Figure 8: Evolution of
tidal asymmetry (\(\gamma\)). Spatially-explicit representation of\(\gamma\) 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 \(\gamma\) for the historical
configurations (1887-2014). (L) Cumulative frequency of \(\gamma\) in
the hypothetical marsh erosion scenarios (2014-E25, E50, E75, E100)
5 Discussion
Our analyses highligth the difficult task of unravelling the
consequences of salt-marsh loss based exclusively on numerical results
obtained for the historical configurations of the Venice Lagoon, because
one can hardly isolate the direct effects of marsh disappearance on the
lagoon hydrodynamics from the indirect, cascade effects due to
morphodynamic feedbacks triggered by marsh loss that are all globally
included in each updated configuration of the lagoon. Therefore,
numerical results must be interpreted based also on the conceptualized
scenarios that assume additional marsh losses without any further
modifying the lagoon morphology. Again, however, caution must be given
when interpretating numerical results, because the simulated scenarios
could be representative of non-morphodynamic equilibrium conditions.
Water Levels
How marsh disappearance affects water levels within the lagoon by
modifying tide propagation is perhaps the most controversial point to
debate. This is because salt marshes are most effective in regulating
tide propagation at high water stages (i.e., in the upper intertidal
frame) due to their characteristic topographic elevations. Conversely,
at lower stages, marshes are not typically flooded and thus have limited
effects on tide propagation, which indeed takes place predominantly
within major tidal channels and across tidal flats.
Our simulations of hypothetical marsh-loss scenarios (E25 to E100 runs,Figure 4 L) suggest that marsh loss overall increases
accommodation in the back-barrier system, thus reducing the average
amplitude of tidal oscillations and, therefore, the mean tidal range
(\(\overset{\overline{}}{\Delta h}\)). This observation agrees with the
results of the numerical experiments carried out in other back-barrier
tidal systems along the continental US coast (e.g., Donatelli et al.
2018), which demonstrated, using an approach similar to that we adopted
here, that tidal amplitudes are reduced when salt marshes disappear.
However, our analyses also highlight a continued increase in\(\overset{\overline{}}{\Delta h}\) (Figure 4K) observed during the last
century. This is most likely a result of both the anthropogenic
modifications imposed on the lagoon inlets and the reduced bottom
friction due to tidal flat deepening (e.g., D’Alpaos and Martini 2005;
Tambroni and Seminara 2006; Carniello et al. 2009; Ferrarin et al. 2015)
(Figure 2A-F). Therefore, increasing \(\overset{\overline{}}{\Delta h}\)is only partially related, both directly and indirectly, to the observed
salt-marsh loss. This is supported by enhancements of\(\overset{\overline{}}{\Delta h}\) occurred in both the northern and
southern lagoon immediately after the construction of jetties at the
Lido (1901, Figure 4B) and Chioggia inlet (1932, Figure 4C),
respectively, together with generalized increases in\(\overset{\overline{}}{\Delta h}\) between 1932 and 1970 (Figure 4D)
when pronounced deepening of tidal flats took place. Excavation of the
Vittorio Emanuele and Malamocco-Marghera waterways also likely
contributed to promoting the observed increase in\(\overset{\overline{}}{\Delta h},\) especially in the central part of
the lagoon delimited by the Malamocco inlet to the South, the city of
Venice to the North, and the industrial area of Porto Marghera to the
West (Figure 4C,D). Finally, slight reductions in\(\overset{\overline{}}{\Delta h}\) between 2003 and 2014 are to be
related to increased hydraulic resistance at the inlets produced by the
modifications associated with the Mo.S.E. works (Figure 4K) (Matticchio
et al., 2017), and are therefore not directly linked to changes in
salt-marsh extent.
