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Much has been written, asked, and observed recently about acqua alta, as the once rare occurrence has become much more frequent in the past few years. A few explanations may help to understand, and eventually plan better how to face the situation so that it has the least impact possible one's time in Venice. Before starting on the details, it should be emphasised that acqua alta does not last for a long time: it is not like New Orleans or Brisbane. The tide comes in and the streets get water in them; after an hour or so, the tide turns and the water goes away. The Venetians are prepared for this, and it is no reason to avoid going to Venice.
“Acqua alta” is the name given to the higher than normal water levels that invade the city of Venice, causing the lagoon's salt water to overflow the edges of the canals and flood the pedestrian walkways. If the levels rise sufficiently, low lying buildings also have their ground floor areas filled with the brackish water and with the backflow of drains as the sewers also fill with the excessive liquids brought in with the tide.
Acqua alta is an exacerbation of the normal astronomical high tide levels (“alta marea” in Italian), influenced by a number of other factors, such as wind, rain and barometric pressure. As such, the level of acqua alta generally corresponds with the astronomical tides, and follows a schedule of six hours rising and six hours falling, with one of the tides generally higher than the other. High water levels are considered “acqua alta” when they rise more than 80cm above sea level, which is the lowest ground level in most all of Venice. Acqua alta can most often occur in the colder Winter months, particularly from November to March, but lately there is no time of the year completely immune to it.
The MOSE Project (acronym for Modulo Sperimentale Elettromeccanico - in English, Experimental Electromechanical Module) is a very large scale public works project currently being built at all of the connecting waterways (Lido, Malamocco and Chioggia) between the Adriatic Sea and the Venetian Lagoon. Its final goal is to block, in case of a forecast of acqua alta, the large incoming quantities of sea water at the inlets of the Lagoon before they can enter and increase the water levels inside the Lagoon. Normal tides will not be interfered with, so hopefully the delicate Venetian Lagoon ecosystem will not be impacted negatively by a decreased exchange of sea water. Unfortunately, while work is progressing the number of occurrences of acqua alta has increased dramatically, with some experts laying blame on the MOSE works themselves, which before being able to position the blocking dam elements, is actually reducing the passageways with the construction sites and support structures, creating greater difficulties for the tides to leave the Venetian Lagoon.
During acqua alta events, raised plank walkways (passarelle) are set out by city sanitation workers over a number of the lowest and most trafficked routes, according to a priority plan, based on the predicted maximum levels.
When the water levels rise in the canals, two things happen to fill the pedestrian streets and squares with the salty water: the levels rise above the height of the canal edges, spilling onto the walkways and the storm drains which are always the lowest spot in an area start to overflow as the storm sewers fill beyond capacity. It is a gradual process, and the rise is generally clearly discernible to the naked eye. Very often, even when flooding occurs, the higher portions of many streets remain passable, though sometimes requiring people to pass one at a time in opposite directions. The most serious problems occur when the levels near 130cm or higher, as then a majority of the land surface of the city remains under water. Practical considerations to deal with the situation in this case include considering alternative routes (including those with raised walkways), using public transportation instead of walking, or arranging your schedule to avoid low areas during the flooding and staying in high areas (or inside raised buildings).
Transportation problems also abound, as would be expected. Some of the ramifications are a little less obvious. When water levels rise above 95cm, the ACTV vaporettos have to modify some of their lines, as the clearance under some of the bridges become unsafe for passage. The main problem is that many visitors from outside the city have no knowledge of this, and thus remain stranded as they are trying to arrive to or leave the city. The circular lines, 41/42 and 51/52 split into two routes each, and no longer make the stops between Piazzale Roma and Tre Archi. So people arriving from Murano or the northern lagoon and headed for the rail station or Piazzale Roma find themselves left at Tre Archi, to walk the rest of the way (with portions of the route quite likely covered with water as well). The ACTV call center (local phone number +39 041 2424 [when calling from Venice, the +39 is not necessary] with English speaking operators) can provide current information, and some of the boat stops have illuminated displays which can, sometimes, give relevant information (though they are not always as up-to-date as they seem).
At the moment this was being written, yesterday was supposed to have 110cm and it barely got above 100cm. Today was forecast (less than 12 hours before the maximum level) at 100cm and it rose to 114cm. So much for the science of weather (and acqua alta) forecasting. The most recent three day forecasts are always available on the City of Venice's municipal website (shortcut: http://www.comune.venezia.it/maree/ - some portions of the site are translated, at least in part, into English. These forecasts are updated every few hours to take into account changes in the meteorological situation, but often are not accurate to more than 5-10cm. It is also possible to listen to an extract of the most recent forecast (only in Italian) by calling the local phone number +39 041 2411996. When the water level is predicted to rise above 110cm sea level, civil defense sirens are sounded to warn of the coming difficulties. There are several levels, depending on the expected maximum, with the siren signal changing with each additional 10cm increase forecast.
As tempting as it may be, the first and most important rule of acqua alta is to avoid thinking of the situation as a “day at the beach” and wade barefoot as a Venetian adventure (since it occurs more often during the winter months, it is usually not that tempting anyway). It is very easy for rubbish or other people's discards to remain submerged and out of sight, presenting serious danger to the unaware. Sometimes areas of temporary construction or excavation along the streets are covered with wooden planks after work has finished for the day, and these may float a short distance away, leaving the holes hidden under the water. If it is necessary to pass an area of high water, there are a number of solutions available, from single-use plastic slip-over boots which cost anywhere from €7.50 on up, to standard below-the-knee rubber boots, which begin at €15 and going into elaborately decorated fashion statements at corresponding fashion boutique prices (as much else, costing more the nearer you are to San Marco or to the train station). These will not lessen your need for caution when walking over unseen ground. For most routes, for most of the acqua alta events, higher hip-boots or fishing waders are not necessary.
One of the easiest to find sources of a good amount of information about acqua alta is the City of Venice website itself. Unfortunately not all the information is available in English. (You may copy and paste the URL into translate.google.com for a reasonable translation). The FAQ page answers a number of questions, but is not completely up to date and underestimates or side-steps some of the more serious questions or issues with typically political PR spin. It is still a good place to start.