Box 10F Venice, a city by the water

Venice

Source: G Arici/Grazia Neri/Milano


Venice is exposed to natural and artificial modifications of the environment at the interface between the Adriatic Sea and the large alluvial basin extending from the Alps to the coast. The combination of a fragile environment with its extraordinary architecture and historical value make Venice a unique case in some respects. However, Venice's hydrological problems are common to many other European cities near coastal waters. Venice is exposed to increasing erosion and flooding which affect the urban fabric. The delicate equilibrium of the lagoon is threatened by increased pollution. The impact of sea-level rise related to global climate change could be enormous.

From the beginning of this century the difference in level between land surface and water surface in Venice has changed by 23 cm. Sea level has risen approximately 11 cm while the land has sunk by 12 cm. On average, flooding now occurs 40 times a year, which is six times the average at the beginning of the century. In Venice, groundwater overexploitation has accelerated the process of salt water intrusion and subsidence, and successive human interventions are also modifying the lagoon bed. According to recent estimates, the lagoon is losing one million cubic metres of sediment a year. At this rate, there is strong concern that the lagoon could, in the future, gradually become a branch of the sea, unprotected from marine action.

Water pollution has also affected the biology of the lagoon. During the last decades, pollution of the water and sediments in the lagoon have reached unacceptable levels. The intense growth of algae that explodes nearly every summer and the decreasing biodiversity of the ecosystem are all symptoms of eutrophication caused by the enormous quantities of nutrients discharged into the lagoon. Since the beginning of the century organic pollution has tripled, and the nutrient load is five times higher. The lagoon receives each year between 10 000 and 12 000 tonnes of phosphorus and nitrogen. Urban pollution accounts for 15 to 20 per cent of the total. The industrial sector accounts for 20 to 25 per cent. However, the largest proportion (40 to 60 per cent) of the nutrient input comes from agricultural, industrial and urban activities taking place in the catchment of the lagoon.

These modifications affect Venice's urban environment and may compromise its future. In Venice, the degradation of historical buildings has increased as an effect of exposure to more frequent and severe storms. Subsidence, eustacy (change in sea level) and storm floods threaten the stability of the old city structure and cause the continuous deterioration of buildings. Pollution of water has substantially constrained its uses. Socio-economic factors, such as the significant change in activities and depopulation that occurred during the last decades, have contributed to the process of degradation. Indeed, accelerated degradation is also a result of inadequate maintenance of buildings and infrastructure.

During the last 30 years, alternative solutions have been studied and proposed for physical defence from flooding as well as for environmental clean-up and restoration. Plans for a project of defence by means of tide gates at three openings of the land barrier separating the lagoon from the sea were completed in October 1989 by the Consorzio Venezia Nuova. In addition, other important measures are under way, including: rehabilitation of lagoon morphology; environmental clean-up; restoration of the shore-line; building of a sewage and rainwater drainage system; and restriction of port traffic (eg, oil tankers). However, concrete measures have still to be taken. The mobilisation of the international community is also important to help protect one of Europe's most valuable historical cities.