Figure 1 : Geomorphological setting. (A) Satellite images of the Venice Lagoon (image ©Google, Landsat). (B,C,D) Close-up views of the three lagoon inlets. (E) Rose-diagram representation of wind climate recorded at the “Chioggia Diga Sud” anemometric station during the period 2000-2019. The two most relevant winds, i.e., the north-easterly Bora wind and south-easterly Sirocco wind, are also highlighted.
The most morphologically and hydrodynamically meaningful wind-storm events are those associated with Bora and Sirocco winds, which are also the prevailing winds in the Gulf of Venice (Figure 1 E). The north-easterly Bora winds blow almost parallel to the major axis of the lagoon, thus producing a pronounced setup in the southern lagoon and generating large waves (significant wave height \(H_{s}\) > 1 m), which promote significant resuspension of fine sediments expecially from the tidal flats located in the central-southern lagoon. In contrast, Sirocco winds blow from South-East and cause large wind-setups in the northern Adriatic Sea, further enhancing high-tide meteorological surges and often leading to extensive flooding of Venice city and other settlements within the Lagoon.
Over the last centuries, the hydrodynamics of the Lagoon was severely affected by anthropogenic interventions (L. D’Alpaos, 2010; Ferrarin et al., 2015). First, by the end of the 16th century, all the major rivers debouching into the lagoon were diverted into the open sea, thus almost completely eliminating fluvial sediment input. Second, between the 1900s and 1970s, extensive land reclamation projects were carried out, especially along the landward margin of the lagoon, to accommodate industrial, agricultural, and fish farming activities, thus importantly reducing the total area open to the propagation of tides (see Figure 2 ). During the same period, extraction of groundwater and natural gas for industrial purposes caused an acceleration in the local subsidence rates, with anthropogenically-induced subsidence reaching cumulative values ranging between 10 to 14 cm in the Venice-City area (Carbognin et al., 2004; Gatto & Carbognin, 1981; Zanchettin et al., 2021). Moreover, in order to allow for increasingly bigger ships to cruise within the lagoon, two large waterways, namely the Vittorio Emanuele and the Malamocco-Marghera channels (Figure 2 C,D), were excavated in the central part of the lagoon in 1925 and 1968, respectively. Finally, massive jetties were built between 1839 and 1934 at the lagoon inlets to maintain water depths suitable for commercial ship traffic (Figure 2 A-D). The jetties at the Malamocco inlet were constructed between 1839 and 1872, whereas at the Lido inlet the northern jetty was completed in 1887 (seeFigure 2 A), with the southern jetty added later in 1892 (seeFigure 2 B). Finally, the jetties at the Chioggia inlet were built between 1910 and 1934 (Figure 2 C). On the one hand, the jetties reduced the width of the inlets, thus resulting in considerable deepening as foreseen during the design phase (Figure 2 A-C). On the other hand, they caused important changes in the lagoon hydro- and morpho-dynamic regimes. Since the construction of the jetties, changes in the tidal regime within the lagoon have been much more sustained than the typical periodic, multi-annual variations induced by the nodal modulation of tides in the Adriatic Sea, which are in the order of 4% of the characteristic tidal range (Amos et al., 2010; Valle-Levinson et al., 2021). Between 1909 and 1973, the tidal range within the lagoon increased as much as 25% on average (L. D’Alpaos, 2010; Ferrarin et al., 2015; Tomasin, 1974), with local changes that can be even more pronounced (Finotello et al., 2019; Finotello, Capperucci, et al., 2022; Silvestri et al., 2018).
All these interventions, coupled with eustatic sea-level rise (average value 1.23±0.13 mm/year between 1872 and 2019; 2.76±1.75 mm/year between 1993 and 2019; see Zanchettin et al. 2021), had important impacts on the lagoon morphological evolution, triggering positive morphodynamic feedbacks. Progressively larger portions of the lagoon became ebb-dominated, especially close to the inlets where the jetties produced strong ebb-flow asymmetries, enhancing the export of fine sediments and preventing the import of sediment carried in suspension by longshore currents (L. D’Alpaos, 2010). This condition, worsened by anthropogenically-induced starvation of fluvial sediment supply, set a negative sediment budget and resulted in a progressive, generalized loss of salt marshes (Carniello et al., 2009; L. D’Alpaos, 2010; Tommasini et al., 2019; see Figure 2A-F,K), the very existence of which is intimately linked to the availability of external sediment supply (e.g., Roner et al. 2021; Willemsen et al. 2021). Reduced marsh coverage lengthened wind fetches, thus favoring the formation of higher, more energetic waves, which further enhanced the erosion of marsh margins (Finotello et al., 2020; Leonardi, Ganju, et al., 2016; Marani et al., 2011; Mariotti & Fagherazzi, 2013), as well as of tidal-flat beds (Carniello et al. 2009; Tommasini et al. 2019; see Figure 2). Deepening of tidal flats (Figure 2 L), exacerbated by the eustatic rise in sea levels and both natural and anthropogenic-induced subsidence, promoted the generation of even higher wind waves, which in turn favored additional erosion of salt marshes and tidal flats through a positive feedback loop.