Extreme events

Coastal risk in the North Adriatic sea: understanding Venice “acqua alta”

The North Adriatic coastal plain of Italy is the most vulnerable to extreme sea events in the Mediterranean due to the shape, morphology and low bathymetry of the Adriatic basin, which cause water level to increase suddenly during coastal storms. The Extreme Sea Level (ESL) in this section of the coast depends mainly on astronomical tide and meteorological forcing, such as low pressure, seiches and prolonged South-Eeasterly winds (Sirocco), which is the first driver of storm surge in the Adriatic, even though highest wave heights are associated with strong N/NE Bora wind gusts. The effect of tides is quite pronounced along the Adriatic, topping about one meter in the Northern-most sector. Aggrevating the situation, all the coastal profile of the Padan plain shows relatively fast subsiding rates, partially due to natural phenomena, but in large part linked to human activities (i.e. water and gas extraction).

The lagoon of Venice, located at the  uppermost part of the basin, has been suffering from aqua alta (“high water”) since centuries; these events are strongly seasonal, occurring 90% of times in late autumn and winter. The most severe ESL event ever recorded happened in 1966, with a record increase of 1.94 m above Mean Sea Level (MSL). More recently, in November 2019, four ESL events exceeding 1.4 m happened within only 5 days, with
the second largest flood ever observed happening during the night of 12-13rd reaching 1.87 m. Around 87% of the historical city center was flooded.

According to historical records from the city, twenty ESL events topping 1.4 m hit the city between 1872 and 2018. In this period, the MSL has risen about 25 cm, mostly due to subsidence, increasing the frequency of high water events (ESL > 1.1 m) from one to 5-6 events per year, triggering major losses to the urban heritage and posing major concerns about the future of the city.

Today, subsidence due to human activities has slowed down, ranging from 3-4 mm/year in the northern portion to 2-3 mm/year in the southern portion. However, eusthatic sea level is expected to rise as effect of anthropogenic global warming. Accounting for the combined effect of subsidence and sea level rise, the MSL in the N Adriatic coast is expected to increase by 21-28 cm before 2050 and by 67-81 cm by 2100, compared to historical values.

The Italian Government funded the realization of the MOSE project (“Experimental Electromechanical Module”), which consists of several mobile barriers placed at the three lagoon inlets to prevent water exchange with the lagoon any time the elevation in the Adriatic Sea achieves the 110 cm height above datum. The construction started in 2003, but MOSE is not expected to become operational before 2021.