6.4 Land and water management

As highlighted in the Introduction, most coastal lagoons in Italy are the remnants of reclamation carried out in swamp and marsh areas, and therefore land and water management interventions have profoundly changed the morphology of most Italian lagoons.

Such is the case of the Pontinian coastal lakes, that were once in communication with one other, and the structure of which drastically changed with the interventions carried out in the 1920s. Water level was controlled and regulated and the tidal channels were completely reorganized. Following reclamation works, the lakes were modified to allow fishery exploitation. In Fogliano, the banks were protected by concrete for about half of the perimeter of the lake and a circumdarial canal was dug, 4-5m deep, to increase circulation and thus oxygenation of the waters and to enhance the efficiency of the fish barrier (lavoriero). In all the lakes, over time,
inputs of freshwater were reduced due to the growing water requirements of intensive agriculture practiced in the surrounding land, which lowered the water layer and depleted the springs. The Corpo Forestale that manages the coastal lakes gradually actuated a reduction and then the complete closure of all freshwater tributaries, because of the poor quality of incoming freshwater and heavy nutrient load, which certainly contributed to the lakes eutrophication.

The Orbetello lagoon also experienced changes linked to reclamation in the surrounding land, and to specific interventions on the lagoon. One of the first interventions was the enlargement of the tidal channels mouths in 1884: hydraulic and sanitary aspects in the whole lagoon improved noticeably, and lasted for about 50 years, till the mid-1900s: waters between 1930 and 1950 were for the most productive and clear. The environmental situation of the lagoon begun to worsen in 1975 due to poor water exchange with the sea, reduced depth, high temperature and salinity oscillations. Supply of nutrients from agriculture and industry resulted in a chronic hypertrophy of the Orbetello lagoon, often bringing about anoxic crises and extensive die-offs (Lenzi and Salvadori, 1986). For these reasons OLERA, the basin authority, set up specific strategies to solve the environmental crisis affecting the lagoon. OLERA action followed three main strategies: (1) macroalgal masses removal from the lagoon; (2) increase of clean seawater amount into the lagoon; (3) reduction of nutrient inputs from anthropic origin. Seawater was pumped into the lagoon at two sea-lagoon canals, and the third canal was used to make the water flow back to sea, according to the hydraulic model proposed by Bucci et al. (1989). In fact, the natural renewal of the lagoon water being extremely low, the pumping was boosted from 8 000 to 20 000 l/s and was concentrated in the warmer months. Between 1993 and 2000, 39 complete water lagoon changes renewal were performed. Furthermore, waste waters from domestic use and intensive land-based fish farms on the lagoons were collected and pumped into treatment plants. This articulated set of measures produced a significant reduction in algal biomass development in the lagoon in recent years.
As it can be expected, the lagoon of Venice is the lagoon that has experienced the most important land and water management interventions over time. Since the first settlements of the Venetians in the area, there was an increase of the natural tendency for sediment filling of the lagoon, due mainly to the deforestation of the mainland. To limit this phenomenon, major rivers (Brenta and Sile) were thus diverted from the lagoon between the thirteenth and seventeenth century, the number of inlets was reduced and the sand bar was reinforced. In 1604 the Po river was diverted southwards, through an artificial delta mouth, to prevent sediment infilling of some areas close to the lagoon of Venice. In the eighteenth century, the combined effects of coastal subsidence and eustatic rise in the sea level led to an increase of the flooding frequency, which obliged to protect the town of Venice: sea defences were built along the coastal strip. Under the Austrian rule and up to 1934, the Lido, Malamocco and Chioggia inlets were altered in shape and pouter dikes were built along the sea. Between the end of the nineteenth century and the beginning of the twentieth, the onset of industrialization determined further major anthropogenic changes by increased urbanization and land reclamation for agriculture, aquaculture and industry, leading to a further reduction of 3 280 ha of the lagoon surface between 1924 and 1960. Two main industrial areas were established, including oil and chemical industries, the first one in 1917 leading to dredging of new navigation channels and the second one in 1950 expanding over reclaimed areas. In 1963, a
third area was foreseen, that brought about the conversion of a large area of the lagoon to solid land, but was never completed due to environmental concerns. Throughout the twentieth century, groundwater withdrawals for the industry, natural subsidence and sea level rise led to the lowering of the town of Venice and of part of the lagoon, with consequent increased frequencies of flooding. In the same period, a progressive marked decrease of the number of fishers and workers in the lagoon occurred, as well as a decline of the number of citizens of the town, up to the present 70 000 inhabitants.
The flooding in the town of Venice, due to high water (high tide), causes an increased flow of water in the lagoon of Venice that invades the town. The frequency of these exceptional high tides has increased, together with the morphological degradation, and this has led to the need to plan operations to allow the safeguarding of the town. These interventions include rising of
banks and paving, qualification of the lagoon by reconstruction and re-naturalization of sandbanks, mudflats and shallow areas, rehabilitation of the smaller islands and excavation of the lagoon channels. The most ambitious intervention is the realization of the MO.S.E. (acronym for MOdulo Sperimentale Elettromeccanico – Experimental Electromechanical Module), a work
in progress, consisting of an integrated system of defence, consisting of rows of retractable 138
mobile gates able to isolate the lagoon of Venice from the Adriatic Sea during the events of high tide over an agreed share (110 cm) and up to a maximum of 3 meters.
An example of massive interventions is the reclamation that has interested the Valli di Comacchio, whose present surface is the remainder of a large and progressive reclamation that occurred across the twentieth century up to the 1990s. Overall surface was reduced from 73 000 ha to the present approximate 10 000 ha (13 000 ha of land area also considered).
In many cases, lagoon structure has been affected by the numerous works that have contemplated the realization of jetties, commercial ports or marinas. Such is the case of the Laguna di Grado e Marano, where the realization of the jetties of Porto Buso and Grado, the creation of the port of Porto Nogaro, with the deepening of the channel connecting the mouth to Porto Buso, the stabilization of the Island of St. Andrea and the realization of a new marina for 6 000 boats significantly affected the circulation of lagoon waters, with a gradual silting of the waterway networks in the lagoon.
The maintenance of tidal channels, dredging of the bottoms, consolidation of embankments and borders are all maintenance works needed to ensure lagoon functionality and even its persistence. Most lagoons have experienced these interventions, in all regions, as in the case of most managed Sardinian ponds.
Water management and land maintenance in the valli of the northern Adriatic area (see following section for a detailed description) are the clearest evidence of the role of human interventions in the maintenance of lagoon ecosystems, and represent an example of positive interaction, standing for a sustainable and compatible use of the lagoon environment.