Venice acqua alta (simply, “high water”) is a phenomenon known around the world and very much in the news in recent weeks. The various environmental and human-affected parameters of acqua alta include lunar-driven tides, wind-driven water pushed into the Lagoon from the Mare Adriatico, and centuries of filling lagoon shallows for industrial development on the mainland, for fish farming, for the development and expansion of Venice itself that reduced capacity and circulation.
Cause and Effect
Islands subside due to the weight of buildings, pavements and bridges compacting the usually water-saturated substrate. Immense modern groundwater pumping for mainland industry also causes subsiding.
Storm water surges from mainland streams was greatly mitigated by centuries-old rechanneling of major rivers around the Lagoon to eliminate their impacts, including the massive depositing of silt that accompanied seasonal and storm flooding. Of course, that re-routing of replenishing fresh water also allowed further intrusion of salt water, removed nutrient flow from the mainland, and changed the nature of the wetlands in the Lagoon.
Dredging of deepwater shipping channels through the Lagoon and up the Giudecca Canal to Marghera, have changed tidal flow characteristics, exacerbating erosion and undermining on some islands while silting natural channels now starved of cleansing action. The “equal and opposite” reaction to every action is not equitably distributed.
The first recorded inundation of the City was in 586.
Over the course of history, despite extraordinary measures like controlling the number and configuration of entrances into the Lagoon from the Sea, rerouting major rivers and providing water piped from the mainland to the City, the islands of the Lagoon in most instances continue to subside. That subsidence accelerated since heavy industrialization of the mainland began and ship channels were enlarged for modern mass-tonnage vessels.
Life Goes On – Or Does It
Several early settlement islands in the Lagoon, including Ammiana and Constantiaca east of Torcello, subsided enough to began to lose inhabitants in the 13th-Century, when their towns were already 600 or 700 years old, and were uninhabitable by the 16th-Century.
Contrary to the visions of giggling tourists experiencing acqua alta as broadcast by some tourism promoters (like morbid gawkers slowing traffic at the scene of an accident?), there are behavior and economic impacts that are not comprehensible in a one or two-day tour stop or cruise ship connection.
The passaralle tables set up on main pedestrian acqua alta routes between key destinations keep feet dry for a while, and the remarkably stoicism of residents and workers repairing damage, cleaning and sanitizing day-to-day (because they must) maintain a good attitude and quality of life as the people adapt to the relentless challenge.
Life is a relentless challenge, every “place” provides variations to that challenge (shovel snow daily every Winter every year!), and Venice is a great place to face it!
Businesses and institutions budget and operate with this “overhead item”. Impacts on high water days include loss of public transit for workers on Canale and Ria due to lack of bridge clearance for boats.
Many residents DO live on the Ground Floor, and have for one-and-a-half millenia, not all enjoying a too-common patrician and visitor notion that “everyone” lives upstairs in palazzi.
Sinking ground and rising water levels, frequent or infrequent, minor or severe, affect every aspect of family life in, to and from home, school and work.
How Much and How Often?
Some areas of the City are affected by acqua alta only once in 150 years, others in 100 years, or 50 years, or 10 or 5. Others are invaded many times per year. The lowest area of the historic center, Piazza San Marco receives water to a greater or lesser degree on up to 250 days per year at this time.
CaGIS, the Cartography and Geographic Information Society, 2009 Best of Show Award was given for the map linked at Venice Versus the Sea: Flood Zone , prepared by Virginia W. Mason and National Geographic Society (USA) staff. Although the partial graphic does not show the entire City, the “where”, How Often – How Much? detail is very educational.
Day-to-day lagoon management and public information services are the work of the Commune’s Istituzione Centro Previsioni e Segnalazioni and its Il Centro Mare (website: http://www.comune.venezia.it/flex/cm/pages/ServeBLOB.php/L/IT/IDPagina/1748 ). Their website is extremely informative, and shares the detailed operations of the agency.
