Venice was hit by high tides of up to 1.5 metres (5ft) on Tuesday after its flood barrier system was not activated due to mistaken forecasts.
Weather bulletins had predicted high tide, or acqua alta, rising to 1.2 metres – lower than the 1.3 metres level at which the 78 mobile barriers of the defensive system, called Mose, would usually be activated.
By the time the water had entered the lagoon on Tuesday morning, completely flooding the narthex of St Mark’s Basilica, it was too late for the system to take effect.
Tides were expected to reach 1.45 metres at about 16.40 local time, although residents estimate the water level to be as high as 1.5 metres.
“The situation is really bad as we weren’t expecting it,” said Matteo Secchi, who leads the activist group Venessia.com. “It’s frustrating as we thought that with Mose this kind of thing wouldn’t happen any more, but instead we’re back to square one. It’s the same old problem.”
Carlo Alberto Tesserin, the head procurator of St Mark’s Basilica, told the Italian media: “The situation is terrible, we’re under water in a dramatic way.”
He said the narthex of the 1,000-year-old cathedral was completely flooded and if the water rises any higher “the internal chapels will also go under”.
Secchi said that bars, restaurants and shops had been affected but the extent of the damage is not yet known.
The long-awaited Mose system, which was given its first full test in July, was praised for saving Venice from recent high tides. In early October, Mose’s huge yellow floodgates, which rise to separate the Venetian lagoon from the sea, succeeded in shielding the city during its first real-time test when the high tide rose to 1.2 metres. The system again functioned successfully a few weeks later in preventing water of up to 1.35 metres from entering the lagoon.
The Mose dams were designed in 1984 and were supposed to come into service in 2011, but progress was blighted by a corruption scandal and cost overruns.
The Italian government came under pressure to finally finish the €6bn (£5.4bn) project, which is designed to protect Venice from tides of up to three metres, after the city experienced its worst flood since 1966 in November last year. The flood killed two people and caused an estimated €1bn worth of damage to monuments, businesses and homes.