Gronda lagunare (lagunar eaves) is a term used to indicate the area of the Lagoon of Venice by its mainland shore. The term is derived from the fact that it receives the waters from the rivers and streams which flow into the lagoon from the drainage basin of the plain of the mainland by the lagoon. It is also reached by the tidal flows form he sea. It is meant to be a belt that forms a transition area between the open lagoon and the mainland.
Since it is between the inflow of freshwater form rivers and sea saltwater, in its natural state it would be an area with freshwater wetlands, barene (saltmarshes), velme (mudflats), and lagunar channels which forms a transitional belt between the mainland and the open lagoon rather than forming a fixed boundary, with its conformation changing according to the interplay of the hydrodynamic forces of the lagoon and their hydrodynamic equilibrium. However, this complex hydrologic and environmental system has undergone dramatic changes, both natural and, especially, man-made and has by and large lost it character as a transitional belt.
Hydrologic characteristics of the lagoon and the gronda lagunare The drainage basin is a mildly inclined plain of alluvial formation to the north and along the sides of the lagoon which drains the watercourses coming from the Dolomites range of the Alps or from outcrop springs in the plain.Outcrop of spring water into the lagoon and the nearby sea coasts. The gronda lagunare receives the waters of these watercourses and the sediments they carry. It has areas of freshwater wetlands near the mouths of these watercourses. It is also reached by the tidal flows which come through the lagoon from the inlets of its coastal shoreline in its south.
A coastal lagoon is formed by the conjoint interaction between the freshwater of the inland watercourses and of the saltwater of the sea and is subject to a delicate hydrologic equilibrium between these. The sea coastline is composed of the peninsulas of Cavallino and Sottomarina, which form the northern and southern part respectively, and the barrier islands of Lido and Pellestrina in the middle. Between these formations, three inlets (Lido, Malamocco and Chioggia) let the tidal flow come into the lagoon.
Over centuries the sediments brought in by the inland watercourses can cause significant silting and eventually fill the lagoon in, which can be turned into it into a delta by the force of the rivers. In the long run, the force of the sea could have two effects. One is to render the inlets non-navigable through the accumulation of sand carried by the current and, ultimately, closing them, turning the lagoon into inland waters closed off from the sea. Pandora – ambiente (Laguna di Venezia) The other effect is an opposite one, a progressive erosion of the barrier islands through exceptional tides and, especially, storm surges. If the sea was to erode the barrier islands to an insignificant size or submerge them, the lagoon would be turned into a coastal bay.
The Republic of Venice built a series of murazzi (imposing walls made of Istrian stone) along the seaward coasts of Pellestrina and Sottomarina to protect them from erosion by the sea between 1748 and 1784.Biblioteca Idraulica Italiana - La Laguna di Venezia The Republic also diverted many of the rivers which drain into the lagoon away from it to prevent silting in some of its areas. This altered the nature of the gronda, decreasing its areas of freshwater wetlands. It also altered the hydrogeographic balance between the input of fluvial and sea water in the lagoon. In the late 19th and early 20th century breakwaters were built at the inlets to stop the accumulation of sand there and ensure their navigability.
The fluvial system of the gronda lagunare was complex and unstable. Several rivers flow into the lagoon along its length. They are, from east to west, the Sile, Zero, Dese, Marzenego, Muson, Bottenigo and Brenta. Moreover, there are affluent streams, ditches and drainage canals. Heavy rain could cause hydrologic stresses which had a cascading effect. The Zero could overflow and pour its waters into the Dese, which in turn would overflow and pour its waters into the Marzenego. Downpours could lead to frequent overflows and floods in the countryside over a wide area and changes in the courses of riverbeds. Due to the low gradient of the pain there was also a tendency for water to stagnate in the lower tracts of the rivers. This created wetlands in the gronda area, forming an amphibious environment between the mainland, lagoon and sea. In addition, the rivers continued on into the shallow lagoon, often reaching the sea, forming rivulets between the lagunar saltmarshes, bringing sediments into it and causing silting and mixing freshwater and saltwater, creating brackish water marshes which were liable to lead to problems with malaria.Il Fiume Marzenego
Hydrologic transformations River diversions
Since the instability of the hydrogeography of the gronda lagunare, which could lead to floods, silting, stagnation of water and the development of wetlands, the Republic of Venice carried out many river diversion works over several centuries. Venice's main concern was to protect the lagoon form silting, which could hamper the navigation of the lagunar channels and the gronda rivers which provided communication routes for her trade with the mainland. Concern about floods in the mainland countryside was not a high priority. As a result, Venice created diversion canals close to the edge to the lagoon as it was cheaper than carrying out such works further inland. Effective flood prevention would have required catching the water of overflowing watercourses further upstream. In addition, the canals further inland would have had a greater incline, guaranteeing a good pace of water flow which would have prevented floods and water stagnation. Instead, canals by the lagoon had an insufficient incline, which meant that they could easily overflow and create floods and pools of stagnant water and wetlands, worsening the hydrologic situation of the gronda lagunare.Il Fiume Marzenego
The Osellino canal was dug between 1510 and 1520 to partially divert the waters of the River Marzenego which flowed into the lagoon at Mestre. It runs along the central part of the lagoon to the east of Mestre, close to its edge, and ends at the mouth of the River Dese, in the northern part of the lagoon. It was intended to prevent silting in the lagoon and to stop the floods in the countryside of this area. However, it did not stop the latter. Being so close to the lagoon, it lacked incline, flowed too slowly and created areas of stagnant waters and wetlands which caused problems with malaria. The Republic of Venice never implemented plans for canals to collect the waters of all the rivers in the area (including the Muson, Dese and Zero) further inland to effectively prevent flooding because it was too expensive. These problems were resolved in the 20th century.
