ASTRONOMICAL TIDE HEIGHT IN THE LAGOON OF VENICE

The Venice Lagoon is historically an environment by its nature in unstable equilibrium, being exposed both to the risk of burial, due to the contribution of river sediments during floods, and to the risk of transformation into a sea arm, with consequent loss of specificity of transitional environment. To avoid the risk of progressive burial, the Serenissima Republic of Venice in the past centuries brought to a successful hydraulic engineering interventions, which determined the diversion of the major rivers that originally flowed inside or in any case near the port mouths (Piave, Sile, Brenta, Adige, Po). The other danger, coming from the sea, was faced by reinforcing part of the sandy coasts that separate the Lagoon of Venice from the Adriatic Sea (the so-called "walls" of the Lido and Pellestrina). During the nineteenth century the port mouths were protected by long dams forane in order to ensure navigability and to prevent the formation of coastal bars of obstacle to navigation, building the so-called “armings” at the mouths. During the twentieth century the large artificial canals were excavated to favor the portualità (the Vittorio Emanuele III and the Malamocco-Marghera) and finally after 2000 begin the work on the port mouths for fixed works and the housing on the backdrop of the movable barriers against floods from high tide (the so-called "Mo.S.E."). Having made this general premise on the historical events that have affected the Venetian Lagoon, it is necessary to define a standard reference condition to which it is possible to be traced back to make comparisons over time. This condition is identified in the tide recorded at the Acqua Alta Water Platform station, located 8 nautical miles off the coast of Venice. This point of observation has the advantage of sharing the general characteristics of the basin of the North Adriatic and of being at the same time exempt from the effects of interventions in the lagoon and the works to the port mouths. It is also not affected by the presence of the coast or piers, with the consequent effects related to the wind and wave setup during the most intense weather events. The tide enters the lagoon through the three mouths of the port of Lido, Malamocco and Chioggia, propagating through the major navigable canals that depart from them and the vast system of meandering natural channels, interacting with the shallow waters, velvesting and barenes, morphological structures typical of a complex lagoon system. The longer the length of the tidal wave path, the greater the effects on the amplitude and phase of the wave itself. In the case of artificial channels, from the rectified course, the inertial effects prevail over dissipative ones and damping are minimal or absent, while the opposite takes place on the slums and along the small channels that ply the areas of velma or barena, 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 have been chosen in the indicator are representative of different conditions, some more natural than other more man-made. It was also tried to give a homogeneous cover to the entire lagoon basin, monitoring stations located both just inside the port mouths, and in locations more inland to the lagoon, up to the contermine lagoon (Figure 1). Compared to the substantially stable trend of the astronomical tide recorded in the Acqua Alta Platform, there are considerable variations that are found within the Lagoon of Venice, with differentiated responses in the various sites proposed in the analysis (Figure 2). The variations, observed in a multi-year range, affected all the stations inside the lagoon during the first decade of the century, while the sea reference station remained substantially constant. This explains the calculation of a “damping index”, which uses that station data as a fixed reference. These variations are observed from 2005 until 2011, while the start of work on the port mouths is to be referred to the two-year period 2003/2004. In recent years, tide heights have stabilized on a new level, lower than that of the last century, with slight annual fluctuations, inherent in the variability of the tidal phenomenon.