Works and restorations: Protecting Venice and its lagoon
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.