Acqua Alta – High Tide in Saint Mark’s Square in Venice
The sky was blue, the air crispy and clear, the sun warmed my cheeks, at least the parts not covered by my mask. I walked along Riva degli Schiavoni in direction of Piazza San Marco. The Straw Bridge, Ponte della Paglia, from where you can admire the Bridge of Sighs, was empty; we were still in the Coronavirus pandemic. From the top of the bridge I recognized the winged lion at the top of the column and behind the National Library.
I continued walking along the Gothic facade of the Doge’s Palace and I turned right around its corner with the marbled statues of Adam and Eve.
I noticed the Piazzetta was flooded in the lower areas parallel to the Doge’s Palace. I already knew I would find the Piazza nearly completely flooded, being lower than the Piazzetta.
13th January 2021, 10 am: Saint Mark’s Square
There was no wind, there was no perception of high tide. We Venetians can ‘sniff’ it, when it will be a high, very high tide; we hope we will no longer have to suffer it now that MOSE gates can be lifted in case of necessity. I squeezed my eyes and checked the lunar phases on the Clock Tower. The moon was blue, this meant it was new moon. 13th January 2021 new moon, it all made sense. Everything fitted.
High tide in Venice: How do you measure it?
What influences / builds up tides? First of all two natural elements.
The first is the astronomical tide related to lunar phases; high tide events are more likely with full moon and new moon. The second is meteorological surge. In case of Scirocco wind blowing from South-East the water is driven into the Upper Adriatic towards the lagoon or in case of Bora wind blowing North-East the water collects in the lagoon. It is favored by the fact that the Adriatic turns less wide and very shallow. I read that this effect can also lead to an increase of 1 meter.
Meteorological surge includes also atmospheric pressure, the lower this is the higher the water will. It is called inverse barometer effect. This pressure can lead to a variation of even 20 cm.
The sun shone on a deserted Venice; all parts of Saint Mark’s Square reflected in the water that bubbled up from the gullies, that day in a very gentle way. I did not resist the temptation and took out from my bag my camera and I started taking pictures… and there came a professional photographer wearing willies, wading through the water, the reflection of the church became soon unrecognizable. I had to wait.
I estimated the tide was around 85 cm above the zero sea tide level, the lower parts of Saint Mark’s flood with little more than 70 cm. ‘Sie ore ea cala, sie ore ea cresse’. Every day the water rolls out for 6 hours, and then rolls in for another 6. I saw no traces of wet haloes around the water pools, thus meaning the water was still coming in, however slowly.
Finally I was able to take a shot of the Gothic crowning of Saint Mark’s Church
High tide events are ‘fascinating’, especially for those who do not live in the lagoon. Frequent questions are how the water bubbles up from gullies? Why doesn’t the water swap in from the water front?
The water front along Saint Mark’s water basin was raised starting from 2003 100 cm above the zero sea tide level. The measurement takes place still nowadays at the Punta della Dogana, the former sea customs, a point chosen back in 1897.
Land subsidence as a result mainly of the drawing of groundwater (from 1950 to 1970 approximately 12 cm) and eustatism, the increasing sea-level (9 cm between 1900 and 1970 and approximately 15 cm from 1970 to 2015) led the point to be today 26 cm higher than above.
Acqua Alta High tide forecasts
In the Seventies the peaks of the Acqua Alta were anticipated by the sound of sirens, then I remember maybe in the Eighties calling a number of the City Council of Venice and a recorded voice on a tape would give the forecast. With the beginning of the internet era we started checking tides on the web and we received a text on our cell phones from the city. Now there is an hi!tide app that give a 3-day forecast and a constant minute-update.
In use as a warning system are still the sirens (now a digital system with 4 progressive sound levels), the city website, the text alert service and of course the app.
The tide forecast is calibrated on Piazza San Marco; you need to anticipate the time according to the area of the lagoon you are in, by the Lido the water comes in approximately 45 minutes earlier rather than in Murano 15 minutes later.
I walked over to Saint Mark’s Square and took other ‘watery’ shots. What a vision despite the sad event.
Securing process of the narthex of Saint Mark’s Church
The area in Saint Mark’s Square is one of the first areas to flood. The water starts coming up through the drains between the one marble slabs and the grayish stone with a tide of 70 cm, then followed closely behind the narthex of the church with its precious floors and mediaeval doors with 75 cm. It is a matter of inches. It has been calculated that between 10 and 30 November 2019 (so in conjunction with the extraordinary tide of Tuesday 12th November 2019 of 187 cm) the narthex was flooded for 250 hours! A real disaster.
Even if the tide is not always high it is very damaging for the old precious marbles of the Church, explosion of salt, crumbling away of materials and detachment of chips are the first evident consequences.
Between 2017 and 2019 a project of valves was developed to seal and protect the narthex up to 85 cm, but by now the water goes beyond so often this level that another solution had to be found.
A temporary project to defend Saint Mark’s Church should start soon, glass slabs will replace the iron railings forming a barrier up to 2 meters. These slabs should then be removed after the waterproofing of the entire square. The first project was refused, considered inadequate from the esthetic point of view, the second by the studio Boeri rejected, as the third one by ingegnere Rinaldo. So back in the race comes the initial project with some variations.
I looked over to the Procuratie Vecchie nearly completely flooded on the ground floor. How sad!
Passerelle wooden walkways
The Passerelle, wooden walkways are displayed throughout Venice from 15th September up to 30th April approximately; the most common months for acqua alta are in order November, December and October. In the summer months the tides reaches sometimes 80/90 cm, reason why the Church has its own walkways to allow visitors walking above the water in the lowest points.
These Passerelle, wooden walkways, some visitors exchange for stands for the daily markets (sic sic) allow to walk around up to a tide of 120 cm, becoming then unpracticable in Saint Mark’s Square with a tide of 130 cm.
A guided tour is possible with a tide up to 130 cm, as a Venetian born guide knows the heights of all streets or nearly… and places for great shots for photographers!