They keep raising it. Plus it is underwater at certain high tides despite their best efforts.
Here is a more honest discussion about a city that builds on swampy sinking land thus been a problem for centuries.
Venice and sea-level rise
Excerpt:
Background
On November 4, 1966, Venice, Italy, was inundated by a flood of 1.94 meters (6.36 feet) above mean high water. The extent of the flood reflected the combination of high lunar tides, heavy rainfall, and fierce, persistent winds. The storm surge overwhelmed the Lidi, destroying sea walls. Meanwhile, flooding rivers poured freshwater into the brackish lagoon, causing ecological harm. Bases of historic buildings were immersed in lagoon water for two days, producing considerable damage. This set a new record for high water (
acqua alta) in the city, although a 2019 flood reached 1.87 meters (6.2 feet) and nearly broke the decades-long record. The 2019 flood killed two people and caused roughly $1 billion (USD) in damage.
Sensitivity to Flooding
Venice is a particularly sensitive indicator of long-term changes in
sea level, because several factors causing the lagoon water to rise, or the land level to sink, are poised against human interventions, such as dike building and diversion of rivers. Sources of flood waters are rivers that originally emptied into the lagoon, including the Piave and the Sile in the east and the Brenta and Bacchiglione in the west. All these rivers have been diverted away from the lagoon, but during episodes of heavy rainfall they manage to overflow into the lagoon anyway. Heavy rains over the ocean can also augment
storm surges in raising sea level.
Venice itself, meanwhile, has been sinking in a process called subsidence. Subsidence can be natural, as when sediment is compressed by the weight of soil and water above it, or artificial, caused, for example, by removal of water from an underlying
aquifer for industrial purposes. Such removal was forbidden by statute in the 1970s, and Venetian subsidence was substantially reduced as a result, however, data from the 2000s revealed that Venice was still subsiding about 1–2 millimeters a year. By 2024, the city had sunk an estimated 23 centimeters (9 inches) over the last century.
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