Acqua alta and acqua granda - the four truths about high tides in Venice

A beautiful morning on Piazza San Marco in winter - or so it would seem ..10:35 am: from the midst of the Piazza, the water is expanding - so the lowest level is the midst of the Piazza (pictures taken by me in November 2013).
The four truths about acqua alta, and how Caffé Quadri deals with acqua alta events: We have seen lately that high water is hitting town ever more often. Starting at Christmas and continuing well into the second half of January, we had a series of bad weather spells, some of which brought on the phenomenon of acqua alta
As usually, the "event" is covered extensively by the latest newspaper and magazine articles, on TV, in blog posts and videos - I even came across a "time lag video" showing how acqua alta conquered Piazza San Marco at night. There is even an i-phone apps available to guide you through Venice during acqua alta: the Venice tides app.
10:45 am: Water level is rising. 
Despite all this broad coverage of acqua alta, I find that the fundamental answers given to questions are not covered. Have we become too accustomed to acqua alta. The answer is - yes: By now, life goes on pretty much normal in town when the sirens announce that in the next 2 hours or so, we'll be in for acqua alta (if you would like to hear these sirens, click here).
10:55 am: water is now fast rising
What is important to know is how amazingly unlevel Venice is - not all areas are hit at once by acqua alta. You can see how slanted the Piazza's surface is, with the lowest point being in the center. Another example is Calle della Mandorla, where you can see the side alleys already covered with water, while you still walk along the "main" calle without shoes getting wet. This brings on a few questions, why do we have acqua alta in Venice in the first place, and since when? Has it gotten more serious over time? Is there even a positive side, or a need for "high tides"in the lagoon?
I have tried to collect "photo evidence" on that, which you can read in the "four truths about acqua alta". You will find an example of a famous cafe included, and now can understand how Caffé Quadri is hit by each acqua alta, as it is located on the low-lying side of the Piazza.
11:05 am: Piazza San Marco is unlevel: To the left is the Procuratie Vecchie area, and Caffé Quadri and Lavena are the first to go under water .. with huge damage done to the floors and walls (read more below).
The first truth is: acqua alta is not a bad surprise coming out of the blue, but it is "simply" an exceptionnally high tide. (though, that was not the case on 4 November 1966 ..).
Acqua alta is a phenomenon of rising and receding tide. Like on all coasts, it has always existed in the lagoon, of course also before Venice was "built" on the mud-covered islands some 1700 years ago. In principle, there is no escaping acqua alta, and its purpose in an amphibious environment is breathing life into the lagoon. But a balance between high and low tides is essential.
The lagoon is protected by two long and narrow islands from the open sea, Lido and Pellestrina, so there are three entrances into the lagoon which are called "bocche di porto".
There have been occasions (the last time was during the terrible acqua alta in November 1966 which brought the problem into the international limelight when newspapers titled "Venice is sinking"), when the wind swept the waters into the lagoon, across the Lido and Pellestrina islands, and the Murazzi stone barriers. No Mose dams could have helped on that occasion. That was when acqua alta was called acqua granda.
Acqua granda: Tides peak twice during 24 hours. In the graph above you can see that instead of receding, the tide rose after 18:00 pm on 3 November 1966, and reached a peak at above 190 cm on 4 November.
11:10 am: By now I am standing on the higher level, as I try to avoid wearing those sacchetti (stivali oppure calzari di plastica) or however you can call these plastic shoes. Water is now communicating between the Piazza and Bacino Orseolo behind it - no way you can get there on foot. Caffé Quadri (left) is flooded.
The second truth is: acqua alta is present in an amphibious territory like our lagoon all the time - twice a day: The high tide is even essential for the survival of plants and animals living on and around the barene and velme (mud islands). 
There are many more islands in the lagoon than the "official" ones numbered 35, plus Venice itself, consisting of 120 islands. The other, "unofficial" ones, uninhabited of course, are called barene and velme, and twice a day during high tide, they are covered with water. Without their daily regular flooding, these plants would die - no sea lavender blossoms would shroud the lagoon in purple blossoms in summer. No sea lavender honey (il miele di barena) would be available, as well as nutrients for many other animals, birds, insects and fish.
11:20 am: enough water to reflect the Campanile .. :-((
The third truth is: During the times of the Venetian Republic, the water surface of the lagoon was expanded to make space for high tides.
At the times of the Venetian Republic, the Savi alle acque surveilled on the state of the lagoon waters. They made sure there was enough space for the high water to spread out into the lagoon, to avoid damages to the city of Venice and the inhabited islands. To maintain this sensitive balance, they even transferred the mouths of rivers (Brenta, Piave) out of the lagoon.
The forth truth is: The water surface of the lagoon has been reduced ever since the beginning of the 20th century, from 550m² to approx. 400m². 
When the industrial area of Porto Marghera was built in 1917, not only the canals communicating with the seas were made larger and deeper, but also contaminated soil was deposited in the lagoon, leading to the existence of "artificial islands" that are called casse di colmata. (BTW: This part of the southern lagoon is being decontaminated, with first tangible results). So the area of the lagoon, which had been expanded during the times of the Venetian Republic, was considerably reduced.
The water surface was further reduced when the bridges (railway and 2 motorway bridges) were built across the lagoon to connect Venice with the mainland.
Damage done by acqua alta: One night, when Caffé Quadri had just been restored from the onslaught of acqua alta, I was told that there were serious cracks on the floor covered with turquoise glass mosacis, created by Murano artists, which where there right from the opening of the caffé in 1775
So what can be done to prevent damaging acqua alta: MOSE will help, but what really happens when the exchange of sea and fresh water is interrupted during high tide, for hours or maybe days on end, remains to be seen. When sea water is shut off for some hours, in the lagoon, both salinity levels fall and temperature may rise. There are some organisms that are responsible for the ecological balance in the lagoon that may be harmed.
Can you see the high tables in the background, covered with a white tablecloth? This is where the valuable chairs are deposited when acqua alta strikes at Caffè Quadri..
Crossing the Piazza, wearing their "calzari di plastica"
This is the third blog post of my series Venezia 2020. Click here to read the first article, where I describe how my grandmother witnessed that ominous 4 November 1966 when the worst acqua granda ever damaged the foundations of Venice.
The next two blog posts, published later this year in the "Venezia 2020" series, will be dedicated to two Venetians who collected unusual but practical evidence and useful views on acqua alta, back in the early 1990s: Marino Potenza (Il mare era più lontano) and by Giovanni and Giuseppe Tagliapietra (Salvare Venezia per chi)