ASTRONOMICAL TIDE HEIGHT IN THE VENICE LAGOON

The Venice Lagoon is historically an environment by its nature in unstable equilibrium, being exposed both to the risk of siltation, due to the contribution of river sediments during floods, and to the risk of transformation into an arm of the sea, with consequent loss of specificity of a transition environment. To avoid the risk of progressive siltation, the Serenissima Republic of Venice in recent centuries carried out powerful hydraulic engineering interventions, which led to the diversion of the major rivers that originally flowed inside it or in any case near the port inlets (Piave, Sile, Brenta, Adige, Po). The other danger, coming from the sea, was faced by reinforcing part of the sandy coasts that separate the Venice Lagoon from the Adriatic Sea (the so-called "murazzi" of the Lido and Pellestrina). During the 19th century the port inlets were protected by long breakwaters in order to guarantee navigability and avoid the formation of coastal bars that would hinder navigation, by building the so-called "armours" at the inlets. During the twentieth century, large artificial canals were dug to facilitate port development (the Vittorio Emanuele III and the Malamocco-Marghera) and finally, after 2000, work began at the port inlets for the fixed works and the placement of mobile barriers against high tide flooding on the seabed (the so-called "Mo. S. E."). Having made this general premise on the historical events that affected the Venetian Lagoon, it is necessary to define a standard reference condition to which it is possible to refer in order to make comparisons over time. This condition is identified in the tide recorded at the Piattaforma Acqua Alta station, located 8 nautical miles off the Venetian coast. This observation point has the advantage of sharing the general characteristics of the Northern Adriatic basin and at the same time being free from the effects of interventions in the lagoon and works at the port inlets. Furthermore, it is not affected by the presence of the coast or piers, with the consequent effects related to wind and wave setup during the most intense meteorological events. The tide enters the lagoon through the three port inlets of Lido, Malamocco and Chioggia, propagating through the major navigable canals that depart from them and the vast system of natural meandering channels, interacting with the shallow waters, the mudflats and the salt marshes, morphological structures typical of a complex lagoon system. The longer the path of the tidal wave, the greater the effects on the amplitude and phase of the wave itself. In the case of artificial canals, with a rectified course, the inertial effects prevail over the dissipative ones and the damping is minimal or absent, while the opposite occurs on the shallows and along the small canals that cut through the mudflat or salt marsh areas, where the propagation of the tidal wave is hindered by the friction effects on the bottom. To account for this complexity, the monitoring sites that were chosen in the indicator are representative of different conditions, some more natural, others more artificial. Furthermore, an attempt was made to give homogeneous coverage to the entire lagoon basin, monitoring stations located both just inside the port inlets and in locations further inside the lagoon, up to the lagoon border (Figure 1). Compared to the substantially stable trend of the astronomical tide recorded in the Acqua Alta Platform, the variations found within the Venice Lagoon are notable, with differentiated responses in the various sites proposed in the analysis (Figure 2). The variations, observed over a multi-year interval, concerned all the stations inside the lagoon during the first decade of the century, while the reference station at sea remained essentially constant. This justifies the calculation of a "damping index", which uses the data from that station as a fixed reference. These variations are observed from 2005 to 2011, while the start of work at the port inlets refers to the two-year period 2003/2004. In recent years, tidal heights have stabilized at a new level, lower than that of the last century, with slight annual oscillations, inherent in the variability of the tidal phenomenon.