Similarly to \(\overset{\overline{}}{\Delta h}\), the continued increase
in mean high water levels (MHWL) observed since 1887 should
be only partially considered as a direct effect of shrinking marsh
coverage (Figure 5 A-F). In this case, however, exploratory
simulations suggest that progressive, additional loss of salt marshes
would result in further MHWL increases (Figure
5 G-J). This effect is most probably related to the progressive fading
of energy dissipations produced by the presence of marshes at high-water
stages. Reduced energy dissipations for high-water levels magnify tidal
peaks, thus enhancing MHWL. Larger MHWLpotentially bears negative consequences in terms of increased flooding
risk of urban settlements (Fletcher & Spencer, 2005; Gambolati &
Teatini, 2014; Rinaldo et al., 2008). One should however appreciate that
marsh loss leads to differential MHWL increases across the
lagoon (Figure 5 G-J), the latter being more pronounced in the
innermost portions of both the northern and central-southern lagoon
where marshes are still widespread nowadays. Conversely, limited changes
in MHWL are observed in the proximity of the inlets as well
as around the major urban settlements found within the lagoon, namely
Venice city, Chioggia, and the islands of Murano, Burano, and
Sant’Erasmo. For these locations, a detailed analysis of the
hypothetical scenarios of additional marsh loss highlights that changes
in MHWL are lower than 3% even if marshes would entirely
disappear (E100 scenario), whereas even lower variations
(<2%) are found when considering the maximum, rather than the
mean, high-water level (\(\zeta_{\text{MAX}}\)) observed during the
investigated period. Hence, the effects of salt-marsh loss on the
hydrodynamics of the tidal systems which host them appears to be
strongly site-specific, with significant hydrodynamic changes being
observed over distances of a few kilometers even within a given tidal
embayment.
Wind waves and bottom shear
stresses
Differently from tidal levels, a much more straightforward
interpretation can be given regarding the direct effects of marsh
erosion on wind-wave fields (i.e., significant wave height\(H_{s_{\text{MAX}}}\), Figure 6 ) and on the associated bottom
shear stresses (\(\tau_{\text{wc}}\), Figure 7 ) within the
lagoon. Before 1932 the generation and propagation of large wind waves
were hampered by the still-extensive presence of salt marshes (Figure
2A-C) that limited wind fetches, as well as by the reduced depths of
tidal flats which promoted important wave-energy dissipation (Figure
6A-C). From 1970 onwards, in contrast, reduced salt-marsh coverage,
coupled with pronounced tidal-flat deepening, led to the observed
increase in \(H_{s_{\text{MAX}}\ }\), especially in the central and
southern portions of the lagoon (Figure 6D-F) (Fagherazzi et al. 2006;
Defina et al. 2007). Therefore, progressively reduced marsh extent -
caused also by tidal flat deepening and expansion - favors larger\(H_{s_{\text{MAX}}}\) due to longer fetches, as confirmed also by
numerical simulations involving additional marsh loss (Figure
6 G-J). The latter also suggests that the most significant increases in
wave heights are to be expected in areas that are nowadays occupied by
extensive salt marshes (i.e., the northern lagoon as well as the
innermost portions of the central-southern lagoon, see Figure 2F), where
the possible disappearance of salt marshes could reduce their
wind-sheltering effect and increase the fetch length, eventually leading
to the generation and propagation of larger waves (Figure 6G-J).
Larger \({H_{S}}_{\text{MAX}}\) also have negative implications from a
flood-risk standpoint, as higher waves could locally increase the risk
of flooding due to overtopping, especially in the more topographically
depressed urban areas (Gambolati & Teatini, 2014; Mel, Carniello, et
al., 2021; Mel, Viero, et al., 2021; Ruol et al., 2020). Moreover,
larger waves threaten the conservation of the remaining salt-marsh
ecosystems, due to the positive feedback mechanism between marsh lateral
erosion and wind-wave power (Carniello et al., 2016; A. D’Alpaos et al.,
2013; Finotello et al., 2020; Leonardi, Defne, et al., 2016; Marani et
al., 2011; Tommasini et al., 2019). Specifically, the salt-marsh lateral
retreat rate is linearly correlated to wave power (\(P_{w}\)) (Finotello
et al., 2020; Leonardi, Ganju, et al., 2016; Marani et al., 2011;
Tommasini et al., 2019), which in turn is a quadratic function of wave
height. Hence, salt-marsh loss leads to higher, more energetic waves
which in turn enhance marsh lateral retreat even further in a
superlinear fashion.
