Changes in Venice Lagoon dynamics due to construction of mobile barriers

doi:10.1016/j.coastaleng.2010.02.009

Coastal Engineering

Volume 57, Issue 7, July 2010, Pages 694-708

https://doi.org/10.1016/j.coastaleng.2010.02.009 Get rights and content

Abstract

The MoSE project (construction of mobile barrier to safeguard the Lagoon of Venice) entails changes to the structure of the lagoon's inlets. This could have consequences for the areas near the inlets and for the dynamics of the lagoon ecosystem as a whole. In order to predict the effects of the proposed alterations on the hydrodynamics of the lagoon, a well-tested hydrodynamic-dispersion model was applied. Simulations were carried out considering both idealised and realistic tide and wind scenarios.

The results show that with the new structures the Lido sub-basin tends to increase its extension due the southward movement of the watershed, at the expense of the Chioggia sub-basin, whereas the Malamocco sub-basin changes its relative position, but not its extension.

The residence time shows variations in agreement with this trend, decreasing in the southern part of the Lido sub-basin and increasing in the inner part of the Chioggia sub-basin.

The variations in residence time and return flow factor indicate that they are caused by changes in both instantaneous current velocities and sea–lagoon interaction. In fact the new breakwaters in front of the Malamocco and Chioggia inlets modify the length and direction of the outflow jet (up to 1 ms^− 1) and the patterns of the currents around the inlets and the nearby coast. The new artificial island in the Lido inlet changes the current pattern and increases the current velocity on the southern side of the channel propagating this effect up to the Venice city.

The risks and benefits individuated from our conclusion are that the Lido sub-basin can improve its renewal time, but the more intense current speeds can be a risk for the conservation of habitats and infrastructures. Finally the micro-circulation between the breakwater and the coast in Chioggia and Malamocco inlets can be a trap for pollutants or suspended sediment.

Introduction

The Venice Lagoon is located in the northwest Adriatic Sea. It is a large lagoon (500 km² in area, 50 km in length) with a complex bathymetry characterised by a network of channels, flats and shoals (Molinaroli et al., 2007). Water exchange between the lagoon and the northern Adriatic Sea takes place through three inlets situated on the eastern side of the lagoon. These inlets are named, from north to south, Lido, Malamocco and Chioggia. The first is around 1000 m wide, and the others about 500 m. The maximum depth is around 8 m for Chioggia and 14 m for Malamocco and Lido.

Most of the lagoon is very shallow, with average depths in the order of 1 m, but there are also a few deep channels (maximum depth around 15 m) leading inwards from each inlet and branching inside the basin. Traditionally the lagoon is subdivided into three sub-basins, one for each inlet, separated by two watersheds through which the residual flow is minimum (Solidoro et al., 2004). The exchange of water through the inlets in each tidal cycle is about a third of the total volume of the lagoon (Gacic and Solidoro, 2004). The main circulation forcing factors are the tide (± 50 cm during spring tide) and the wind. Stratification of water masses is seen only at some distance from the inlets, where the tidal energy is low. Inside the inlets, water velocities are high (over 1 m s^− 1) and the vertical shear creates enough turbulence to mix the water column. Consequently, water exchanges between the lagoon and the sea are essentially barotropic (Gacic et al., 2002).

The MoSE project (from the Italian acronym for Experimental Electromechanic Module, short description in http://www.veniceword.com/news/8/mose.html) is a long-debated project (Nosengo, 2003, Bras et al., 2001, Ammerman & McClennen, 2000) to defend the city of Venice and the surrounding lagoon from “high water” events. The project entails building mobile barriers at the bottom of each inlet which, when tidal events threaten to become critical, will rise and shut off the lagoon from the sea.

At the time of writing the project is still being implemented, and the configuration and bathymetries of the three lagoon inlets are being altered. These changes are likely to modify the interactions between the lagoon and the sea, the local hydrodynamics around the inlets, and the general circulation of the lagoon basin. All these aspects could have direct and indirect effects on the Sites of Community Interest (SCIs) around the inlets and on the quality of the lagoon environment as a whole (Spiro and Rizzardi, 2006).

The available literature includes studies of various aspects of the MoSE project: the Department of Hydraulics of Padua University (IMAGE — Padua University, 2006) analysed the hydrodynamic effects of various inlet configurations. Berrelli et al. (2006) explored the dynamics of the basin under different wind forcing scenarios and predicted the possible consequences of the mobile barrier closures. Umgiesser and Matticchio (2006) considered the potential negative effects of the MoSE project on commercial activity in Venice harbour. Rosatti et al. (2002) examined the effects of the mobile barriers on the transport of a passive pollutant. Bendoricchio and De Boni (2005) used a statistical model to quantify the effects on water quality.

