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Summary Report No. 15


Änderung von Hochwassercharakteristiken im Zusammenhang mit Klimaänderungen - Stand der Forschung

A. Bronstert (April 1996)

In the last five years, dramatic river floods have been reported in many European countries (e.g. Germany, Netherlands, France, Italy) as well as in other countries such as the USA. There is much debate about whether these floods are partly generated or aggravated by anthropogenic caused changes of the climatic system, of vegetation and land-use or of the river systems.

There has been a lot of reports by insurance companies that the number of damage events due to river flooding and in particular the material losses have been steadily increasing over the past decades.

The increasing damage has different causes. It is commonly agreed that four different topics have to be distinguished if one wants to investigate and evaluate the reasons for flooding and the related damage:

• Meteorological factors, i.e. amount and intensity of rainfall and the occurrence of snow melt conditions.

• State of the river catchment, such as vegetation cover, topography, and soil surface conditions.

• State of the river system, such as hydraulic roughness of the river bed, geometry of the cross sections, and hydraulic interactions with flood plains.

• Potential damage to human life and to property within the flood plains.

Only situations with all four factors occurring in an unfavourable constellation may lead to river flooding events with the feared damage and losses. Climatic change is mainly affecting the meteorological conditions and - to a lesser extent - the situation of the catchment surface, such as vegetation and soil conditions. The state of the river system and the damage potential in the catchment are not affected by climatic changes.

The current knowledge about the impacts of Climate Change on river flooding generation is quite uncertain. This is mainly due to the fact that even the natural - so to say non-anthropogenically affected - weather conditions show a wide range of variation, which has led and may lead to floods of any extent. If and to what extent a possible climatic change is going to aggravate (or mitigate) this variation is not possible to predict in detail. However, some qualitative conclusions might be drawn already from the existing state of knowledge.

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