The Brussels-Capital Region updated its flood zone mapping in 2025, assessing risks associated with three event probabilities: a 1 in 25, 100, or 1000 chance of occurring annually. The results indicate significant vulnerability for the population, particularly to basement flooding caused by sewer backflows, potentially affecting 25% of Brussels residents every 25 years and 6% every 100 years. Pluvial flooding also poses a major risk, with 20% of inhabitants potentially affected.
Three main phenomena cause flooding in the Region: rainwater runoff on impermeable surfaces, overflowing watercourses and the Canal, and sewer system backflow during heavy rains. A fourth risk, rising groundwater levels, can also cause dampness problems in basements after prolonged wet periods.
Urbanization and increased soil impermeabilization are identified as the primary causes of flooding observed in Brussels. Nearly 53% of the regional area is impermeable, a rate that has doubled in half a century, leading to an increase in the volume and speed of runoff water. The predominantly combined and aging sewer system struggles to manage these volumes, especially when rainwater is added.
Climate change, with a likely unfavorable evolution of rainfall patterns, and the disappearance of natural overflow areas also contribute to this risk. However, soil impermeabilization remains the predominant cause of flooding, often linked to intense, short-duration thunderstorms in spring or summer.
The new flood zone maps, based on hydraulic and hydrogeological model simulations, provide for the first time estimates of water depths and submersion speeds for pluvial floods. They also map for the first time the risk of basement dampness due to rising groundwater. These maps are essential awareness tools for the public.
In response to these risks, individual and collective protective measures are crucial. Soil des-impermeabilization and disconnecting rainwater from the sewer system are key strategies to reduce the Region's vulnerability. The Brussels Region has set the European objective of 'net zero land take' by 2050, aiming to restore the natural water cycle.
Data indicates that 91% of industrial zone areas are partially or fully within pluvial flood zones, posing a risk of accidental pollution to the environment. Transport infrastructure, listed monuments, fire stations, and police posts are also affected by these risks.
Between 2007 and 2023, 35 significant floods were recorded in the Brussels Region, most occurring in spring or summer. The year 2021 was particularly marked by six flood events. These events, often localized and short-lived, caused considerable material and economic damage, sometimes exceeding 11 million euros.