Several investigations have been carried out in the past to evaluate the effect of different inlet structures on the tide levels inside the lagoon. The methods employed are the analysis of measurements (Pirazzoli, 2004), or the application of numerical models (Maticchio, 2004, Umgiesser, 1999, Benetazzo, 2004). Other works handle theoretical aspects on the application of numerical models (Delfina, 2004), or evaluate the effect of different arrangement of the inlets and of the lagoon on its residence time (Umgiesser, 2004). The configuration of the inlets, to which most of these studies are referred, has been recently changed, and in the previous modelling implementations simplified forcings, domains and set-ups have been chosen.

No investigations have yet been carried out, with the inlet structure recently projected, of the effects on water circulation in the Venice Lagoon resulting from modifications of the inlet structure in itself. Only Mosquera et al. (2007) analysed the timeseries of estimated monthly mean flows through the inlets and highlights the increased amplitude of the three tidal constituents in Chioggia inlet, starting from the second half of the year 2004; he suggests the possible impact of inlet narrowing on water flows.

After the MoSE project is completed, the most common situation in the Venice Lagoon will be one in which the new structures have been installed — thus changing the configuration of the seaward inlets — but are not in operation. The effects of this new inlet configuration are an important aspect of the question.

In this study, numerical modelling techniques were applied in order to predict the consequences for lagoon hydrodynamics of modifications to the geometry of the inlets. This approach makes it possible to analyse various spatial and temporal scales and verify local and global effects on the lagoon's dynamics. In addition, numerical modelling enables calculation of complex indices, such as residence times, which characterise the behaviour of the lagoon.

A coupled hydrodynamic and tracer-transport model was applied. Several simulations were carried out in order to compare the results obtained using two different numerical grids representing the post and ante operam configurations of the inlets, and to contrast the responses of the new and old configurations under different environmental forcing scenarios.

Section snippets

The SHYFEM hydrodynamic model

The SHYFEM model is a hydrodynamic model developed at ISMAR-CNR and applied successfully in the Venice Lagoon and in numerous coastal basins (Umgiesser, 2000, Melaku Canu, 2001, Umgiesser et al., 2004, Ferrarin & Umgiesser, 2005, Cucco et al., 2006, Zemlys et al., 2008, Ferrarin et al., 2010, Cucco et al., 2009). For spatial integration the model uses finite elements in the horizontal discretization and z-layers in the vertical discretization and a semi-implicit algorithm for integration in

Validation of the hydrodynamic model

The 3-D hydrodynamic model was validated by comparison with measured water fluxes at the inlets. The empirical water discharge data derived from ADCP measurements collected inside each inlet reflected both the influence of tidal and meteorological forcing (Gacic & Solidoro, 2004, Kovacevic et al., 2008). The comparison was 20 days long and was carried out with respect to 2002 and 2004 by adopting the ante operam grid and with respect to 2005 (when the work inside the inlet was almost complete)

Conclusions

The implementation of the MoSE project has entailed alterations to the structure of the inlets in the Venice Lagoon, with consequences that are both local (affecting the area around the inlets) and lagoon-wide. Our results indicate some of these consequences and make it possible to identify the potential risks and benefits for coastal management.

From model results, the mobile barrier construction does not affect water levels, while small differences can be detected analyzing velocities and a

Acknowledgements

This research was funded by the Osservatorio della Laguna. It was also partially been carried out in the framework of the VECTOR and CMCC projects. The wind data set from the CNR Platform and the tide level timeseries were provided by Venice Municipality. The flux data employed to validate the model were provided by Dr. Zaggia (ISMAR-CNR). Special thanks to Dr. Sarretta (JRC, Ispra) for technical assistance with the Geographical Information System and to Dr. Bellafiore and Dr. Ferrarin

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Short communicationChanges in Venice Lagoon dynamics due to construction of mobile barriers

Abstract

Introduction

Section snippets

The SHYFEM hydrodynamic model

Validation of the hydrodynamic model

Conclusions

Acknowledgements

Ecol. Model.

Ecol. Model.

Ecol. Model.

Ecol. Model.

J. Mar. Syst.

J. Mar. Syst.

J. Mar. Syst.

J. Mar. Syst.

J. Mar. Syst.

J. Mar. Syst.

Saving Venice

Science

Hydrodynamic coastal processes in the North Adriatic investigated with a 3D finite element model

Ocean Dynamics

Modeling the water exchanges between the Venice Lagoon and the Adriatic Sea

Ocean Dynamics

Rescuing Venice from a watery grave

Science

Water circulation and transport timescales in the Gulf of Oristano

Chem. Ecol.

Alcune considerazioni sulla stima delle dissipazioni di energia prodotte da opere fisse in una bocca lagunare

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Short communication
Changes in Venice Lagoon dynamics due to construction of mobile barriers