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The FOEN uses the five-level hazard scale stipulated in the Ordinance on alerts and warnings to warn of floods. The hazard levels provide information on the severity of the event, its potential effects and recommended actions.
Mandatory broadcasting may be stipulated for level 4 and 5 warnings and they must then be broadcast by the state-licensed radio and TV stations.
The thresholds separating the hazard levels are determined by knowledge of the river behaviour (level at which the river bursts its banks, when the initial damage can be seen). These thresholds are approximately equivalent to discharges with recurrence intervals of 2, 10, 30 and 100 years, i.e. recorded once every 2, 10, 30 or 100 years on average.
At the stations which have some experience of the hazard posed by flooding, the thresholds are adapted selectively by agreement between the Confederation and the Cantons. The thresholds for the other stations, including those with small catchment areas, are based on the rounded flood recurrence intervals HQ2, HQ10, HQ30 and are updated approximately every five years.
The hazard thresholds may differ from the statistical thresholds, as in the example below:
|Recurrence interval||Statistical threshold||Rounded threshold||Adapted threshold|
Flood statistics are not compiled for lakes. Therefore the warning thresholds are defined differently: The variance between summer high (SK) and flood limit (HWG) is divided into three equal ranges corresponding to hazard levels 1, 2 and 3. The flood limit forms the transition between hazard levels 3 and 4. Hazard level 5 is reached if the lake level rises above the "Flood limit + 25 cm" warning threshold. The thresholds for lakes are also adapted if there is some experience of the hazard from flooding, by agreement with the canton.
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