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Shallows of the Venice lagoon
THE PROJECT CONCERNS THE DESIGN OF NEW MORPHOLOGICAL WORKS TO PROTECT THE BOTTOM OF THE VENICE LAGOON FROM EROSION AND THE TRANSPORT OF SEDIMENTS FROM THE SHALLOWS (CALLED BASSIFONDALI) TO THE NEARBY CANAL IN THE AREA OF SAN LEONARDO- MARGHERA.
Project: Morphological works for the protection of the shallows of Venice lagoon- Italy
Location: Venice, Italy
Client: Venezia Nuova Consortium for Italian Ministry of Public Works – Water Board, Venice
Cost of total Works: €47,000,000 (approx) for the Preliminary Design, and €17,154,000 for the Detailed Design
Services: Mathematical Model Studies; Preliminary, Final and Detailed designs
Over the last century, the morphology of Venice lagoon has completely changed. In particular, the shallow waters increased in depth and the canals were constantly filling with sediments causing them to be reduced in depth and consequently require constant dredging. This led to an ever-increasing cost of canal maintenance, and the risk to the environment itself because the lagoon was turning into a lake. The erosion of the shallows is mainly due to the waves generated by wind and ships and to the artificial interventions of man. One of the areas in which these phenomena are most evident is the central part of the lagoon. At the beginning of 1970s, the dredging of the canal (diversely known as the Petrol tankers canal or the Littoral canal), and the increasing waves caused the shallows to more than double in depth, from about 0.90 m to around 2 m. The traffic of big ships along the Littoral canal also increased the waves in the central part of the lagoon. This area of the lagoon has undergone a drastic change in appearance, the increasing depths and erosion having led to the loss of sea grass, algae and salt-marshes. Technital has been involved in the past twenty years in the planning and design activities to improve the lagoon morphology and to halt the process of degradation of the lagoon itself. The project included several studies involving multi-disciplinary experts to analyse all the different aspects: hydraulic, environmental (particularly regarding water quality, the vegetation system and the aquatic and bird life), geological and geotechnical, navigation, and economic. Among these activities was the development of studies, the Preliminary Design and the Detailed Design of the morphological structures along the Littoral canal.
The aim of the study was to identify the layout of the structures to reduce the transit of sediment from the shallow water of the central lagoon to the Littoral canal, and thereby reduce the continuous sedimentation in the canal and the onerous dredging operations. The analysis carried out with different mathematical and physical models led to the definition of six different structures, which became the object of the Preliminary Design. The confining works, consisting of a wooden palisade with a gravel bank, were designed on the experience of the previous similar works. In the subsequent Detailed Design, this type of confining structure was changed, and gabions (modular cylindrical elements made of high resistance geomesh or woven natural fabric with varying degrees of degradability filled with small riprap or sand) were introduced in accordance with the request of Italian Superintendence of Cultural Heritage. The Detailed designs were developed for the first two northern structures for a total of about 380,000 m³ of fill materials for an area of about 14.2 ha with some 5,600 m of boundary structure.
Further specific information
The type of boundary defined in the Preliminary Design (1999) was significantly changed in the Detailed Design (2003). The former were made of palisades of poles joined together, while the latter were made of gabions, of different types according to water depth and wave exposure. In the Detailed Design six different types were used, differing from one another in the number of rows, and in the fill material of the gabions. On the Littoral Canal side, the gabions used were filled with stone, while on the lagoon side of shallow water, in some stretches and in the top rows, the gabions were filled with biodegradable materials. The two types of structures shown in the Detailed Design were built in the years between 2004 and 2010. During the construction of the first structure, a monitoring program was carried out to verify the level of the structure and the behaviour of the boundary; a year later the same monitoring program was repeated. The results were positive and satisfactory. The first type of boundary (Section A) consist of a pyramid of three rows of stone gabions of 60 cm diameter, resting on two stone mattresses, 30 cm thick. Behind this gabion structure is a filter in draining material up to the height of the top gabion, overlaid with two stone mattresses 10 cm thick. The total width of this structure at the top is 4 meters. At the back of this structure there is a wooden palisade of poles 25 cm in diameter and about 7.5 m long, placed at 50 cm intervals, and a polyester mesh to contain the fill material during the discharge operation. On the shallows side, the boundary structure (Section D) consists of a pyramid of three rows of stone gabions of 60 cm diameter, resting on two stone mattresses, 30 cm thick. Behind this gabion structure is filter in draining material up to the height of the top gabion, overlaid with two stone mattresses 10 cm thick. The total width of this structure at the top is 2 meters. Behind the mattress is a stone gabion 60 cm in diameter. At the back of this structure there is a wooden palisade of poles 25 cm in diameter and 7.5 m long, placed at 50 cm intervals of 50 cm, and a polyester mesh, capable of containing the fill material during the discharge operation. For this section, which extends along the lagoon side, there will be no mattress placed behind the top gabion row at the end of the fill operation.