Dam failure
The water alarm – 12 deep steady tones – applies only below dams and means: act immediately, do not wait for further information. Leave the endangered, lower-lying area at once and move to higher ground. Do not return to get something and warn others on the way.
Switzerland is a country of dams: more than 200 large reservoirs are subject to federal supervision, including the Grande Dixence in Valais, at 285 metres the highest dam in Switzerland. The failure or overtopping of such a structure would trigger a flood wave that can reach settlements lower down the valley within a short time. Causes can be a structural failure, overtopping or – as at Vajont in Italy in 1963 – a rockslide into the reservoir.
Supervision of the large dams lies with the Swiss Federal Office of Energy (SFOE); the structures are continuously monitored. For emergencies the federal government operates the water-alarm system: in the endangered areas below dams there are dedicated sirens – around 630 installations – covering the area a flood wave could reach within two hours. The water alarm is triggered manually by the operator, never automatically.
The water alarm is a distinct signal – twelve low continuous tones of 20 seconds each with pauses of 10 seconds. Unlike the general alarm, there is no time here to wait for information first: anyone who hears the signal leaves the endangered area immediately and moves to higher ground.
Risk rating
Likelihood: Low · Impact: High → Overall risk: Medium.
| Impact | |||
|---|---|---|---|
| Low | Medium | High | |
| High | |||
| Medium | |||
| Low | This hazard's rating | ||
Legend: Low (green) · Medium (yellow) · High (red). Overall risk is likelihood multiplied by impact.
If a dam breaks or overtops, a flood wave races down the valley. In the endangered area the water alarm sounds – then head for higher ground immediately.
Immediate measures
- The water alarm (12 low tones) means: act immediately
- Leave the endangered area without delay
- Get to higher ground as quickly as possible
Warning signs
- Water-alarm siren: twelve low continuous tones of 20 seconds each, pauses of 10 seconds (only below dams)
- An unusually fast rising or falling water level below the dam
- Visible cracks, seepage or deformations in the dam wall
- Slope movements or rockfalls above the reservoir
- Authority notices via Alertswiss and radio on the state of the dam
Before · During · After
Before
- Know whether you live in a water-alarm zone below a dam
- Be able to tell the water-alarm signal from the general alarm
- Define an escape route to higher ground in advance
- Note the municipality's leaflets on the water alarm
During
- On a water alarm, do not wait for further information
- Leave low-lying areas immediately, flee to higher ground
- Do not return to the endangered valley, do not go to fetch anything
- Warn and take others with you, help vulnerable people
After
- Return only when the authorities give the all-clear
- Have undermined/damaged buildings checked before entering
- Watch for further debris flows, mud and damaged infrastructure
This really happened
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2023 Dam collapses in Derna (Libya)
On the night of 11 September 2023, after storm "Daniel", two dilapidated dams above the city of Derna failed; the flood wave destroyed entire districts and claimed several thousand lives — an example of the consequences of neglected dams.
Source -
2023 Destruction of the Kakhovka dam (Ukraine)
On 6 June 2023 the Kakhovka dam on the Dnipro was destroyed during the war; the flooding lasted almost three weeks, affecting around 80 communities and the water supply of hundreds of thousands of people.
Source -
2019 Brumadinho dam collapse (Brazil)
On 25 January 2019 the tailings dam of an iron-ore mine near Brumadinho failed; around 12 million m³ of sludge buried facilities and settlements and killed at least 270 people.
Source -
1963 Vajont disaster (Italy)
On 9 October 1963 around 270 million m³ of rock broke off the Monte Toc and fell into the reservoir, displacing a huge wave over the wall. It destroyed Longarone and neighbouring villages and killed over 2000 people – the dam wall itself remained largely intact.
Source -
1965 Mattmark disaster (VS)
On 30 August 1965, during construction of the Mattmark dam, part of the Allalin glacier broke off; around 2 million m³ of ice and debris buried the construction site and killed 88 people. It was a glacier collapse onto the site, not a failure of the dam.
Source
Frequently asked questions
How do I recognise the water alarm and how does it differ from the general alarm?
The water alarm consists of twelve deep steady tones of 20 seconds each with pauses of 10 seconds and sounds only in endangered areas below dams. Unlike the general alarm (ascending and descending howl), there is no time to wait for information: if you hear it, leave the area at once and flee to higher ground.
How do I prepare for a possible dam failure?
Find out whether you live in a water-alarm zone below a reservoir and establish an escape route to higher ground in advance. Learn to distinguish the water alarm from the general alarm and heed your municipality's information sheets on the water alarm.
How likely is a dam failure in Switzerland?
Switzerland has over 200 large dams under federal supervision; they are continuously monitored, with supervision under the Federal Office of Energy (FOE). Failure is very rare, but the consequences would be serious. For emergencies, the confederation operates its own water alarm system with around 630 sirens covering the area that a flood wave could reach in two hours.
Official sources
- Alertswiss What does the water alarm mean?
- FOCP Sirens and alerting
- SFOE Dam supervision
- Guide: Protection shelters in Switzerland Protection and assignment in emergencies