Additionally, higher waves also produce larger bottom shear stresses,
especially across the extensive tidal-flat areas that characterize the
lagoon morphology (Figure 7 ). Indeed, our simulations
demonstrate that additional losses of salt marshes would further enhance\(\tau_{\text{wc}}\) proportionally to the marsh area being lost (Figure
7G-J, L) because the wind sheltering effect typically offered by marshes
would progressively be reduced, allowing for increasingly higher waves
to winnow the lagoon bottom (Carniello et al., 2014, 2016; A. D’Alpaos
et al., 2013; Tommasini et al., 2019). From a morphodynamic standpoint,
the implications of increasing \(\tau_{\text{wc}}\) can be manifold.
Unquestionably, larger \(\tau_{\text{wc}}\) will enhance the entrainment
of fine sediment from the lagoon shallows, in this way leading to higher
concentrations of suspended sediment (SSC) (Tognin et al.,
2022). Notably, wave-driven resuspension from tidal flats represents a
key source of sediment for salt marshes in sediment-starving shallow
tidal embayments, where the majority of mineral sediments are delivered
to the marsh surface during storm surge events concomitant with strong
wave activity (Tognin et al., 2021). Thus, enhanced SSC could
ensure higher resilience of salt-marsh ecosystems in the face of rising
relative sea levels (Elsey-Quirk et al., 2019; Mariotti & Fagherazzi,
2010; Tognin et al., 2021). However, such a beneficial effect is likely
to be offset by the above-recalled marsh loss via lateral retreat, which
would reduce the total marsh area and promote fragmentation, in this way
hampering the marsh’s ability to capture suspended sediment and cope
with sea-level rise (Donatelli, Zhang, et al., 2020; Duran Vinent et
al., 2021). Besides, enhanced SSC, coupled with the generally
ebb-dominated character of tides (Figure 8 and see Section
5.3), are likely to negatively affect the lagoon net-sediment budget,
leading to further tidal flat deepening and salt marsh losses.
Tidal asymmetries
Loss of salt-marsh areas appears to feedback into tidal asymmetry
(\(\gamma\)) mostly in an indirect fashion, with manmade modifications
on the lagoon inlet morphologies playing, in contrast, a critical role
in driving tidal asymmetry changes (L. D’Alpaos, 2010; L. D’Alpaos &
Martini, 2005; Matticchio et al., 2017; Tambroni & Seminara, 2006).
Indeed, pronounced \(\gamma\) changes are observed after the completion
of the jetties at the Lido (Figure 8B) and Chioggia inlets (Figure 8C),
both of which resulted in more marked ebb dominance in the inlet
surroundings. Indirect effects of salt-marsh loss on \(\gamma\) could
instead arise from the positive morphodynamic feedback between marsh
loss and tidal-flat deepening (Carniello et al., 2007, 2009; Defina et
al., 2007), which is likely responsible for the shift from flood- to
ebb-dominance observed in many portions of the lagoon between 1932 and
1970, especially in the area facing the Malamocco inlet where the
Malamocco-Marghera shipway was also excavated (Ferrarin et al., 2015)
(Figure 8D-E). Nonetheless, numerical simulations demonstrate that the
direct effects of additional marsh losses on tidal asymmetry are
potentially not negligible, with progressive marsh erosion leading to
more widespread flood-dominated areas in the innermost portions of the
lagoon (Figure 8G-J). This result is consistent with evidence from
previous studies showing that the decrease in intertidal storage
capacity associated with gradual marsh losses will re-establish flood
dominance typically observed for progressive tidal waves (Dronkers,
1986; Rinaldo et al., 1999). While enhanced flood dominance associated
with marsh disappearance could potentially limit sediment export to the
open sea and promote marsh accretion, one should appreciate that the
tidal regime in most of the lagoon is likely to remain dominated by ebb
tides, especially in the surroundings of the inlets where ebb-dominance
would be even further exacerbated compared to present-day conditions
(Figure 8G-J). This could hardly lead to an inversion of the ongoing net
sediment loss driven by ebb-dominated tides, as sediment entrained by
higher, more energetic waves will likely keep on being carried in
suspension by ebb-dominated tidal currents and transported outside the
lagoon for the most part. Given the non-linear dependence of sediment
transport processes on both tidal flow velocities and asymmetry,
however, these hypotheses should be verified by ad hoc coupled
hydrodynamic and sediment-transport numerical simulations to investigate
how sediments will be redistributed within the basin following changes
in the dominant tidal regime.
