Conservation Comment Italy

Cambridge scientist challenges the Italian government to show that the cruise ship solution is safe

“A simple question: show us the evidence”, says Tom Spencer

The Venice lagoon is fast losing sediment and is becoming more and more like open sea

I very much look forward to seeing the observational data and mathematical modelling on which the decision to widen, deepen and straighten the Canale Contorta has been made and on the steps that have been taken to obtain proper, independent assessment of these lines of evidence. This is what would be expected of responsible, evidence-based policy making in the 21st century.

In modern times, there has been a serious loss of the ecological goods and services provided by the Venetian lagoon. These benefits include hazard regulation—the lagoonal damping of tidal surges and dissipation of wave energy by shallow waters, meandering channels and wetlands—and these losses are thus critical for the city of Venice itself.

Coastal lagoons are transitional environments between fully terrestrial and fully marine conditions; in the absence of direct human intervention, their long-term tendency is to infill with sediments. Over the centuries, the Venetian Republic was instrumental in maintaining this vital yet delicate land/sea balance, starting with the huge undertaking to divert the main rivers in the 16th century and stop the region from silting up altogether. Since that time, however, historical and near contemporary records of changing patterns of lagoonal topography and water depth; tidal currents; and sediment transport from the lagoon to the sea all show unequivocally that the current lagoon is moving in the opposite direction, becoming a downward-eroding, sediment-exporting system. It is thus on a trajectory that will turn it into a fully marine bay. That this process is well underway is evidenced by the appearance of plant and animal species in the lagoon that are characteristic of marine environments.

We may argue about the velocity of this trajectory but the evidence for such a trend, clearly related to a whole series of human interventions from the late 18th century to the present, is not in doubt. As wave height and tidal flows are strongly influenced by water depth, such a shift has critical importance for the sustainability of the historic core of Venice itself. If we drill down into the detail behind this general trend, it is clear that the excavation of two large canals (Canale Vittorio Emanuele, around 1925, and Canale Malamocco Marghera, around 1969) produced strong transversal currents across the original tidal network, with consequent siltation of channels and erosion of adjacent shallows.

There is one simple question that needs to be answered. Can we be assured that the large-scale excavation of the Canale Contorta will not have the same effect and not give the Venice lagoon a further shove in the direction of yet more environmental degradation and urban vulnerability? Where—in terms of proper scientific assessment, not simply talk—is this assurance that all Venetians, and all those concerned with the future of Venice, need?

Tom Spencer is a Reader in coastal ecology and geomorphology, University of Cambridge and the director of the Cambridge Coastal Research Unit

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Comments

7 Sep 14
12:33 CET

LINDA FRESCO , PORCIA

D'accordo

25 Aug 14
12:22 CET

DAVIDE ROCCHESSO, VENEZIA

On the measured effects of ship wakes on the lagoon, check: J. Rapaglia, L. Zaggia, K. Ricklefs, M. Gelinas, H. Bokuniewicz, Characteristics of ships' depression waves and associated sediment resuspension in Venice Lagoon, Italy, Journal of Marine Systems, Volume 85, Issues 1–2, March 2011, Pages 45-56, and M. Gelinas, H. Bokuniewicz, J. Rapaglia, and K. M.M. Lwiza (2013) Sediment Resuspension by Ship Wakes in the Venice Lagoon. Journal of Coastal Research: Volume 29, Issue 1: pp. 8 – 17.

23 Aug 14
13:49 CET

DAVIDE ROCCHESSO, VENEZIA

On the measured effects of ship wakes on the lagoon, check: J. Rapaglia, L. Zaggia, K. Ricklefs, M. Gelinas, H. Bokuniewicz, Characteristics of ships' depression waves and associated sediment resuspension in Venice Lagoon, Italy, Journal of Marine Systems, Volume 85, Issues 1–2, March 2011, Pages 45-56, and M. Gelinas, H. Bokuniewicz, J. Rapaglia, and K. M.M. Lwiza (2013) Sediment Resuspension by Ship Wakes in the Venice Lagoon. Journal of Coastal Research: Volume 29, Issue 1: pp. 8 – 17.

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