ENVIRONMENTAL ENGINEERING IN VENICE. Past and Present

Transcription

1 ENVIRONMENTAL ENGINEERING IN VENICE Past and Present

2 the Venice lagoon is the largest in the Mediterranean sea, about 50 km long and 10 km wide the area is about 550 km 2. At the center of the lagoon over a group of about 100 small islands there is the city of Venice whose topography is very flat: the highest pointof its soil is slightly higher than 1.5 meters on m. s. l; the lowest parts are below the meter. Piazza San Marco has an elevation about of 70 cm. on m. s. l.

3 The Mediterranean Sea is a closed basin, and therefore does not have important tides, usually a few centimeters (20-30 inches), but the location of Venice at the bottom of the Adriatic is the one that has the highest tides in the Mediterranean. The maximum astronomical tide is 114 cm.

4 But to the astronomical tide it can add a phenomenon caused by setup of water due to the different barometric pressure between low and high Adriatic which often adds to the effect of wind from the southeast (scirocco) blowing strong (acqua alta). The highest acqua alta check on the 11/04/1966 reached the altitude of 1.92 m on m. m. While the astronomical tide is perfectly periodic acqua alta is linked to the weather conditions then it is distributed at random and modern models give a forecasting only with an advance of three days and with a certain degree of uncertainty.

5 With large acqua alta, noticeable areas of Venice are flooded.

6 The current situation of the lagoon is the final result of several centuries of human activity. To understand it is therefore necessary to remember even if briefly its history. Venice was an independent republic for almost a thousand years and from the beginning it has take care of the lagoon, but the first reliable documents we have start from 1500 to get to 1797, the last year of the republic. This is an old map of the first years of 1500, the west coast of the high Adriatic was occuped by a chian of lagoons and marshes that stretched from Ravenna to Aquileia for about 200 km so that it was possible to trevel by boat. The problem was that the trend of all this lagoons asto be filled by the sediments transported by the many rivers.

7 The effect is, see below, that these lagoons are cancelled except for the lagoon of Venice and the lagoon of Marano which have been maintained by human activity.

8 In 1500, Venice was a powerful maritime republic with a large fleet and its wealth was trade imports and exports between Europe and the Middle East. The ruling class was made up of merchants there was no nobility or warrior The city had not a commercial port since the area of the city was flat (at about one meter on mean sea level) and the city was covered by a dense network of canals so that it was easy to load and unload ships anywhere: The city was a port widespread. Venice gave rilevance to have a large lagoon around the city for different reasons. Military reasons: water was an excellent defense from enemies, so that Venice never had a city wall; commercial reasons: it was essential to have deep channels that allow navigation inside the town ; igienic reasons: lagoon was a good sewage plant. In the lagoon we had two types of water: fresh and salt water, but venetians liked only salt water was liked. Fresh water of rivers, in fact, was carrying much solid material, then deposited in the lagoon, and favored the Anopheles mosquito carrying the malaria disease

9 For these reasons, the Government of the Republic was heavily involved in the works for the conservation of the lagoon First, it was decided to divert the course of rivers, moving them away from the lagoon. beginning with those who had the mouth near the city: the estuary of Brenta wos brought in the sea south the lagoon with seven subsequent interventions. The mouth of the Piave was moved about 12 miles north; Sile was diverted to the north in the ancent estuary of Piave.

10 Po river, which had a large Delta with the main mouth near Chioggia, was the subject of an important "cut" that diverted the main stream south.

11 Following the displacement of the estuary of Po the littoral isle in the southern part of the lagoon was heavily eroded by sea and the Republic decided, in the seventeen to defend it with a wall of natural stones long over 10 km. This was the last important work of the republic.

12 Venice also had to deal with the salt water entring from the mouths rich of sand, depositing it in the principal channels. The battle with the salt water lasted nearly four centuries without great success: the sea was too strong for the techniques of the time The victory arrived on the second half of 1800, when the Republic of Venice was gone, when the technological tools allowed the robust construction of dams at the three inlets. In 1997, at the hands of Napoleon, the Republic of Venice ceased to be independent. After a short period of dependence by the French, Venice was ceded to Austria and stayed there for about sixty years.

13 This was a period of strong crisis for the city, with no significant business ;become a peripheral city of a great empire on the European continent. The his period produced only two important works: the railway bridge 6 km long between Mestre and Venice, ended in 1846 and the two dams that stabilize the mouth of Malamocco, built during the years

14 In 1866 Venice became part of the Kingdom of Italy As first work, the Italian government took charge of stabilizing the seabed to the two mouths of the Lido and Chioggia building their sides of the breakwaters about 2 km long.. Lido was completed in 1910, finally Chioggia was completed around 1934

15 At the end of World War 1, the Venetian economy restarts and the development of the industrial area of Marghera begane occupying the period , with the gradual occupation of about 25 square kilometers of lagoon.

16 In the period , the new canal Vittorio Emanuele was dug 10 m deep, to allow access to the industrial area.

17 In 1934 the road bridge was finished parallel to the railway. In 1958 he took another peripheral area of the lagoon (almost 3 square kilometers) for the construction of the airport in the city.

