The Mose Project - To Isolate Venice From High Tides

 

Sudeshna Mukherjee 
Assistant Editor
Civil Engineering and Construction Review

 

 

MOSE, is an acronym for Modulo Sperimentale Elettromeccanico, which means Experimental Electromechanical Module. The project is an integrated system consisting of rows of mobile gates installed at theLido, Malamocco, and Chioggiainlets that can isolate the Venetian Lagoon temporarily from the Adriatic Sea during acqua alta high tides. Together with other measures, such as coastal reinforcement, the raising of quaysides, and the paving and improvement of the lagoon, MOSE is designed to protect Venice and the lagoon from tides of up to 3 metres.

Requirement

The Great Flood of 1966, which caused massive loss of life and property, and the sinking of the city by 11 inches during the course of the last century, provided the momentum and necessity to protect Venice. The reasons for the sinking of the city of Venice are principally attributed to the rise in the sea level and extraction of ground water and methane gas within the vicinity of the Venetian Lagoon. In 1982 Consorzio Venezia Nuova was entrusted by the Water Authority to design and implement the measures to safeguard the city, which was presented in 1989 under a project named Riequilibrio EAmbiente (REA). It provided an abstract design of the mobile barriers at the lagoon inlets and was finally approved in 1994 by the Higher Council of Public Works.The first environmental impact study was accepted in 1998 and was improved in 2002. Construction work of MOSE finally started in 2003.

The aim of MOSE is to protect the lagoon, its towns, villages and inhabitants along with its iconic historic, artistic, and environmental heritage from floods including extreme events. Although the tide in the lagoon basin is lower than in other areas of the world (where it may reach as high as 20 metres (66 ft)), the phenomenon may become significant when associated with atmospheric and meteorological factors such as low pressure and the bora, a north-easterly wind coming from Trieste, or the Sirocco, a hot south-easterly wind. Those conditions push waves into the gulf of Venice. High water is also worsened by rain and water flowing into the lagoon from the drainage basin at 36 inflow points associated with small rivers and canals.

 

 

Floods have caused damage since ancient times and have become ever more frequent and intense as a result of the combined effect of eustatism (a rise in sea level) and subsidence (a drop in land level) caused by natural and man-induced phenomena. Today, towns and villages in the lagoon are an average of 23 centimetres lower with respect to the water level than at the beginning of the 1900s and each year, thousands of floods cause serious problems for the inhabitants as well as deterioration of architecture, urban structures and the ecosystem. Over the entire lagoon area, there is also a constant risk of an extreme catastrophic event such as that of 4November 1966 when a tide of 194 centimetres submerged Venice, Chioggia and the other built-up areas. Flood effects are exacerbated due to greater erosion by the sea caused by human interventions to facilitate port activities (e.g. through the construction of jetties and artificial canals); establishment of the industrial Porto Marghera area; and increased wash from motorized boats, which all aggravate erosion of morphological structures and the foundations of quaysides and buildings.

 

 

In this context, MOSE, together with reinforcement of the barrier island, has been designed to provide protection from tides of up to 3 metres in height. MOSE will therefore effectively protect the lagoon, even if the most pessimistic hypotheses are proven true, such as a rise in sea level of at least 60 centimetres (recent estimates from the IPCC (Intergovernmental Panel on Climate Change) predict a rise in sea level of between 18 and 59 centimetres over the next 100 years). MOSE is flexible and can be operated in different ways according to the characteristics and height of the tide. Given that the gates are independent and can be operated separately, all three inlets can be closed in the case of an exceptional event, the inlets can be closed one at a time according to the winds, atmospheric pressure and height of tide forecast, or again, each inlet can be partially closed.

Following the flood of 4November 1966 when Venice, Chioggia and the other built-up areas in the lagoon were submerged by a tide of 194 centimetres, the first Special Law for Venice declared the problem of safeguarding the city to be of “priority national interest”. This marked the beginning of a long legislative and technical process to guarantee Venice and the lagoon an effective sea defence system.

To this end, in 1975 the State Ministry of Public Works issued a competitive tender, but the process ended without a project being chosen from those presented as no hypothesis for action satisfied all the mandated requirements. The Ministry subsequently acquired documents presented during the call for tender and passed them to a group of experts commissioned to draw up a project to preserve the hydraulic balance of the lagoon and protect Venice from floods (the “Progettone” of 1981).

