Freighter enters Malamocco Channel with the doors of Mose Project, prior to installation, in the foreground, Venice in the background

Venice, Italy

The city of Venice has had a problem with rising water forever. With the projected ocean rise, the threat is critical, leading to the idea of the Mose Project. The ambitious design is for a series of gates across each of the three channels into the lagoon, which when high water threatens the city, will be filled with air causing them to rise and close the channels. Instead of the initial estimate of 1.6 billion Euro, the final cost was over 6 billion. The projected date of completion was 2011, but corruption and construction setbacks delayed it repeatedly. In 2020 it was put into service, and does protect the city, though in December 2020, the computers failed to predict the height of the tides, and because the gates were not activated, the city flooded again.

The realization of the MOSE required approximately 220,000 m3 of concrete. If we calculate 410 kg of CO2 emitted per m3 of concrete, that means that 90,200 metric tons of carbon were emitted in the making of the concrete for the project. That’s equal to the climate change impact of 41,000 economy flights from New York to Venice.
It is a supreme irony that we are causing significant ocean rise by making barriers to protect our coastal cities against ocean rise.

Another hazard of closing an estuary from the ocean is the elimination of the constant refreshment action of regular tidal flow. The sewerage and toxics created will thus remain and increase in the closed system.
A recent article in the New York TImes discusses this issue in detail

Some Specifications:

MOSE has a total of 78 gates
There are 156 hinges, two for each gate and a number of reserve elements
Each hinge weighs 42 tons
The gates can withstand a 3-metre (9.8 ft) maximum tide (to date, the highest tide has been 1.94 metres)
Mose has been designed to cope with a 60 centimetres (24 in) rise in sea level