18 Finally, to prevent the passage of commercial vessels in front of St. Mark's Square, it was decided the digging of the new canal Malamocco Marghera 14 m deep in its initial part and 12 m in the nearest part in Marghera. The work was carried out during the period

19 On November 4, 1966 the largest historic high water occured: almost the entire city and its surrounding islands were flooded with some victim and severe property damage. Public opinion and the media wondered what was the origin of this disaster never seen in history, and soon saw the cause was all these transformations occurred in the last two hundred years. It was asked in a loud voice that all work in progress had stopped and someone even thought to bring the lagoon in the old days. The authorities decided to carry out a wide range of studies to understand the real situation, studies which occupied several following years

20 The first important result of the studies was the demonstration that the entire lagoon was subjected to a combined process of subsidence and eustasy. In part due to natural causes, but in most subsequent to extraction of water from the subsoil to the Industrial Zone.

21 In the period there was an increase in the average sea level of 23 cm. The number is small, but, due to the flat topography of Venice, it was sufficient to increase the frequency of flooding and the extent of the flooded areas as a result of the high water. The number of times per decade the tide leve l is greater then 60 cm.

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23 An other group of studies concerned the morphology: due to removed estuaries of rivers and due to dams at the mouthes, the tendency of the lagoon morphology was changed, in the sense that the trend to be filled was finished, and there was a generalized erosion. A significant areas of the emerging parts of the lagoon (called barene) was eroded and has now disappeared,.

24 bathymetric surveys conducted over succesive years indicate a loss of sediment to the sea of about a million of cubic meters per years.

25 The general trend was towards the flatting of the bottom of the lagoon. Depending on the overall increased depth of the lagoon, some significant environmental changes were noted on both the fauna (with a shift to marine species and depletion of freshwater species) and flora (with expansion of alophile species)

26 The final issue emerged from studies on water quality showed a sensible pollution of water and sediments of the lagoon. The presence of the industrial area, the traffic of cargo ships (including tankers) have caused environmental degradation heard with the pollution of peripheral areas of the lagoon. Release of pollutant from the banks of the South Industrial Canal.

27 The results of the various studies carried out, gradually acquired, were the basis of projects for flood defenses and restoration of the lagoon. The works are decribed now according to the purpose they had.

28 One of the first issues addressed was the repair of the damage caused by the flood. It ' been so necessary to restore many of the side walls of the city's canals and islands. Works to repair A bank of a channel In Venice

29 An isle in he lagoon with new lateral wall

30 a stretch of coastline of lagoon before the flood

31 The defensive wall destroyed by flood

32 the same stretch of coastline with the wall repaired and with a new artificial beach.

33 The bigest work was devoted to defend the residential areas inside the lagoon From frequent floodings. After long discussions the choised solution was to build a series of barrages at the mouthes, to control the level in the lagoon when the sea "trigger level" decided by the local authority. The battle has focused on the type of barrier to use because people wanted to have not effects on the landscape in a normal situation (no piers in the mouthes). To satisfie this condition, it was decided an original solution for the gates, i.e.the mobile parts of the dams. The MOSE gate essentialy is a metal casson resting on the bottom of the channel and hinged on one side to to ground, so it can rotate around a horizontal axis. Compressor can pump air into the casson till it becomes floating, bringing out its top. More cassons arranged in series may constitute a barrier of indefinite length ithout the need for intermediate piers resting on the bottom of the channel and hinged on one side to the bottom to ground, so as to rotate around a horizontal axis. Compressor can pump air into the casson that becomes floating bringing out its top. More cassons arranged in series may constitute a barrier of indefinite length without the need for intermediate piers Under resting conditions the gates are submerged and do not interfere with the landscape.

34 The following layout clearly expliques the principle of Mose.

35 Now the barrages are in advanced construction, the 80% is finished. The Simple aerial view of the mouths clearly shows the solution Lido mouth

36 Malamocco mouth

37 Chioggia mouth

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41 During the project one of the most discussed point was the choice of the alert level at which to close the barriers. To protect the whole town it had to close when the sea reaches cm, with too frequent maneuvers. It was therefore decided to close when the sea level exceeds 110 cm, which means close 5-6 times a year on average. This makes it necessary to defend the lowest points of the city with local defenses (project insulae)

42 The diagram below explains the usefulness of insulae

43 Unfortunately, one of the lowest points of Venezia is the Basilica of San Marco, which is the subject of a very delicate and complex insula project- An aerial view of the insula divided into several zones with different protection depending on the type of buildings

44 After speaking of defense against flooding (which is the most important problem for the population), let's talk briefly about two other issues discussed in the defense of the lagoon: morphology and pollution We have seen before that the lagoon is in erosion, in part for natural reasons, but in part for the maintenance of the channels where the dredged material was discharged into the sea The pubblic administration decided to keep all the dredged materials in the lagoon. The material thus obtained was to be used for the reconstruction of artificial barene" in place of those destroyed by erosion.

45 An artificial Wall of a New barena

46 the containment wall completed

47 Filling the new barena

48 The new barena finished

49 The new obtained surfaces that are submerged only with the most high tides, have become an excellent environment for flora and fauna, especially for aviofauna.

50 We arrived last topic : the pollution, produced mainly from the industrial area.. The most interesting for you but simply to tell. Some areas of the lagoon was treated by a capping of the surface using unpolluted sand. To delete the pollution it was decided the closure of all industrial discharges into the lagoon. The contaminated water was collected by a new sewer that leads them into a treating plant..

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52 The problem of pollutant coming from the banks of the South Industrial Canals was solved closing any connection between the lagunar channels and the underground of the industrial zone, waterproofing the banks of the channels, where necessary, with continuos steel sheetpile of adequate length. As an example I show you the Tresse island, right in front of the industrial zone, before (top) and after work

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55 THANKS FOR YOUR ATTENTION