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The Italian Republic defines the problem of protecting Venice and its
lagoon as "of pre-eminent national interest".
Protecting the lagoon involves the participation of: the State to protect
Venice, Chioggia and other towns from high water, the protection of coastal
areas from high tides, environmental readjustment of the eco-system; the
Regione del Veneto to clean up pollution and town councils for the socio-economic
development, maintenance and restoration of its architectural and construction
fabric.
The protection work system is direct, co-ordinated and controlled by a
committee (ex art. 4 L 798/84), chaired by the Presidente del Consiglio
dei Ministri (Premier), which involves all competent bodies and institutions,
on a local and national scale.
The following outlines the project to protect the St. Mark area from the
most frequent water incursions. This project has been devised by the Ministero
delle Infrastrutture e dei Trasporti - Magistrato alle Acque (Waterway
Superintendent) of Venice - Consorzio Venezia Nuova and provided for in
the General Plan of Works under State control, which is the main planning
and financial body for protection works.
Local protection of St. Mark's Square is a fundamental part of the protection
works program to defend the city and lagoon from flooding and includes
the MOSE system to defend the entire lagoon area from all high water,
including extreme flooding.
Where
The lowest areas of Venice, mostly the oldest, are subject to increasingly
frequent flooding. The "island" of St. Mark's is particularly prone to
high water, a one-time political, religious and administrative centre
of the Serenissima (Most Serene Republic); today symbolizes the city in
the eyes of the world, an extraordinarily striking architectural and monumental
area, characterised by the city's most precious and representative buildings.
How the Piazza is flooded away
The high water problem has seriously worsened over the last century
due to a rise in sea level (eustasism) and sinking of the land (subsidence).
Together these two phenomena have led to a loss of approximately 23 cm
in Venetian land. Over the last decades, high water in St. Mark's square
has become almost a daily occurrence. In fact, about 250 times a year,
when the tide reaches the 60 cm level, water begins to flood the Basilica's
narthex and the pavement in front of the entrance. As the tide rises,
the flooded areas gradually extend even further. With an 80 cm tide, the
water laps over large areas of the Piazza and at 90 cm nearly two thirds
of its surface is flooded. Furthermore, at 100 cm (7 times a year on average),
the Piazza and surrounding areas are almost entirely submerged.
Besides the inconvenience for Venetians and for socio-economic activity,
continuous high water causes damage to the pavement and the foundations
of the Piazza. The deterioration of the "masegni" (trachyte slabs), general
instability and subsidence of underground passages used for rainwater
drainage are the main signs of widespread deterioration, which has accelerated
in recent decades.
Water invades the Piazza in three ways: rising through manholes, overflowing
banks and filtering from underground.
The aim of the works
To protect the "island" of St. Mark's from the most frequent high water
guaranteeing pedestrian passage up to + 110 cm (according to the main
marigraph), by eradicating the destructive effects of flooding; to restore
the pavement in the Piazza and reclaim the area behind it.
How works are to be implemented
To prevent flooding due to water overflow the quay along the basin and
the pavement behind will be raised by 100 cm, whereas the level of the
pavement in the Piazza will not be changed. The quay will be raised by
means of consolidation works for the banks, to counteract deterioration
due to wave movement caused by very heavy motorboat traffic.
Instead, to avoid flooding due to water rising from manholes and filtering
the old network of underground passages will be separated from the water
rising out of the "rii" that surround the "island". The structural instability
of the underground passages, which has also led to subsidence of the pavement
at many points, will make complete restoration essential. At the same
time a new collection and drainage system for rainwater must be provided,
connected to a pumping station to be installed at the Giardini Reali (Royal
Gardens) to allow water to run off into the lagoon in the event of high
tides. An evaluation will be made of the possible insertion of an impermeable
bentonite layer under the Piazza's pavement to counteract flooding due
to filtering. As part of this work, the linking service network will also
be re-organized, by directing existing lines along pre-determined routes.
In this way future maintenance operations will be made easier and the
risk of general mishandling of the pavement, which occurs today, will
be avoided.
The first phase
The first phase, begun in March 2003, involves the quay along the basin
for a stretch of 150 m. Works include raising, restoring and reinforcing
the bank also to protect it from wave motion; drain location in preparation
for the new network for rain water drainage and restoration of the pavement.
Criteria and work strategies have been agreed with the Comune di Venezia
(Municipality) and the Soprintendenza per i Beni architettonici e il paesaggio
(Architecture and Landscape Office). The order of work has also been established
with the involved parties: the construction site, organized so as not
to hinder access to the Piazza, will proceed in two successive phases
and involves, among other things, the provision of temporary landing spaces
and protected access for boats.
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