Current legislation establishes the objective of guaranteeing the total defence of all built-up areas of the lagoon from all levels of high water, including extreme events. In selecting the intervention which best fulfils this objective, the designers assessed the effectiveness and technical and economic feasibility of a number of alternative design hypotheses grouped into the following categories:

•  interventions acting on the physical structure of the  lagoon (morphological);

•  intervention for the opening of the fishing valleys;

• interventions for the reduction of the volume of water exchanged with the sea;

•  interventions aimed at defending built-up areas of the lagoon by raising shores and banks

•  interventions to temporarily close one or two lagoon inlets

•   interventions to temporary close the three inlets.

Numerous possible combinations of these categories of solution were also studied as illustrated in the general Plan and in the Environmental Impact Study.

The solution chosen to provide a full response to the complex problem of high waters involves a combined system of interventions including: temporary closure of all three lagoon inlets by means of a row of mobile gates; local measures to raise shores and banks, compatible with the architectural and socio-economic structure of the individual built-up areas; and wide area morphological measures compatible with the environment to protect against the most frequent flooding.

This is a flexible strategy which differentiates between the various prevention measures in order to cope with a possible significant increase in sea level in the future without growing impact.
 

INTERVENTIONS FOR THE PROTECTION AGAINST FLOODING
Local protection works

Many alternative solutions were studied such as: raising of pavements; raising of perimeters of the islands; raising of each building; raising of large areas acting in to the soil. The fist two solutions were finally adopted the other two being demonstrated practically unfeasible.

Local defences involve than “raising” the level of shores, banks and public paved areas through complex operations designed to avoid filtration of water from the subsoil and flow back from drains; protection of ground floor areas of buildings from flooding; modernisation of the waste water drainage system and reorganisation of the network of underground service infrastructure. This type of measure has precise limits dictated by the urban, architectural and monumental contexts of the individual areas. Local defences must thus be integrated with the mobile barriers.

The different urban situations require different types of intervention, with the dual aim of eliminating the problems caused by the most frequent flooding as far as possible, while at the same time avoiding invasive measures affecting the built fabric by forcing the raising height. In the towns and villages along the barrier islands, for example, the sparser and less fragile nature of the built-up areas enables the raising to be carried out to a greater height, providing a relatively high level of protection (between +130 and +180 cm).

In the historic cities, towns and villages within the lagoon, in other words the lowest lying areas of Venice and Chioggia together with the islands of Murano and Burano, the operation is much more delicate and complex and the level of defence possible is considerably lower than in the built-up areas on the coastal strip. The projects approved and measures implemented to date demonstrate that without unacceptable alterations to the architectural elements and the relationship between the level of public areas and the level of ground floors, Venice can be protected up to a generalised and homogeneous level (therefore functional in terms of management of the mobile barriers) of +100/+110 cm.

These heights have, for example, been achieved along certain sections of Tolentini and are specified in the final design for the defence of the San Marco "insula", approved (among others) by the City Council. This confirms that the oldest and most valuable areas cannot be raised or otherwise protected above these levels. 
 

INTERVENTIONS FOR THE PROTECTION AGAINST FLOODING
Mobile barriers at the lagoon inlets 

Objectives

The objective attributed by legislation to the Design for Mobile Barriers is of the utmost importance - defence of the cities of Venice and Chioggia and other historic towns and villages in the lagoon together with the entire lagoon basin itself from the damaging effects of medium high tides and the devastating effects of exceptional tides. In line with the principles expressed forcefully in the special legislation, the plan to provide protection against high waters was given immediate priority. A selection was made from among the various alternatives and the design was dimensioned according to the system of relationships and constraints inherent in protecting and restoring the balance of the lagoon ecosystem and in accordance with the precise instructions of the Higher Council of Public Works, namely, to avoid interfering with port activities and the landscape, modifying water exchange between the sea and lagoon or damaging lagoon morphology and water quality.

Although with an extremely precise role, the Design for Mobile Barriers is nevertheless just one element in a much vaster Plan of Interventions which has never lost sight of, or attributed less importance to, other specific problems emerging with urgency from the general crisis affecting the lagoon environment, the oil tanker emergency in particular.

The solution involving separating the lagoon from the sea during exceptional high tide events has been elaborated at such length that few projects in the world have been studied and experimented in such depth. The hypothesis advanced more than once that a tidal wave some hundred or so kilometres long could be opposed simply by acting on the physical structure of a relatively limited and narrow lagoon by means of wide range morphological measures throughout the lagoon basin has, on the other hand, been shown to be completely ineffective in reducing water levels in Venice.  The wide range morphological measures, compatible with the environment, are on the other hand indispensable to achieve various general objectives (and with this scope in mind are currently being implemented) such as the safeguarding and restoration of the lagoon environment and improvement of water quality.

Local morphological measures affecting the cross section of the lagoon inlets, with compatible modification of volumes exchanged between the sea and the lagoon, are on the other hand able to reduce high water levels in Venice only by few centimetres, depending on the characteristics of the tidal events. On its own, this type of measure, although it may partly limit the number of floods, cannot therefore resolve the problem definitively and, in particular, is not able to provide any form of protection from exceptional events.

Nature and operation

The mobile barriers consist of rows of gates installed on the seabed at the lagoon inlets. They are defined as "mobile" as under normal tidal conditions they are full of water and rest in recessed caissons on the seabed. Each gate is attached to the caisson via hinges. When tides above the established height are forecast (if the height is 100 cm, an average of 7 times a year; if the height is 110 cm, an average of 3-5 times a year), compressed air is introduced into the gates to expel the water. They thus rise, rotating around the axis of the hinges, to emerge and block the tidal flow.

The inlets remain closed for the duration of the high water only and for the time required to manoeuvre the gates (on average, a total of four and a half hours). The feasibility and effectiveness of the proposed solution have been studied during years of tests on mathematical and physical models with the contribution and control of leading hydraulic model laboratories.

A life size prototype of a gate has been constructed (Modulo Sperimentale Elettromeccanico – Mo.S.E.) and subjected to four years of testing, enabling operation of the gate to be optimised and the individual components of the system, the constructional materials and their reaction to the aggressive marine environment to be verified.

Time scale, costs and job creation

The mobile barriers could be completed within 8 years from assignation of the work (and activities could begin on the first sites two years from the start of the final design phase) with a gross cost of about 1,900 million Euro (unforeseen events and vat excluded).

Construction of the barriers would create a total of about 1,000 direct jobs per year, plus a further 4,000 jobs in the indirect or spin-off sectors in addition to jobs associated with work to raise the level of paved areas. In the operational phase, maintenance and running of the mobile barriers and regulation of the lagoon ecosystem in association with defence from flooding would generate a further 150 jobs per year.

This type of project requires a high percentage of unusual work and advanced technology. This suggests that it could lead to local development of specialised new activities and professions. In short, execution of the design, with massive funding concentrated in a short space of time and limited geographic area, would have major repercussions on the economy of Venice, representing an important objective for the targeted conversion of production in the city and industrial area.

Interventions for the protection of the coastline from the sea storms      [Click here]

Interventions for the restoration of the environment                               [Click here]