The sea is a fickle mistress. Built on marshland within a salt-water lagoon off the Adriatic, Venice owes her fortune to the sea. At her birth, at 12pm on 25th March 421 AD (according to legend), the lagoon protected Venice from her enemies. Later, in her heyday, water-based trade and naval warfare made the city very rich. But water can also bring damage and destruction as Venice knows all too well. Venetians call the phenomenon Acqua Alta, or high water.
So what causes acqua alta? Here comes the science bit! Acqua alta is produced when a number of factors combine to cause an exceptionally high tide which floods the city. These factors include –
- Normal and seasonal high tides
- Low pressure weather and prevailing winds pushing the high tide into the Venetian lagoon like a storm surge
- Subsidence or sinking of the city by around 25cm over the last 150 years
- The rise of sea levels over the last 100 years
- Natural daily Adriatric oscillations which can add another 0.5m to a high tide if the timings coincide
Acqua alta officially occurs when a tide rises more than 80cm above the average sea level recorded in 1897. Normal high tides rise around 40-79cm and cause no problems, you won’t notice a thing! Even when high tides reach acqua alta status, at 80cm or above, most do little more than affect the lowest lying parts of the city, primarily around Piazza San Marco (St Mark’s Square), which becomes a temporary paddling pool for tourists. But once the tide exceeds 130cm, as measured by the hydrographic stations around the lagoon, over 40% of the city can be inundated with water. And at “exceptionally high waters”, when tides rise over 200cm, the entire city would be affected causing millions of Euros worth of damage.
To put this into context the Venice Comune (Council) helpfully shows a cartoon on their website to illustrate how high acqua alta water comes on a person, peaking at 1966’s record of a 194cm tide. (The table also shows how much of the city floods via the allagamento figure.) But don’t worry, this isn’t Atlantis and you won’t need your snorkel! Acqua alta is an occasional and temporary inconvenience. The city is mainly susceptible between November and April, although acqua alta can occur at any time of the year as this summer has shown, and also varies in frequency from year to year but the adept Venetians have become used to its ebb and flow.
As luck would have it for me, factors combined one evening late June this year to raise the tide for my first mini-acqua alta! Fortunately, though, the effect was less dramatic than I had anticipated! At the Rialto water was gently over-flowing the Grand Canal to flood the waterfront around the bars in the old spice market but apart from a couple of empty tables it was business as usual. In Piazza San Marco, however, the water seeped up through the drains, creating less of a “tidal surge” and more of a big paddling pool much to the amusement of hundreds of over-heated tourists! It was a very gentle introduction to acqua alta!
The effects usually only last a few hours until the tide subsides and for most of the city and its citizens, life goes on as normal – the vaporetti still run, shops are still open and so are most attractions. Venetians have lived with acqua alta in various forms for hundreds of years and have learnt to make adjustments when the forecast is high and sirens warn of impending risk. And once the high tide arrives, the main methods to avoid wet feet are to use the temporary raised walkways, or passerelle, across the worst hit areas and water-tight gates across doorways to minimise damage to houses, shops and offices.
Within the last 15 years the Council has also approved and commissioned a €7 billion project to build tidal barriers across the 4 entrances to the lagoon (similar in principle to the Thames Barrier) to combat tidal surges of up to 3m and keep the city safe from the worst of the floods. The MOSE project (which stands for Modulo Sperimentale Elettromeccanico, i.e. “Experimental Electromechanical Module”) employs over 4000 staff and is hoped to be completed in 2016. The project is, however, already 4 years behind schedule and has recently been very much in the news due to allegations of multi-million Euro bribes and corruption around the awarding of contracts, leading to the arrest of several high level politicians including Venice’s Mayor who later resigned.
In the meantime, tourists continue to flock to take photos of the seasonal spectacle and have a paddle although this is hardly a blue-flag beach and the water quality is very dubious! Some even cough up €10 for the luminous plastic knee-high overshoes on sale from numerous street vendors to keep them dry, although most preferred the novelty of splashing about in the warm summer water in June! Rather them than me! The same cannot be said during Winter acqua altas, when wellies, waders and waterproofs are undoubtedly the fashion must-haves for everyone, visitors and “bella figura” Italians alike!
Overall, my summer acqua alta experience was a good one. The water was warm, the pavement was worn smooth by centuries of footsteps and the novelty value was high. Plus I’ve learnt alot about the geography of the city and which areas are hit worst! If you want to experience it for yourself, you might have to be quick as the MOSE project should prevent the worst of the acqua alta flooding within a couple of years – although St Mark’s Square will still be susceptible to low flooding. But if you’re visiting between November and April this year, check the tidal tables, pack your wellies, winter waterproofs and wrap up for a paddle!
The Venice Comune or council hosts a website giving advice on the lowest areas of the city and FAQs of aqua alta here
Warning sirens – If you hear the acqua alta siren, you can determine the predicted water level from the tones given. For example –
110 cm : an old siren followed by a long tone on the same note
120 cm : an old siren followed by two tones in a rising scale
130 cm : an old siren followed by three tones in a rising scale
140 cm and above (i.e. serious flooding!) : an old siren followed by four tones in a rising scale
To hear what the different sirens sound like & familiarise yourself with what to expect check out the council website here. The sirens usually go off 3-4 hours before high tide so you have time to prepare if you need it!
Plastic overshoes can be bought from street vendors for around €10 or bring your own wellies!