Between 1674 and 1683 Taglio del Sile canal was dug. It diverts the waters of the River Sile which flowed into the northern part of the lagoon to the east of the above-mentioned rivers and takes them out of the lagoon, running along its edge. Silting in the lagoon was prevented but, again, the problems of floods and the formation of wetlands worsened. The river delta behind the canal progressively became eroded and the ground got lower due to sediment compaction. As a result, the countryside was on average half a metre lower than the canal which, with rain, would overflow. In 1871 the left bank of the canal was raised and in 1889 a siphon underpass was built to allow two local rivers (the Vallio e Meolo) to flow into the lagoon. This only alleviated the problems which were finally solved in the 1930s with the construction of a water pumping station.La Bonifica tra Basso Sile e LagunaArtificial fluvial diversions in the mainland of the Lagoon of Venice during the 16th and 17th centuries inferred by historical cartography analysis, Geomorphologie, Vol 18, No 2, 2012 : Alluvial geomorphology in Italy
To the west of Mestre, the River Brenta and other nearby watercourses caused floods and silting in the central area of the lagoon which led to disruptions in that part of the mainland countryside and obstructed navigation on the watercourses which connected Venice with the mainland. To counter this, between the 14th century and the early 16th century, the Republic of Venice diverted the waters of this fluvial system, especially the Brenta, with a series of canals. After a number of ineffective interventions two long canals were created.
Between 1489 and 1507 the Brenta Nova canal was dug to divert the Brenta away from its original mouth at Fusina, near Venice, to Conche, in the southern lagoon. It started at Dolo and run roughly parallel to the edge of the lagoon a few miles further inland. As this canal was close to the lagoon, it did not have a sufficient gradient and caused repeated floods upstream. Moreover, the new mouth brought sediments which created a large delta (over 30 km2) which partially buried the southern lagoon.
Between 1554 and 1577 an embankment (the Parador di Brondolo) was built to take the mouth of the Brenta at Conche, together with the mouth of the nearby River Bacchiglione, away from the lagoon of Venice. The two rivers were made to flow into the lagoon of Brondolo, which was to the south of the lagoon of Venice and connected to it. This lagoon, which did not have a direct access to the sea, was turned into a freshwater lake and, later, it became completely silted up. The sea shore in this area advanced into the sea, forming a cuspate foreland. The southern part of the Lagoon of Venice, having lost the sediment input for the Brenta, became exposed to the erosive effect of the tidal flow, which led to a decrease in its above-water lands, a reduction of its saltmarshes and the progressive erosion of its inner deltas and of the fluvial channels that cross it.
A new canal, the Taglio Novissimo, was dug between 1610 and 1791 to divert the waters of the rivers Magra, Muson, Bottenigo, Volpago, Bionca and Tergola as well as those of the Brenta. It started further east than the Brenta Nova (at Mira) and was thus longer than the Brenta Nova. It run a similar course and most of it was along the edge of the lagoon. Its objective was to resolve the problem of flooding. However, as it run along the edge of the lagoon, it incline was low and did not stop the floods. This problem was solved only in the late 19th century with the creation of the Cunetta canal which starts much further inland and to the east of the lagoon (at Stra, near Padua) and joins the Brenta Nova canal at its southern end and thus has the required incline.
These river diversions also altered the hydrodynamic balance of the lagoon and new problems arose. Large areas of the lagoon became exposed above water during low tides and the inlets which provided communication to the sea became poorly navigable because of an accumulation of sand brought by the sea currents. In the early 17th century Benedetto Castelli, an engineer, argued that the former was due to the scarcity of the inflow of freshwater into the lagoon caused by the diversion of the River Brenta and that the latter was due to the sand brought by the sea current no longer being counteracted by the freshwater flowing into the lagoon from the opposite direction. He proposed to bring back the waters of the Brenta in a controlled manner so as to bring back the inflow of water into the lagoon but in a way that would prevent silting. He was also opposed to the planned diversions of the River Sile and four other mentioned rivers. This was rejected.