Deep trouble: can Venice hold back the tide?

By Neal E Robbins

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The next day, another acqua alta of 1.56 metres broke a record, flooding 75% of the city, giving Venetians a real scare, but now, a year on, that has paled in the face of a new record flood recorded on 12 November 2019, of 1.87 metres, the highest in more than 50 years, flooding over 85% of the city. Lesser record highs hit in the following days. The flood caused millions in damage, and two deaths – one man who tried to restart a water pump was killed by electrocution, and another was found dead in his home. The extended flooding has disheartened many among the city’s depleted population of 53,000, with some now thinking that there is no future for Venice.

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The average sea level rise predicted by the IPCC, the United Nations body for assessing climate change, is 0.43 metres by the end of the 21st century, and it could be as high as a metre or more. In Venice, higher water levels, adding to the effect of subsidence, are creating new, possibly unsustainable, stresses on the lagoon defences and the city. Already, higher water levels cause rising damp in Venice’s ageing walls, crumbling the bricks and rusting the ties that hold up the buildings. The effect of higher water is also aggravated by lagoon erosion. While the tides took between 90 minutes and two hours to enter a century ago, now they enter in an hour. With the lagoon an average of 1.5 metres deep, twice what it was, the tides not only rise higher but also move faster and in greater volume on entering and leaving. The number of acque alte have also been rising since the last century: the number of floods over 1.1 metres have doubled since the 60s, due not only to subsidence and sea level rise, but also to increase in wind, waves and storms related to the climate crisis.

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The 1966 flooding disaster that led to special laws for Venice launched decades of studies and planning and opened a multibillion-euro tap of funding that would go into housing refurbishment, art restoration and a two-part programme to save the lagoon. One part dealt with acqua alta up to 1.1 metres by bolstering the shock-absorbing effect of the salt marshes and sea fronts while building smaller barriers and localised adaptations in Venice and on other islands. The other part, for flooding over 1.1 metres, when the sirens sound, envisioned the massive dams dubbed Mose (pronounced Mosé), a strained acronym for Modulo Sperimentale Elettromeccanico (Experimental Electromechanical Module).

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The major challenge for the dams is that acqua alta is never flat. When there is wind – and there is always wind – the surface is high here and low there, and it can even push water out of one port opening and into another, said Rusconi [Venice University Civil Engineer Professor]. “The wind crams the water into certain places, raising it up by as much as 40cm. Controlling this system will involve fights and legal issues. It will be impossible to manage, not just technically. This is where things will get unclear and unclean, because this is a political choice – and we know that these choices get made because the big companies are paying.” The waves will cause the raised barriers to flap, creating “multi-directional stress” that will cut the lifespan of the hinges, he said, while every storm will send sand, mud and debris into the wide grooves that cradle the barriers under the water, preventing them from re-closing.

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