Acqua Alta in Venice Article
Acqua Alta in Venice
The acqua alta - literally “high water” - in Venice refers to periods of more or less significant flooding of the city during high tides. By convention, in Venice it denotes a tide exceeding the attention threshold of +80 cm relative to the Punta della Salute tide gauge zero.
Waterproof boots or overshoes are sometimes essential when these phenomena flood the streets, depending on the level reached and whether the MOSE has been activated.
Indeed, the water level can be kept lower than the tide level thanks to the MOSE barriers.
The historical increase in acqua alta episodes is linked to the combination of subsidence - the sinking of the ground, particularly pronounced in the mid-20th century - and the rise in average sea level. According to the Centro Maree, the average sea level in Venice today is about 32 cm higher than at the beginning of the 20th century.
Tide forecasts
The Centro Maree also provides real-time observed levels, the tide bulletin and various alert services on its website. In particular, it distributes its forecasts and alerts via Telegram.
Forecasts for water levels in Venice over the coming days
To keep informed about the level of forecast tides over the coming days:
- Home page of the Centro Previsioni e Segnalazioni Maree of the City of Venice
- Tide forecasts from the Centro Maree - Comune di Venezia
The MOSE and Acqua Alta today
Since December 2020, the situation has changed profoundly with the operational commissioning of the MOSE. When sufficiently high sea levels are forecast, its mobile barriers can temporarily close the three inlets connecting the lagoon to the Adriatic and limit the inflow of the tide.
It is therefore now necessary to distinguish between the forecast sea level and the level actually reached in Venice: during many MOSE activations, a strong outside tide can be kept at a significantly lower level within the city. The acqua alta phenomenon has therefore not disappeared, especially for more modest levels or the lowest areas, but major floods are now much less systematic when the system is activated.
Acqua Alta levels and alerts
Conventionally, acqua alta refers to a level of +80 cm or more relative to the Punta della Salute tide gauge zero (ZMPS).
Between 80 cm and 100 cm, flooding is minor and affects only the lowest parts of the city, such as around St Mark's Square, but above that level streets begin to be affected.
| Tide level | Public pedestrian area potentially flooded (historic centre) |
|---|---|
| +100 cm | approximately 5% |
| +110 cm | approximately 12% |
| +120 cm | approximately 28% |
| +130 cm | approximately 46% |
| +140 cm | approximately 59% |
| +150 cm | approximately 70% |
| +160 cm | approximately 77% |
Warning codes are used to characterise alerts:
- Normal tide: from -50 to +80 cm
- Sustained tide: from +80 to +110 cm - traditionally associated with the yellow code
- Very sustained tide: from +110 to +140 cm - traditionally associated with the orange code
- Exceptional high tide: from +140 cm - traditionally associated with the red code
Alert sirens are triggered when a very sustained tide is forecast, generally from +110 cm. After an initial signal drawing attention, a sequence indicates the expected level: one sound for +110 cm, two sounds of increasing pitch for +120 cm, three for +130 cm and four for +140 cm or more. In practice, it is therefore enough to count the successive sound signals.
Walkways during Acqua Alta
To allow pedestrians to move around dry in certain areas, raised walkways may be installed as part of the pedestrian viability plan. The service normally operates during the season most exposed to acqua alta, from 15 September to 30 April. The planned routes in principle allow dry movement up to a tide of +120 cm; beyond that, some may become impassable.
Sources and further information
- Consult the website of the Centro Previsioni e Segnalazioni Maree of the City of Venice for official forecasts and information.
- The feature dedicated to acqua alta on e-venise for further information and photographs.
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