Century event

Century events are weather events containing on average only occur once a century ( return period T = 100 a), as the flood of the century or the century heat .

detection

For a more precise determination z. B. the hundred-year flood event of a river is not enough to observe a single flood, but longer observations are required. In simple terms, a 100-year flood would be a discharge that exceeds the tenth largest flood event in the last 1000 years.

In general, every event changes the previous calculations (such an adjustment takes place, for example, after every flood in Switzerland): An event that is currently classified as 100 years old could just as well turn out to be a 50 or a 200 year later. exposing yearly.

The exact determination is made using statistical methods. For this purpose, existing level recordings for floods , but above all historical sources such as high water marks, are used in order to record even more rare and therefore more distant runoff and weather events. In the case of storms, on the other hand, the measured values ​​are not particularly meaningful, here the damage pattern is mostly assessed (e.g. the extent of windthrow in forests) and the total damage is taken as a criterion. This also applies to other unspecific, extended events, such as extreme snow events (where avalanches usually account for the main damage). Therefore, weather event categories are differently difficult to put in a longer-term context.

The problem here is the consistency of the historical data, closed weather records have only existed since the second half of the 19th century, measurements in the modern sense only since the early modern period (i.e. a maximum of 5 centuries), older records are mostly verbal recordings in annuaries - the high water marks are one of the few exceptions to reliable, precise measurements that can go back many centuries. Climatology is therefore in the unfavorable position of diagnosing events of the century within a framework that only slightly exceeds the interval of the century.

Archaeo- geoscientific findings are therefore becoming increasingly important in order to understand and interpret the relationships of the previous centuries. It can be:

• geological traces of recent flood and mass movement events ( sedimentation , erosion )
• geophysical data, e.g. from climate archives (e.g. sediment samples, ice cores )
• up to the evaluation of old cartography on river banks or vegetation.

These historical findings, however, are only local “flashes of light”, which, even with current data, represent a poor basis for geographically comprehensive analyzes: Outlier values occur frequently and frequently , especially in meteorology .

Therefore, the numerical model calculation takes on the central role today , whereby the models are calibrated by the historical evidence .

meaning

The values ​​regarding the annuality are important for the dimensioning of flood protection facilities , or in the case of rain for drainage systems , as well as for the development of emergency plans .

They are also of central importance for the insurance industry , where risk assessment forms the basis of the calculations. Therefore, in addition to weather and climate science and the authorities, the relevant insurance associations and reinsurers are particularly involved in this sector.

objectivity

The century is typically the period of time that corresponds to the expression "since living memory". Surprisingly, however, people forgets borderline experiences and setbacks if they are not directly affected, and especially puts those whose stories are often only alive for one or two generations into perspective. Extreme natural events occur more frequently than one would suspect based on personal experience. For the past 500 years, 25 floods, some of which have been very damaging, have been reported by the Alpine Rhine . H. every 20 years on average. The term event of the century is therefore not always appropriate. Accurate event documentation is a prerequisite for objective risk perception . The term “event of the century” is often overused in the press today - also in the context of global warming and its consequences.

See also

• climate
• Climate catastrophe

Individual evidence

1. ^ Event analysis flood 2005 . Part 1: Processes, damage and initial classifications , FOEN 2007 ( page no longer available , search in web archives: web link to PDF) - flood classification statistical change after an event there findings point 2.7.
2. ↑ unknown document on prostollen.ch ( original link ( memento of the original of 30 March 2013, Internet Archive ) Info: The archive link is automatically inserted and not yet tested Please review the original and archive link under. Instructions and then remove this notice. ; PDF , 134 kB; no longer available); there "The probability of a flood increases with every event." 
3. ↑ The longest existing series worldwide are the temperature data from Kremsmünster , which are homogenized back to 1760 (250 years) and form the basis of the HISTALP data. Unfortunately, the series only reflects the small-scale conditions of the Alpine region on the border of three basic climatic zones .
4. ↑ Fact sheet of natural hazards in the Canton of Graubünden  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.   (pdf, on gr.ch).
5. ↑ unknown document on bafu.admin.ch ( original link ( memento of the original on 31 May 2012 at the Internet Archive ) Info: The archive link is automatically inserted and not yet tested Please review the original and archive link under. Instructions and then remove this notice. , unavailable); Exact documentation there ( damage profiles , point 2) to obtain relevant data.