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).