Why Build a City in the Water?
Venice started as a small fishing village but started to grow when refugees from the Roman Empire fled as the conquerors, such as Atilla the Hun, invaded.
What started as a less-than-ideal situation and undesirable marshland became a beautiful thriving city. They were able to make and trade salt for money.
The ground was soft and the buildings would settle and sink if built directly on the ground, so the Venitians engineered a way to keep the buildings supported by driving long wood piles into the ground beneath the water and building on top of them. This snippet below from Architecture of Cities is much more in depth about how this system has held up for so long. read below:
"Long ago the buildings were built by using long wooden piles (about 60’ long) driven deep into the ground. These piles go deep down into the soil, reaching past the weak silt and dirt to a portion of the ground that was hard clay which could hold the weight of the buildings placed on the piles above. The piles were also driven into the water which normally would have been a disaster as wood rots normally. The wood used in the construction of the piles was very water resistant but even so the wood should have rotted away eventually. However, several things happened which kept the wood intact for over 500 years. The first is that wood rots only when both air and water are present, so in the oxygen starved environment of the water underneath the buildings, the wood was protected until the second thing happened. The waters of the lagoon carried an extremely large amount of silt and soil and the wood was being blasted by this sediment for years. The wood absorbed the sediment and quickly petrified into basically stone at an accelerated pace. The wood used in the construction of the piles was also very water resistant such as oak or larch. " - https://architectureofcities.com/venice#elements
Rainwater has to be collected for drinking water since the city is on the ocean.
Gothic and Byzantine
Basilica of San Marco, Constructed 828-832, rebuilt 1063-1094
Doge's Palace, Constructed 1424
Santa Maria dei Miracoli, Constructed 1481-1489
Ca’ d’Oro, Contructed 1428-1430
Venice is sinking slowly and salt from rising waters has gotten into the brick and has caused damage. High tides and rising sea levels pose a threat to the City.
A 6 Billion Euro Project called MOSE has been constructed in an attempt to save Venice from rising tides. 78 mobile barriers raise up from the water blocking the water. So far the project has already saved Venice from the rising waters.