Bassa Marea - Low Tide in Venice
It's not often that you get to see in Venice a picture like the one above - algae carpeting the low-lying steps that a hundred years ago, were the level from which people boarded boats ... Back then, the water line was lower, and today, these stairs are just exposed with low tide - bassa marea.
Steps once used are now fully covered with algae, and exposed during bassa marea
A few visitors to Venice reacted positively when I told them that marea bassa had been around in the last few days - but you need to know that this state can be as detrimental to the fundaments of our town as is acqua alta. Marea bassa is when the low tide recedes as much as minus 50 centimeters from the reference level measured at Punta Dogana / della Salute. All levels below minus 90 centimeters are even called bassa marea eccezionale (extreme low tide). In the last decades, Venetians expected bassa marea to happen in late winter and early spring - February and March, but nowadays, the lagoon is unpredictable with regard to tides. Hydrological balance - with little variance in water levels - is what the buildings of Venice need to survive in the long term. With bassa marea, their fundaments are exposed to oxygen and may even rot. Boats may not be able to move because they may be stuck in the ground. Certain species of flora and fauna may suffer from lack of water.
Exposing the fundaments of the palaces ... bassa marea - an unusual view of the Doge's Palace and Ponte della Paglia (on the left)
But how do we distinguish high and low tides in the lagoon ? Take a look at this table classifying tide levels in Venice:
Source: Comune di Venezia
All tide levels ranging between -50 cm and +79 cm from the reference level are considered "normal high and low tide". All levels below are bassa marea, all levels above are alta marea (high tide).
Bassa Marea - view of the Bridge of Sighs
When the tide rises above 80 cm at the reference level at Punta della Dogana, which corresponds to Level 0 in Piazza San Marco (quota calpestio), water puddles (pozzanghere) will form in the Piazza. With the water level rising, in winter, people cross the Piazza on passerelle (wooden planks). This is possible until the water level reaches 120 cm. You see that the passerelle are arranged 40 cm above the ground. When the water rises above this 40 cm level, the passerelle are closed to the public and 28% of the surface of Venice is flooded, as you can see in the table above. On that 6 November 1966 when the worst acqua alta hit Venice in its history, the water rose to level 194 cm - which was 114 cm above the ground in the Piazza, with about 90% of Venice flooded.
Acqua alta level 85 cm in the Piazza
There are some factors that man can influence to promote hydrological balance in the lagoon, while others cannot be changed like the general rise in the sea level around the world due to climate change. 
What has always interested me is how the hydrological experts (Savi alle Acque) of the Republic of Venice (421-1797) succeeded in keeping the lagoon alive for 1400 years. During the last years of the Republic, the Murazzi (stone embankments) on Pellestrina were built to strengthen the narrow sandy buffer islands. And this miracle work by the Republic of Venice, the Murazzi saved Venice later in 1966, and is still effective !!!!
Lidi sottili sabbiosi - the long islands of Lido and Pellestrina separate the lagoon from the sea (you can see them in the background)
The Savi alle Acque (Wise Men, experts nominated by the Republic of Venice with the task of surveying and maintaining the delicate hydrological balance of the lagoon, within the government body called Magistrato delle Acque) separated the problems of interramento (the lagoon becomes a swamp) and allagamento (the lagoon becomes sea). 
Valle da Pesca - Fish basins - closed off from the rest of the lagoon
To avoid the lagoon becoming a swamp and the extension of the acque morte - those parts of the lagoon not regulary flooded by the tide, the Magistrato alle Acque diverted the rivers Brenta and Piave out of the lagoon. They even saved Torcello and its surroundings, though these measures took longer to work - in the meantime, most inhabitants moved to Venice and so Torcello lost its population.
Large areas of the lagoon were filled up to create the airport and the port of Venice
The Republic of Venice took to heart the principle of la FUOSSA - la sinuosità naturale dei canali - the naturally winding canals of the lagoon which allow for the tides to disperse into micro-canals called GHEBI, and thus avoid flooding Venice. In that manner, no direttisima caused the high tide rush towards the town, Each area of the lagoon was used and not filled in as was done after WW2 to create land for the industrial areas, the airport and the port of Venice. Also in the 20th century, a considerable portion of the southern lagoon was closed to create fish farming zones (valli da pesca). 
The Republic of Venice didn't want to eradicate high tides, but they wanted to avoid acqua alta without closing the lagoon, which was considered a "living organism".
Solid stone embankments at the airport built part of the lagoon 
The essence of the approach taken by the Republic of Venice is summarized as respecting the SFASATURA DEI COLMI: Based on the time the water needs to enter and exit the lagoon, the length of protective land tongues at the bocche di porto (lagoon entrance points) were calculated. These land tongues were maintained to prolong the path the water would take to reach Venice - il mare era più lontano - the sea, under hydrological aspects, was farther away than it is today. The main land tongue was called SCANNO DELLA PISSOTA - who knows that name today? 
During the times of the Republic, this is where the Scanno della Pisotta was located.
Scanno means sand bank created next to the mouth of rivers and lagoon. Scanno della Pissota was the name of a sandy land tongue protecting the main entrance into the lagoon, reaching far beyond the Lido from Sant'Erasmo causing the high tide to lose speed and disperse. Below you can see a picture of where it was located, in a map from the year 1735: The length of this land tongue was calculated based on the above-mentioned sfasatura dei colmi concept. All three bocche di porto (lagoon entry points) were protected by sand tongues like this.
Scanno della Pisotta, map of 1735 (private library source)
The morphology of the lagoon changed considerably since those times ... its surface was decreased by 30%, and the depth of the lagoon entrance poins was excavated from an average of 4 meters to reach 14 meters, to allow in le petroliere (oil tankers) when the Canale dei Petroli was excavated between 1961 and 1969.
Traffic of all sorts in Venice
It's not enough to immortalizzare la città - to immortalize the environment and buildings of the lagoon and of Venice - so much more is required to bring about quality in life for Venetians ... and what is done in this respect will be the topic of next week's post La Lavagna dei Desideri 2015 - Wish List for Venice in 2015.