Bart Schultz
The city has been dealing with floods since its foundation in the fifth century. These floods are caused on the one hand by the rising of the Adriatic Sea and on the other hand by extreme precipitation and river discharges. Due to the rising sea level and subsidence, the floods gradually increased over the centuries. It is estimated that from the foundation of the city until the beginning of the twentieth century the relative sea level rise was about two metres.
During the twentieth century, subsidence accelerated due to groundwater extraction. Because this problem was recognized, groundwater extraction has been prohibited since the sixties. However, it is estimated that during the twentieth century, the water level at the sea has risen by 23 centimetres in relation to the built-up area. Flooding is therefore becoming more frequent. The worst flooding occurred on November 4, 1966, when the water rose to 1.9 metres above the average level in the lagoon due to backwater from the Adriatic Sea and excessive precipitation in the hinterland.
Following this flooding, a number of measures were decided upon, such as strengthening the beaches, jetties and quay walls. The most important project in this regard, however, is the MOSE project (Modulo Sperimentale Elettromeccanico), in which a storm surge barrier consisting of hinged steel caissons was built in the three openings between the Adriatic Sea and the lagoon. These caissons are filled with water in concrete tanks in the bottom of the three openings of the lagoon. If a flood is expected, the water can be pumped out of the caissons, causing them to tilt upwards and thus close off the three openings.
Construction was completed between 2003 and 2020. The entire system consists of 78 of these caissons with the associated facilities and has a total length of 1.6 kilometres, divided over the three openings. Pumping out takes half an hour and refilling after the flooding takes 15 minutes. Shortly after completion, the storm surge barrier had to be closed for the first time on October 3, 2020 and the famous St. Mark's Square was not flooded for the first time during such an event. A year earlier, the water had been almost two metres high in similar conditions in places.
The sea level rise will most likely continue in the coming years, perhaps even at an accelerated rate. In addition, the subsidence, albeit perhaps at a slower pace, will also continue. Sooner or later, the question will arise whether a polder should be created. The question will then be which area exactly would have to be reclaimed. Very diverse interests play a role in this. On the one hand, people will want to preserve the skyline along the large body of water in the lagoon as much as possible. On the other hand, the lagoon cannot be completely closed off, for example in connection with shipping to and from the ports in the hinterland and the discharge of excess water to the Adriatic Sea. It will require a great deal of insight and wisdom to arrive at a good and balanced decision. The unique city is certainly worth it.
