The 5 flood danger levels and recommended behaviour

The FOEN has the legal mandate to warn of floods using a five-level danger scale. The danger 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 meaning of the flood danger levels

Danger level 1: Low danger or none

Effects

Transport disruption and damage are not normally to be expected.

Behaviour

It is not dangerous to remain near rivers, but vigilance is required at all times.

Description of the danger level

Watercourses: The discharge in the river is below an event occurring once every two years on average. This threshold value is based on both statistical data and experience of how the rivers behave.
Lakes: The lake level is below danger threshold 1.

Danger level 2: Moderate danger

Effects

At this discharge level, local occurrences of rivers bursting their banks and flooding are unlikely, but cannot be ruled out. In exceptional cases, local flooding of underpasses, underground car parks and basements is possible. Local occurrences of disruption on exposed transport routes and minor damage are unlikely, but cannot be ruled out.

Behaviour

It may be dangerous to remain near rivers. As a precaution, maintain a safe distance.

Monitor the situation and take initial precautions.

Stay calm, exercise good judgment and, as far as possible, act independently. Do not expose yourself to unnecessary danger; leave the endangered area immediately.
When danger of flooding exists, do not enter basements or underground car parks, and do not drive a vehicle or ride a bicycle on flooded roads.
Do not drive a vehicle or ride a bicycle on flooded roads.
Keep away from rivers and lakes that carry flood water. Surging water can not only catch you by surprise, but also erode banks and cause their collapse.
Listen to the radio and follow instructions issued by the emergency services.

Description of the danger level

Watercourses: The discharge in the river is within the range of an event occurring once every 2 to 10 years on average (HQ2, HQ10). These threshold values are based on both statistical data and experience of how the rivers behave.
Lakes: The lake level is above the quota by 1/3 of the range between the maximum seasonal mean and the flood limit but below the danger threshold 2.

Danger level 3: Considerable danger

Effects

At this discharge level rivers may burst their banks locally in exposed places and flooding may occur. Local flooding of underpasses, underground car parks and basements is possible. Local disruption may occur on exposed transport routes and minor local damage is to be expected.

Behaviour

It is dangerous to remain near rivers. Keep well away.

Monitor the situation and take initial precautions.

Stay calm, exercise good judgment and, as far as possible, act independently. Do not expose yourself to unnecessary danger; leave the endangered area immediately.
When danger of flooding exists, do not enter basements or underground car parks, and do not drive a vehicle or ride a bicycle on flooded roads.
Do not drive a vehicle or ride a bicycle on flooded roads.
Keep away from rivers and lakes that carry flood water. Surging water can not only catch you by surprise, but also erode banks and cause their collapse.
Listen to the radio and follow instructions issued by the emergency services.

Description of the danger level

Watercourses: The discharge in the river is within the range of an event occurring once every 10 to 30 years on average (HQ10, HQ30). These threshold values are based on both statistical data and experience of how the rivers behave.
Lakes: The lake level is above the quota by 2/3 of the range between the maximum seasonal mean and the flood limit but below the flood limit.

Danger level 4: High danger

Effects

At this discharge level rivers are likely to burst their banks and flooding may occur. Buildings and infrastructure may be affected. Transport disruption is possible and damage is likely to occur.

Behaviour

It is extremely dangerous to remain near rivers. Keep well away.

Monitor the situation and take initial precautions.

Stay calm, exercise good judgment and, as far as possible, act independently. Do not expose yourself to unnecessary danger; leave the endangered area immediately.
When danger of flooding exists, do not enter basements or underground car parks, and do not drive a vehicle or ride a bicycle on flooded roads.
Do not drive a vehicle or ride a bicycle on flooded roads.
Keep away from rivers and lakes that carry flood water. Surging water can not only catch you by surprise, but also erode banks and cause their collapse.
Listen to the radio and follow instructions issued by the emergency services.

Description of the danger level

Watercourses: The discharge in the river is within the range of an event occurring once every 30 to 100 years on average (HQ30, HQ100). These threshold values are based on both statistical data and experience of how the rivers behave.
Lakes: The lake level is between flood limit and the "flood limit + 25 cm".

Danger level 5: Very high danger

Effects

At this discharge level rivers may burst their banks and flooding may occur in many places. Infrastructure of national importance, such as railways, villages and towns and industrial plants, may be significantly affected by floods. Significant transport disruption is likely in some cases. Some widespread major damage is to be expected.

Behaviour

It is extremely dangerous to remain near rivers. Keep well away.

Monitor the situation and take initial precautions.

Stay calm, exercise good judgment and, as far as possible, act independently. Do not expose yourself to unnecessary danger; leave the endangered area immediately.
When danger of flooding exists, do not enter basements or underground car parks, and do not drive a vehicle or ride a bicycle on flooded roads.
Do not drive a vehicle or ride a bicycle on flooded roads.
Keep away from rivers and lakes that carry flood water. Surging water can not only catch you by surprise, but also erode banks and cause their collapse.
Listen to the radio and follow instructions issued by the emergency services.

Description of the danger level

Watercourses: The discharge in the river is within the range of an event occurring once every 100 years on average (HQ100). These threshold values are not based solely on statistical data, but may be adjusted based on experience of how the rivers behave.
Lakes: The lake level is above the range "Flood stage + 25 cm".

Definition of the danger levels for rivers and lakes

For rivers:

At the stations which have some experience of the danger 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.

Example:

The danger thresholds may differ from the statistical thresholds, as in the example below:

Recurrence interval	Statistical threshold	Rounded threshold	Adapted threshold
HQ2	572	570	590
HQ10	819	820	880
HQ30	943	950	1050
HQ100	1065	1100	1250

For lakes:

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 danger levels 1, 2 and 3. The flood limit forms the transition between danger levels 3 and 4. Danger 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 danger from flooding, by agreement with the canton.