Playing Both Defense and Offense
Venetians undertook massive heavy engineering projects to re-route rivers and move sea inlets centuries ago, as the City recognized the critical need to monitor and manage every possible aspect of Lagoon health and impacts in its earliest days. The modern defenders of the City, carry the authority and burden of the very powerful Magistrato alle Acque di Venezia, founded in the early years of the Commune.
The MOSE project (MOdulo Sperimentale Elettro-meccanico), at a cost of many hundreds of million Euro, is building giant articulated dams at the three Lagoon entrances to help defend against wind-driven and tidal surges, more dangerous in combination, by moving sea-bed gates upward to block the worst events.
Routing heavy ship traffic to Maghere via a southern Lagoon entrance would greatly lessen vibration and churning damage in the Giudecca Canal through the City. The concept is old, but planning, politics, economics and convenience put off action again and again.
The most visible defense waged in the City requires expensive, repetitive, intrusive action. One significant way in which land subsidence effects are offset is reconstruction to raise pavement levels in campos and calles, and inside many buildings.
I’ll See Your Bet and Raise You One Floor
In his book Venice on Foot, Hugh A. Douglas recounted substantial work “raising floors” over many centuries (Venice on Foot, Charles Scriber’s Sons, New York, 1907), including:
- Basilica San Marco and the Piazza: One used to walk up many steps to enter the church, not cross a watery moat in front of a few little steps as today (the interior Vestibule is today much lower than most of the Piazza it used to be safely above). During pavement elevations, the Piazza was sloped down to the Basilica, exacerbating the moat, since the Basilica has continued to settle. First paved in 1264, the Piazza was repaved and raised in 1382, again in 1519, again in 1772 and since… On page 14 of his book, Douglas quotes Francesco Sansovino, writing in 1581,
“One ascended from the level of the piazza to the facade – or rather to its doors – when the church was first founded, by some steps; now there is barely one, so much have the shallows of the lagoons increased, making it necessary to raise the level of the piazza so that it may not be inundated.”
- San Vio was perhaps first constructed in 912. During a 1745 reconstruction, the original church floor was found 8 feet below the 1745 floor level.
- San Simeon Piccolo, first built in the 800’s, has three floors below the 1904 floor level – each one “a great distance” below the other.
Many calli have been raised, and some bridges must be raised to continue passage of boats. Gravity fed utility lines suffer gradient changes or settlement breaks and must be relaid or pumps installed (Wouldn’t want stuff to back up, you know).
Reports, such as the investigation of the 1904 San Marco Campanile collapse, identify the oldest surface levels in many parts of the City at 2 meters below 1904 levels, with medieval levels at minus 1m-70cm, an average settlement of 9 centimeters per century up to 1904 – a century ago. During the 20th-Century, however, subsidence in the Piazza San Marco area increased by about 23cm, versus that previous 9cm per century average!
Consider today’s acqua alta challenge, and add 2 meters to the water depth! Or, raise your floors and eventually rebuild to raise the roof!
The Commune and individual property owners can never stop their expensive labors to raise and replace sinking pavements, as well as programs to raise floors in residences in areas of most Sestieri. We have seen such work in houses on Sant’Anna, Campo Ruga and elsewhere in the historic core of the City. Doors, vents and utilites must also be modified, sometimes pumps installed, and portable water-dams fitted to openings.
In a few centuries, if your successors do not totally rebuild your house or business, life will become rather constrained under either “lowered” ceilings or deeper water!
In the early years and mid-life of the City, such “raise or rebuild” work occurred during a willing replacement or enlargement of many early, more modest structures during eras of economic prosperity. Those now-historically protected buildings need continued care today, without replacement, in a vastly different economic climate for the City.
A significant percentage of buildings are seldom affected by water inundation to a damaging degree today, although other forms of water intrusion and land subsidence can be even more insidious. However, consider the work necessary to “keep up” with the Lagoon, understanding that, in the historical Sesetiere of Venice, there are…