Flood events are becoming more frequent, affecting even long-established settlement areas with increasing frequency. While this trend is often attributed to CO?-driven climate change, observed and predicted changes in precipitation from climate change alone are too insignificant to explain the rapid rise in flood frequency and severity, including loss of life. Land-use changes are likely a far more significant driver. This paper focuses on one such change: soil sealing. Soil sealing reduces rainwater infiltration, thereby increasing runoff. Given that sealed surfaces account for only a small share of total land area (e. g. about 6 % in Bavaria), this direct effect is moderate. The greater impact arises from the drainage infrastructure-culverts, sewers, and ditches-made necessary by this runoff. Such infrastructure accelerates water flow and increases flood peaks because a given amount of runoff flows within a shorter time. The increase in peak flow is directly proportional to the increase in flow velocity. The increase in flow velocity by a roadside ditch can be tenfold compared to grassland, which dominated the areas where modern field roads can now be found. Roads are thus a peculiarly critical part of this amplification because they capture the surface runoff from the rural landscape and direct it quickly towards the next settlement. Modern settlement patterns, which require widespread motor vehicle use, and the mechanisation of agriculture and forestry, have created a dense road drainage network that effectively channels heavy rainfall into nearby settlements. Near forests that will be harvested for timber, for instance, truck-accessible road networks are now as dense as those in large cities, meaning these areas-unlike (semi)natural forests-also contribute to flooding. In short, we have overlaid the entire country with a highly efficient road network that facilitates the rapid flooding of our settlements. Numerous cost-effective measures exist to reduce soil sealing and slow runoff. Small and medium-sized towns bear a particular responsibility, as they account for 90 % of the ongoing loss of permeable land due to sealing.

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Der Artikel "Stadt, Land, ... alles im Fluss. Ursachen zunehmender ?berflutungen von St?dten" von Prof. Dr. Karl Auerswald (TU München) und Dr. Lena Katharina ?ttl (Universit?t Augsburg) kann nur kostenpflichtig in der Zeitschrift "Forum Stadt" gelesen werden: https://www.forum-stadt.eu/zeitschrift

Auerswald, K., ?ttl, L. K. (2026) Stadt, Land, ... alles im Fluss. Ursachen zunehmender ?berflutungen von St?dten. Forum Stadt - Vierteljahreszeitschrift für Stadtgeschichte, Stadtsoziologie, Denkmalpflege und Stadtentwicklung (1.2026): 31-41. ISSN 2192-8924.