How Venice intends to keep the Adriatic Sea at bay

The natural phenomenon of acqua alta (‘high water’) happens several times a year in Venice when high tides from the Adriatic Sea combine with winds and long waves to flood the city. In 2019, Venice experienced the worst acqua alta since 1966, with 1.87m-high tides flooding two-thirds of the city. The number of high tides over 1.4m has been increasing, with 14 in the last 20 years.

In 1984, a scheme to protect the historic city’s inhabitants and buildings was designed, consisting of barriers at the mouths of each of the three inlets to the Venice Lagoon.

The MOSE (Modulo Sperimentale Elettromeccanico, or Experimental Electromechanical Module) is a network of barriers located at the Malamocco, Chioggia and Lido harbours. Each barrier is a series of metal gates, which are raised when the high tide exceeds 1.1m and can protect from tidal waters of up to 3m.

There are 19 gates at the Malamocco inlet, 18 at Chioggia harbour, and the Lido harbour is separated by 20 at the South Lido and 21 at the North Lido inlet. Between the two lies an artificial island, which houses the gates’ operation centre.

In Malamocco harbour, the deepest of the inlets, the building work includes a navigation lock to allow ships to pass. In the Lido and Chioggia harbours there are refuge havens with small locks that allow fishing trawlers, leisure boats and emergency vessels to pass when the barriers are raised.

The metal gates are hollow but with internal columns to ensure resistance for the sea waves. They are painted to protect them from the marine environment and secured in watertight structures called caissons, which are installed on the seabed. Gate dimensions vary according to the depths of the canal, but they can be between 18m and 28m long, 3m to 6m thick, and all are 20m wide.

To activate the defence system, compressed air is pumped into the gates, emptying them of water and rotating them around their axis to raise them to form a barrier at the entrance of each harbour. As the water ebbs and reaches the same level as the lagoon, the gates fill with water and return to their original position in around 15 minutes. The varied sizes of inlets and different locations means that installation is different for each harbour.

Chioggia is to the south of the Venice Lagoon and its defence system is called Baby MOSE. It includes two gates on the Canal Vena which will rise to isolate the canal and prevent the tide from flowing over the quayside or rising up through the drains. The project also includes reinforcing the banks of the canal and modernising the rainwater and drainage systems.

At Malamocco, the deepest of the inlets, the caissons were rolled along a track to a Syncrolift, in a system designed by engineering company Fagioli. The caissons were lifted and sunk below the water level to be collected by a custom-designed catamaran, created using two 190-ton river barges and a pair of steel crosshead beams. This catamaran drags the caisson to its final position in the harbour, where it is lowered to the seabed.

At Chioggia, the caissons were built in a dry dock and, once they were completed, the dock was filled with water. A barge was positioned over the caissons and, together with four tug boats, dragged the caisson to its position in the inlet.

GPS was used to ensure the caissons are accurately positioned. Fagioli also created launching gantries, each with four towers and two crosshead beams and a fishing beam to hook each metal gate. Self-propelled modular transports carried the gate to a barge, which transported it into the harbour entrance. There, a mobile crane set the top sections onto another set of tower legs, connected to a frame already in place in the harbour. This launching structure was then used to lay the gate on to the caisson, where it was fixed in place.

Hydraulics company Enerpac created caissons for three of the four barriers. It developed a damping and levelling control system, with four cylinders at each corner of the caisson and hydraulic accumulators, which absorb forces as the huge structure is placed onto metal sliding plates mounted on concrete mats positioned on the seabed. The control system cushions the momentum caused by lowering the caisson and an algorithm stops residual wave momentum from dislodging the caisson while it is being positioned. A watertight seal between caissons allows a service tunnel for maintenance once the project is complete.

Construction work for the MOSE project began in 2003 and was scheduled to be completed in 2011, but delays – from environmental and conservationist challenges as well as bribery and corruption scandals, have resulted in a 10-year slippage and an increase in budget from €1.6bn to €5.5bn (around £4.8bn).

Raising the gates takes 30 minutes and they remain closed for around four to five hours each time. The barriers have been tested every day since December last year, when they proved effective against high tides of 1.35m. The barriers are raised when there is a forecast of high water above 1.3m. In December 2020, incorrect forecasts of 1.2m-high tides meant that the gates were not raised and 1.37m-high waters flooded parts of the city.

At the time, Venice’s mayor, Luigi Brugnaro, said that the project is still being tested and that protocols would be developed to activate the defence system sooner. It was activated again shortly afterwards to protect the city from a 1.2m tide the following week.

MOSE is scheduled to be fully operational by the end of this year to protect historic buildings and Venetians’ businesses from the Adriatic Sea.