Canal-laden Venice may be famously in peril from flooding, but thanks to work begun Tuesday, that peril may not last much longer.

Earlier this week, four retractable gates were dragged into the Malamocco inlet, a seaway into the Venice Lagoon that will ultimately be protected by a string of 57 flood barriers that can be raised at high tide. When all these gates are in place by June 2018, every single inlet that could allow flood waters into the Venetian Lagoon will be pluggable when the waters in the Adriatic Sea rise. For a heritage site long under threat, the completion of the huge MOSE project (short for “Modulo Sperimentale Elettromeccanico”) could mark a huge success—a scheme on a pharaonic scale that if effective will definitively prevent Europe’s most beautiful city from disintegrating into salty sludge.

Even now, however, there are dissenting voices on the project, which began way back in 2003. Do the works still underestimate the magnitude of the threat? And can we be sure that what has been constructed will actually do what it’s supposed to?

The barrier certainly proposes a bold solution to a long-standing problem. Venice has always been vulnerable to king tides, but subsidence around its lagoon (over 9 inches in the past 100 years) and rising global sea levels have made the danger more acute. Presented with annual photos of its ancient central piazza, St. Mark’s Square, under water, external viewers might be tempted to see the place as a bizarre mash-up of backdrops from Waterworld and Game of Thrones.

The reality is somewhat less alarming. Between autumn and spring, the city has always experienced acqua alta, or “high water” caused by a combination of astronomical tides, seasonal rain and strong winds that hinder water outflow from the lagoon. These high tides nonetheless leave most of the city unscathed (St Mark’s Square is the city’s lowest point) and typically subside after three or four hours. Even in the central square, the city has adapted: raised walkways are temporarily installed, and businesses (where power points are always installed at least halfway up the wall) simply block their doorways until the waters sink.

Of far greater concern are the exceptional high tides that occur every three years or so. The last of these, which took place in February 2013, saw water levels rise by 1.43 meters (4 ft 8 inches), enough to affect over half the city. Waters like these can wreck buildings, with residents retreating (often permanently) to upper floors and saltwater corroding masonry.

Venice is already fragile and under-populated—the old city’s population dropped from 120,000 in 1980 to 60,000 in 2009. While the city has taken protective measures such as raising quaysides, Venice needs far more than just a few extra walls to preserve its viability long-term.

As it happens, Venice does have some natural protection against flooding. The lagoon within which the city nests is enclosed by a string of barrier islands that help to shelter it. This barrier of sandbars is nonetheless breached by three inlets, which are essential to water flow and shipping but still leave the lagoon vulnerable to a battering from high tides. Venice has been repairing some of the damage by reconstructing areas of salt marsh and mudflat that could help manage high tides. These limited measures are still far from enough to protect the city overall.

To spare Venice from the highest water, Italy has thus been creating a barrier of retractable gates since 2003. The inlets at Chioggia and at the tip of Lido island (the closest to the city of Venice) have already seen their gates installed. A jaw-like row of barriers now sits on the seabed at these inlets, lying flush with the earth during normal weather but rising to form a barrage during unusually high tides. Plugging the final inlet at Malamocco will complete the string.

With all these barriers in place, the lagoon should be able to deflect tides of up to three meters. So that ships can still reach Venice Lagoon ports while the gates are raised, the Malamocco inlet, through which most ships passes, is being fitted with a lock gate.

Protection of this nature could nonetheless have negative side effects. Damming the inlets during peak tides will seal pollution inside the lagoon, potentially poisoning it if water outflow is disrupted for a long while. But in reality the gates are only intended to combat freak tides, not the standard seasonal acqua alta that will still be allowed to wash the city’s lowest sections. As a result, the disruption of tidal flows should be minimal.

That said, if sea level rise exceeds predictions, the gates may have to be used more frequently, risking stagnation and pollution build-up within the lagoon. Meanwhile, there are some question marks hanging over both the barrier’s construction costs and its functionality. An initial budget of €1.5 billion ($1.67 billion) has ballooned to a total cost of €5.5 billion ($6.15 billion). This rise was made harder to stomach when it was revealed in 2014 that at least €20 million ($22.4 million) had been skimmed off the budget in the form of kickbacks, leading to the arrest of Venice’s mayor.

It also turns out that barrier tests in 2013 that were initially declared successful actually had some problems. While the barriers were raised effectively, they were blocked from returning into their previous position due to a build-up of debris, and could not settle back until a team of divers had cleared the site. With the final barrier now being installed, it remains to be seen whether these setbacks are merely growing pains for a supremely ambitious plan or evidence that there are some flaws inherent to it. Let’s hope that the project’s ambition turns out to be fully justified.

http://www.citylab.com/tech/2016/09/venices-vast-new-flood-barrier-is-almost-here/498935/