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.