Extremely heavy or prolonged rain may lead to flooding. Humans cannot control these natural events, but they can prepare themselves for them and minimise their impact.
For the last centuries, Berlin has done quite the opposite really. The growing number of buildings on an ever spreading area have made it impossible for precipitation to infiltrate the ground. If you live near the Panke, for example, you should watch out during heavy thunderstorms in summer. Although they pass quickly and rarely affect the entire city, they can lead to rapidly rising flood waves. On the Spree, Havel or Tegeler Fließ, however, the water tends to rise to new heights, simply if it has been wet for a long time.
Berlin has prepared itself for such events by defining flood areas, in places where flooding is to be expected at least once in 100 years. The potential use of an area depends on this status. Immense efforts are also being put into retaining the water before it floods the basements if push comes to shove.
Areas along the Tegeler Fließ, Panke and Erpe, along the Untere Havel/Untere Spree and along the Müggelspree are prone to flooding. Their shores are therefore considered flood areas. In total, this involves 6.8 square kilometres, or just under 0.8 percent of the city area.