Figure 7: Evolution of
maximum bottom shear stress (\(\tau_{\text{wc}}\)). Spatially-explicit
representation of \(\tau_{\text{wc}}\) in 1887 (A), 1901 (B), 1932 (C),
1970 (D), 2003 (E), 2014 (F), 2014-E25 (G), 2014-E50 (H), 2014-E75 (I),
2014-E100 (J). (K) Cumulative frequency (CDF) of \(\tau_{\text{wc}}\)for the historical configurations (1887-2014). (L) Cumulative frequency
of \(\tau_{\text{wc}}\) in the hypothetical marsh erosion scenarios
(2014-E25, E50, E75, E100)
Tidal asymmetries
Larger values of bottom shear stress increase the chance for sediments
to be resuspended and transported elsewhere by tidal and wave-induced
currents, thus affecting the overall lagoon sediment budget. In
particular, previous studies have shown that asymmetries in tidal
currents (Aubrey & Speer, 1985; Friedrichs & Aubrey, 1988) are
critical in determining the ultimate fate of sediments carried in
suspension, with ebb-dominated tidal flow leading to sediment export to
the open sea and, therefore, to a net erosion of the lagoon (L.
D’Alpaos, 2010; Finotello et al., 2019; Sarretta et al., 2010). To
investigate how the lagoon’s morphological changes affected tidal
asymmetries (\(\gamma\)), we quantified the latter following the
formulation proposed by Nidzieko (2010). This formulation allows for a
spatially-explicit computation of \(\gamma\) in estuaries with mixed
diurnal/semidiurnal tidal regimes based on the normalized skewness of
the tidal water level time derivative\((\partial\zeta/\partial t\ =\zeta^{\prime})\):
\begin{equation}
\gamma=\frac{\mu_{3}}{\sigma^{3}}=\frac{\frac{1}{\ \tau-1}\sum_{t=1\ }^{\tau}\left(\zeta_{t}^{{}^{\prime}}-\ \overset{\overline{}}{\zeta^{\prime}}\right)^{3}}{\left[\frac{1}{\ \tau-1}\sum_{t=1\ }^{\tau}\left(\zeta_{t}^{{}^{\prime}}-\ \overset{\overline{}}{\zeta^{{}^{\prime}}}\right)^{2}\right]^{3/2}}\nonumber \\
\end{equation}where \(\mu_{3}\) is the third sample moment about the mean, \(\sigma\)is the standard deviation, and \(\tau\) is the sampling timeframe.
Negative values of \(\gamma\) indicate ebb-dominated tides, whereas
flood-dominated tides are characterized by positive values of\(\gamma\). Our results suggest that progressively larger portions of
the Venice Lagoon became ebb-dominated over time (Figure 8A-F).
Pronounced changes in tidal asymmetry in the surroundings of the Lido
inlet can be observed between 1887 and 1901 (Figure 8 B),
immediately after the construction of the jetties. Similarly, an
extensive expansion of the areas dominated by ebb tides is highlighted
after the construction of the jetties at the Chioggia inlet in 1932
(Figure 8 C). Afterward, the hydrodynamic regime of many other
portions of the lagoon shifted from flood- to ebb-dominated, especially
around the Malamocco inlet where the Malamocco-Marghera canal was
excavated (Figure 8 D-F). Frequency distributions of \(\gamma\)highlight that the most pronounced changes in the hydrodynamic regime of
the lagoon occurred between 1932 and 1970 (Figure 8 K), when
most of the salt marshes had already been lost and the deepening rate of
tidal flats accelerated (see Figure 2 K,L). Numerical
simulations demonstrate that the effects of additional marsh losses on\(\gamma\) are not negligible (Figure 8 G-J). Overall, ebb
dominance is slightly reduced as salt marshes are progressively eroded
(Figure 8 L), but distinct trends of \(\gamma\ \)changes are
observed depending on the position of the individual site relative to
the lagoon inlets. Specifically, while ebb-dominance is either
maintained or enhanced in the portions of the lagoon closer to the
inlets, a shift to flood dominance is observed in the most landward
regions where extensive salt marshes are found in the present-day lagoon
configuration (Figure 8 G-J).