A few years later, a further Special Law (Law no. 798/1984) emphasised the need for a unified approach to safeguarding measures, set up a committee for policy, coordination and control of these activities (the “Comitatone”, chaired by the President of the Council of Ministers and consisting of representatives of the competent national and local authorities and institutions) and entrusted design and implementation to a single body, the Consorzio Venezia Nuova, recognising its ability to manage the safeguarding activities as a whole. The Venice Water Authority –Consorzio Venezia Nuova presented a complex system of interventions to safeguard Venice (the REA “Riequilibrio E Ambiente”, “Rebalancing and the Environment” Project), which included mobile barriers at the inlets to regulate tides in the lagoon. In this context, between 1988 and 1992, experiments were carried out on a prototype gate (Modulo Sperimentale Elettromeccanico, hence the name MOSE) and in 1989, a conceptual design for the mobile barriers was drawn up. This was completed in 1992 and subsequently approved by the Higher Council of Public Works then subjected to an Environmental Impact Assessment procedure and further developed as requested by the Comitatone. In 2002 the final design was presented and on 3April 2003, the Comitatonegave the go-ahead for its implementation. The same year, construction sites opened at the three lagoon inlets of Lido, Malamocco and Chioggia.

 

 

Construction

Work began simultaneously and continues in parallel at the three inlets of Lido, Malamocco and Chioggia. Work on the structural parts (foundations, mobile barrier abutments, and gate housing structures), associated structures (breakwaters, small craft harbours, locks) and parts for operating the system (technical buildings, plant) has been completed.Restructuring of the buildings and spaces in the area of the Venice Arsenal where maintenance of MOSE and management of the system will be located is also underway.

Lagoon Inlet Construction Sites

Construction of MOSE at the inlets necessitates complex logistical organisation. These are located in a highly delicate environmental context so as to avoid interfering with the surrounding area as far as possible. The sites have been set up on temporary areas of water in order to limit occupation of the land adjacent to the inlets and reduce as far as possible the effect on activities taking place there. Materials (for example, site supplies) and machines are also moved via sea to avoid overloading the road system along the coast. Since the sites opened, all work has been carried out without interrupting transit through the inlet channels.

Lido Inlet

There are two rows of gates at the Lido inlet (21 mobile gates for the North barrier Lido-Treporti and 20 mobile gates for the South barrier Lido-San Nicolò).

To the north of the inlet (Treporti), a small craft harbour consisting of two basins communicating through a lock, allows small craft and emergency vessels to shelter and transit when the gates are raised. The sea-side basin was temporarily drained and sealed for use as the site to construct the gate housing structures for this barrier. Once the housing structures had been completed, the area was flooded with water to allow the housing structures to be floated out.

The housing structures for the gates in the north barrier (seven housing structures and two for the abutment connections) were positioned on the seabed. Four of this barrier’s gates were installed and manoeuvred for the first time in October 2013.

At the south of the inlet (San Nicolò), the launch and the positioning of seven housing structures and two for the abutment connections has been completed. At the centre of the inlet, a new island has been constructed to act as an intermediate structure between the two rows of mobile gates. This island accommodates the building and plant for operating the gates.

Malamocco Inlet

A temporary construction site has been set up alongside the basin to fabricate the gate housing structures to be positioned on the sea bed (Malamocco and Lido San Nicolò barriers, seven housing structures and two for the abutment connections for each barrier have been built).

In April 2014, the lock for the transit of large ships became operative to avoid interference with port activities when the gates are in operation.Positioning of the gate housing structures for the Malamocco barrier started in June 2014.

Outside the inlet, a 1,300-metre long curved breakwater designed to attenuate tidal currents and define a basin of calm water to protect the lock has been completed.

 

 

Chioggia Inlet

Work has been completed to construct a small craft harbour with double lock to guarantee transit of a large number of fishing vessels when the gates are in operation.The sea-side basin has been temporarily drained and sealed for use as a construction site to fabricate the gate housing structures, as for the Lido north inlet barrier.Positioning for the gate housing structures started in June 2014 (expected to end in October 2014).

In the inlet channel, the seabed in the area where the 18 gates will be installed has been reinforced. Outside the inlet, a 500 metres long curved breakwater has been completed.

Alternatives

Over the years, various proposals have been presented as an alternative to MOSE. Some offer widely different technological solutions while others suggest technologies to improve the efficiency of the system of mobile gates. At the request of the Mayor of Venice, Massimo Cacciari, approximately ten of these projects were examined in 2006 by round tables of experts appointed by individual responsible bodies, including the Higher Council of Public Works. In November 2006, negative assessments of the alternative proposals by these round tables led the government to give definitive approval for the MOSE project with the alternative proposals deemed ineffective or inappropriate to guarantee the defence of Venice.

References

  1. https://www.citylab.com/environment/2018/04/will-a-huge-new-flood-barrier-save-venice/556226/
  2. https://en.wikipedia.org/wiki/MOSE_Project#Chronology