Implications for the hydrodynamics of back-barrier tidal
lagoons
Our analyses highlight both direct and indirect effects of salt-marsh
deterioration on the hydrodynamics of the Venice Lagoon. Despite being
generally consistent with previous studies carried out in different
tidal settings (e.g., Donatelli et al. 2018, 2020b, a), care should be
given to generalizing the results reported here to back-barrier tidal
embayments morphologically and hydrodynamically different from the study
case at hand. This is a general remark which holds for previous studies
and should be taken into consideration for future ones.The reasons
behind this caution are manyfold, though all broadly related to the
morphological and hydrodynamic peculiarities that characterize each
tidal environment, as well as to the conceptualization we adopted in our
numerical simulations.
First, the hydrodynamic response of a tidal system to marsh erosion
depends on i) the planform geometry and hypsometry of the basin (Deb et
al., 2021; Van Maanen et al., 2013); ii) the characteristics of the
tidal waves, especially in terms of tidal range and progressive vs.
standing character of the system (Van Maanen et al., 2013; Ward et al.,
2018; Zhou et al., 2018); and iii) the wave climate, affecting both the
basin hydrodynamics and its sediment transport regime (Carniello et al.,
2011; A. D’Alpaos et al., 2013). Particularly important is the spatial
distribution of salt marshes within the back-barrier basin. As
demonstrated by Donatelli et al. (2020b), different hydrodynamic changes
due to marsh loss are to be expected in back-barrier systems where most
marshes fringe the mainland compared to systems characterized by the
presence of extensive marsh areas detached from the mainland.
Second, in our exploratory numerical simulations, we did not account for
hydrodynamic changes due to rising sea levels, and assumed that marshes
are ineluctably destined to disappear. In other words, we implicitly
assumed that sea-level rise will outpace vertical marsh accretion, or
that, more realistically, wave-driven lateral erosion will lead to marsh
disappearance. This is strictly true only if the combined rate of
eustatic sea-level rise and soil subsidence is higher than the rate of
marsh vertical accretion, and if no significant mineral sediment supply
is available from either longshore currents or fluvial sources, which
could potentially allow marshes to prevent drowning and expand laterally
even in the face of rising relative sea level (Ladd et al., 2019; Roner
et al., 2021). Besides, salt marshes might be less vulnerable than we
hypothesized even in the absence of a significant inorganic sediment
supply (Kirwan et al., 2016). Indeed, even in systems characterized by
negative sediment budgets, there might potentially be a transitory
regime in which sediments are supplied to marshes from the adjoining,
eroding tidal flats, in this way mitigating marsh drowning (Donatelli,
Kalra, et al., 2020; Kalra et al., 2021; Tognin et al., 2021).
Third, our simulated scenarios do not account for the fact that salt
marshes can potentially survive sea-level rise by migrating landward
(e.g., Feagin et al. 2010; Field et al. 2016; Enwright et al. 2016;
Fagherazzi et al. 2019; Kirwan and Gedan 2019). While this process is
hindered in the Venice Lagoon, as well as in most salt marsh ecosystems
worldwide, by the presence of fixed seawalls, levees, and dikes at the
interface between marshes and the upland, it cannot be disregardeda priori . Clearly, the colonization of new intertidal areas by
marsh upland migration would profoundly change the hydrodynamics and
sediment budget of the whole back-barrier system, since new areas would
be periodically flooded by tides and additional sediment volumes would
become available as marshes expand landward.
Finally, the timescale required for the system to morphodynamically
adapt to changes in marsh coverage is generally difficult to quantify.
