Port Activities and their Effects on the Lagoon and Therefore Venice

13/10/2009

Port Activities and their Effects on the Lagoon and Therefore Venice


There has been much discussion of the effects of the Marghera-Malamocco navigation channel on the Venice Lagoon, originally dredged in the late 1960s for petrol tankers and cargo ships to access the industrial development.

With broader inlets between the sea and the lagoon, together with rectilinear and deep navigation channels, more water has been coming in and going out of the lagoon and the central part of the lagoon is especially affected by more intense erosion that has made the area deeper overall and removed the characteristic diversity of lagoon forms (marshes, mudflats, shallows, tidal channels and creeks). The saltmarshes especially are vital to the wellbeing of Venice inasmuch as they help attenuate the incoming currents and thereby dampen water levels in other parts of the lagoon. Saltmarsh extent is now just a third of what it was about a century ago and Venice is suffering from chronically high water levels which accelerate the deterioration of buildings rather than attracting media attention to dramatic scenes of flooding or "acqua alta".

Erosion of the lagoon and disappearing saltmarshes due to port traffic is evident all the way along the main navigation canals. The wave energy produced by the passage of ships and tankers spreads laterally across the shallows and mudflats bordering on the deep thoroughfare, resulting in re-suspension of sediments. This material is then dragged along by the transverse currents induced by the ships' motion, and deposited in the deep channel, where the strong tidal currents carry the material out to sea. Anything that is not taken away by the current has to be periodically dredged anyway by mechanical means (and at great expense) to maintain the depth necessary for tanker traffic.

In the light of this scientific understanding of how deep navigation channels and port traffic can be detrimental to the lagoon environment, the prospect of intensifying port activities and allowing the transit of even bigger ships (as long as 400m) is deeply worrying for those concerned with safeguarding Venice since decomposition of the lagoon morphology weakens the ecological resilience of the entire lagoon system and threatens the processes on which the wellbeing of the city and lagoon depend.

Even without the planned dredging to increase depth in the Malamocco-Marghera channel the hulls of larger ships intensify the transverse currents and associated sediment resuspension leading to erosion. Together with wave energy throughout the lagoon, generated by wind and other boat traffic, this is a principal cause of lagoon erosion that could exacerbate the already precarious ecological state of one of the largest and most important Mediterranean wetlands.

As explained by Dr Tom Spencer in The Venice Report, the proposal to enlarge the port and deepen the navigation lock at Malamocco while entrusting the mobile barrier (MOSE) system (still under construction) with a significant role in maintaining lagoon morphology is misleading. From a scientific and technical point of view the only use of MOSE is to protect the lagoon and inhabited islands from extreme flooding.

Since there is now significant consensus among researchers that the main cause of morphological degradation lies in the effects of wave energy in the lagoon basin, the presence of the mobile barriers at the inlets cannot be expected to have any effect on the phenomenon, irrespective of how they are manoeuvred or managed.

The only possible way to safeguard the lagoon and its ecological functions, which are vital to the survival of the buildings of Venice, is by carrying out works that restore and protect the morphology by reducing wave height (which means reducing water depth where possible and reconstructing saltmarshes within the large areas of open waters to reduce the fetch); containing the hydrodynamics of the large navigation channels by building borders to limit sediment re-suspension on either side; and obliging motorboats and water buses to respect speed limits in the lagoon.

Luigi D'Alpaos and Jane da Mosto
 
FURTHER READING:

  •  D'Alpaos, L. Fatti e misfatti di idraulica lagunare - La laguna di Venezia dalla diversione dei fiumi alle nuove opere alle bocche. Istituto Veneto di Scienze Lettere ed Arti (presented 28 feb 2009). In press.
  •  Molinaroli E, Guerzoni S, Sarretta A, Masiol M and Pistolato M Thirty-year changes (1970 to 2000) in bathymetry and sediment texture recorded in the Lagoon of Venice sub-basins, Italy. Marine Geology 258:115-25.
  •  Sarretta A, Pillon S, Molinaroli E, Guerzoni S and Fontelan G Sediment budget in the Lagoon of Venice, Italy. Continental Shelf Research doi: 10.1016/j.cr2009.07.002.

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