IMPACT ASSESSMENT
Mobile surge barriers at the three inlets of the Venice Lagoon - Italy 

Client: Venezia  Nuova  Consortium -  Ministry  of Public Works

Services: Basic and Preliminary Design of flap-gates barriers at the lagoon inlets of Chioggia, Malamocco, S. Nicolò di Lido and                    Treporti and all related marine electro-mechanical and civil works.

Period:  1990- 92

Construction cost:  € 1,549,370,697

The project:
The works at the lagoon inlets for the defence of Venice and its lagoon from tidal flooding represent one of the largest hydraulic engineering projects of our times.

The optimal solution to solve this problem, as identified by the Preliminary and Basic Design phase, is the construction of four fixed barriers with buoyancy flap-gates, two at the Lido mouth, one at the Malamocco mouth and one at the Chioggia mouth, with the following dimensions:

• Lido S.Nicolò:           width 400m., depth 11m., 20 flap-gates
• Lido Treporti:             width 420m., depth  6m., 21 flap-gates
• Malamocco:               width 400m., depth 15m., 20 flap-gates
• Chioggia:                   width 360m., depth 11m., 18 flap-gates

Each flap-gate will consist of a rectangular metal caisson, 20 metres wide, 4 metres high and of variable length depending on the depth of the canal.

The four sets of buoyancy flap-gates, with an overall length of almost two kilometres, will be anchored to pre-cast concrete foundation caissons floated to the site place and laid in special trenches, and will be raised by the introduction of compressed air during exceptional tidal conditions to prevent the lagoon from flooding.

Six refuge basins, with navigation locks for small and medium sized craft, will allow the harbouring and transit of marine traffic even when the flap-gates are raised.

The preliminary design of the work at the inlets was performed by Technital during 1990-1992, with the co-operation of numerous specialised national and international institutes which, under the designer's co-ordination, carried out innumerable tests on physical models, studies with mathematical models and measurement campaigns.

Following the analysis of the results of the physical and mathematical model tests and of those derived from monitoring the full-size experimental module (MO.S.E.), the "heart" of the system , the buoyancy flap-gates, were designed.

The definition of the functional and structural dimensions, the selection and scaling of special components and of the corrosion protection system, and the definition of the tolerances on the basis of the construction methods to be used were performed separately for each inlet.

The dimensions of the principal structures of the barrier were then defined: these include the flap-gate housings, the fixed works on either side of the barriers, the gantry crane shelters and the control building.  Since these are the only above-ground structures of the complex, a special  study was carried out for their insertion in the environment resulting in the definition of singular architectural solutions for the buildings, compatible with local morphology and the historical tradition of the city. By means of mathematical solutions of the problems of building-ground interaction, the related settlements and the safety coefficients for various alternative solutions, a type of foundation was selected consisting of caissons pre-cast off-site and sunk in situ with flexible watertight joints.

The design of the refuge basins and their connection with the mobile works and that of the locks for small craft involved, in addition to the hydrodynamic and wave motion model studies and the traditional dimensioning, the use of  models for simulating the successive arrival of craft and models for the access and evolution  manoeuvres within the internal basins. The design of the bottom protection, the auxiliary technical plants and the study of the most suitable  realisation stages in relation to the construction methods and to minimal interference with the anthropic activities concluded the design activities.
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