfall events under both current and future climate conditions.

The effect of climate change on the statistical distribution of extreme precipitation in Berlin is assessed by analyzing a single-model set of climate scenario simulations at convection permitting resolution (COSMO-CLM). Three 30-year periods are simulated: The historical period under observed greenhouse gas concentrations from 1971 to 2000 and two RCP8.5 scenario periods from 2031 to 2060 and from 2071 to 2100. For the historical period, the estimated 1-hour rainfall sum for a 100-year return level (referred to as ‘Historical 100a’) agrees well with the statistical values from station observations. For the period 2031–2060 under RCP8.5 conditions the respective rainfall sum of the 1-hour 100-year event (referred to as ‘Future 100a’) increases by 46 % and the strongest hourly intensity in all three simulated 30-year periods (referred to as ‘Strongest’) is increased by 123 % compared to the Historical 100a event.

The impacts of these increases in extreme precipitation on flooding characteristics in a Central-Berlin region around the Gleimtunnel, which is known for frequent pluvial flooding, are studied by conducting simulations with the 2D surface flow model hms++ coupled to a 1D drainage model. The Future 100a event result in a 51 % increase in the simulated maximum water depth, a 43 % increase in maximum surface runoff at the local flooding hotspot Gleimtunnel, and a 33 % increase in the volume of combined sewer overflow. For the Strongest event, the respective increases are 137 % (maximum water depth), 296 % (maximum surface runoff), and 74 % (combined sewer overflow).

The effects of the existing drainage system and infiltration under different rainfall scenarios are highlighted by comparing simulation results with and without their consideration. Neglecting the drainage system results in a 170 % increase for the Historical 100a event and a 110 % increase for the Strongest event, compared to the reference simulations. While the drainage system strongly reduces flooding, especially at hotspots, it cannot fully prevent severe flooding, and its effectiveness decreases with higher rainfall intensity. Studying infiltration reflects potential impacts of surface sealing or, conversely, desealing as a climate adaptation strategy. Neglecting infiltration increases the maximum water depth at the Gleimtunnel by 33 % for the Historical 100a event and 18 % for the Strongest event compared to reference simulations. Infiltration significantly reduces flooding, though its effectiveness decreases with higher rainfall intensity.

As a potential adaptation strategy, the impact of replacing all roofs with retention roofs is examined. For this best-case adaptation scenario, the maximum water depth at the local hotspot is reduced by 22–24 %, and the volume of combined sewer overflow by 15–20 % in the different scenarios. Since full retention on all roof surfaces is considered for all rainfall scenarios, the effects are almost the same. Remarkably, the retention roofs significantly reduced the maximum surface runoff in the Gleimtunnel during the Strongest event to below the stability threshold for pedestrians, which was clearly exceeded in the simulation without retention roofs.

The results of this study highlight the potential local impacts of ongoing global warming in terms of heavy rainfall and urban flooding in the city of Berlin and emphasize the need to combine grey infrastructure, retention roofs, and other blue-green measures.