This is because sediment volumes liberated by marsh lateral erosion can
be redistributed within the basin by tidal currents and wind waves, thus
affecting both the lagoon sediment budget and related morphological
changes, besides potentially contributing to marsh vertical accretion
(Donatelli, Kalra, et al., 2020; Elsey-Quirk et al., 2019; Kalra et al.,
2021). In our simulations, in contrast, sediments eroded from marshes
are instantaneously removed, and can no longer contribute to the lagoon
morphological evolution. Moreover, although numerical simulations
considering hypothetical scenarios may be useful to isolate the sole
effects of salt-marsh loss on the hydrodynamics of back-barrier systems,
it should be noted that salt-marsh loss seldom occurs without inducing
modifications to other back-barrier landforms. This is clearly
highlighted by historical field data from the Venice Lagoon, which
suggest mutual feedback between marsh erosion and tidal-flat deepening.
Therefore, generalizations of results obtained from hypothetical erosive
scenarios should be treated with caution, since the modeled hydrodynamic
changes could be mitigated or magnified by other morphodynamic
adjustments induced by marsh disappearance on the tidal back-barrier
system as a whole.
In view of the above, the effects of salt-marsh loss on the hydrodynamic
and morphodynamics of shallow back-barrier tidal systems are likely to
be extremely site-specific, and therefore difficult to generalize.
Moreover, biogeomorphological feedbacks, which are key drivers of marsh
spatiotemporal evolution, are likely to vary geographically as a
consequence of distinct ecological assemblages of metacommunities and
different climatic forcings. This further complicates predicting the
morphodynamic effects of salt-marsh loss, as well as the timescales over
which they would manifest themselves (Bertness & Ewanchuk, 2002;
Finotello, Alpaos, et al., 2022; Pennings & He, 2021; Wilson et al.,
2022).
6 Conclusions
In this study, we focused on the microtidal Venice Lagoon (Italy) to
disentangle the role played by the loss of tidal wetlands on the
hydrodynamics of tidal back-barrier embayments. Numerical simulations
were performed considering both past morphological configurations of the
lagoon dating back up to 1887 and hypothetical scenarios involving
additional marsh erosion relative to the present-day conditions. This
allowed us to highlight both direct and indirect effects of salt-marsh
loss on the evolution of the lagoon hydrodynamics. Direct effects
include enhanced mean-high water levels due to reduced energy
dissipation at high-water stages, as well as the formation of higher and
more powerful wind waves due to longer wind fetches promoted by reduced
marsh extent. Moreover, historical data and numerical results suggest
that marsh disappearance is likely to trigger tidal flat deepening, thus
indirectly feeding back into hydrodynamics, leading to increased tidal
ranges due to reduced energy dissipation of the lagoon, and modifying
tidal asymmetries across the entire back-barrier system. We also
speculated on the potential impacts of the observed hydrodynamic changes
on the lagoon ecomorphodynamic evolution, as well as on the associated
risk of tidal flooding in urban settlements. Although further
investigations will be needed to conclusively address these hypotheses,
our analyses suggest that in the absence of sea-level rise, further
losses of salt marshes are unlikely to critically affect urban flooding
risk, whereas it may promote additional marsh lateral erosion through
positive feedback between reduced marsh extent and higher wind-wave
power.
The findings of this study provide novel insights into the hydrodynamic
effects of salt-marsh loss in sediment-starving, shallow tidal
embayments morphodynamically dominated by wind-driven sediment transport
processes, with far-reaching implications for the conservation and
restoration of coastal ecosystems that extend well beyond the study case
at hand. However, we stress that care should be given to generalizing
the results presented here to tidal embayments that are morphologically
and morphodynamically different from the Venice Lagoon. In doing so, we
support the idea that the response of back-barrier systems to changing
external forcing is highly dependent on site-specific morphological and
ecological features, as well as on the characteristic of the local tide,
wave, climate, and fluvial processes. This clearly prevents one from
drawing general conclusions regarding the future of coastal back-barrier
systems worldwide based on the analyses of individual study cases.
Acknowledgments
[reviewers will be acknowledged]. This scientific activity was
performed in the Research Programme Venezia2021, with the contribution
of the Provveditorato for the Public Works of Veneto, Trentino Alto
Adige and Friuli Venezia Giulia, provided through the concessionary of
State Consorzio Venezia Nuova and coordinated by CORILA, Research Line
3.2 (PI Andrea D’Alpaos).