In the event of short but very heavy rainfall, so-called heavy rainfall, large volumes of water must be drained off above ground if they can no longer be fully absorbed by the sewer network or adjacent bodies of water. Such events, for example during a summer thunderstorm, often only affect a few square kilometers. This is another reason why they are almost impossible to predict; instead, they come as flash floods and so suddenly that the response time for the city and the affected residents is extremely short.