± these authors contributed equally to this
work
Key Points:
- Past and future feedbacks between salt-marsh loss and back-barrier
hydrodynamics are investigated in the microtidal Venice Lagoon
- Marsh disappearance affects the lagoon hydrodynamics both directly and
indirectly through cascade effects due to morphodynamic feedbacks
- Hydrodynamic changes depend on site-specific morphological and
ecological features of the back-barrier system, as well as on local
climate
Keywords
morphodynamics, salt marshes, salt-marsh erosion, back-barrier system,
Venice Lagoon
Abstract
Loss of salt marshes in back-barrier tidal embayments has been widely
documented worldwide as a consequence of land-use changes, wave-driven
lateral erosion of marsh margins, and relative sea-level rise compound
by mineral sediment starvation. However, how salt-marsh loss affects the
hydrodynamics of back-barrier systems and feeds back into their
morphodynamic evolution is still poorly understood. Here we use a
custom-built, depth-averaged hydrodynamic model to investigate the
mutual feedbacks between salt-marsh erosion and hydrodynamic changes in
the Venice Lagoon, a large microtidal back-barrier system facing the
Adriatic Sea in north-eastern Italy. Numerical simulations were carried
out for past morphological configurations of the lagoon dating back up
to 1887, as well as for hypothetical scenarios involving additional
marsh erosion relative to the present-day conditions. We demonstrate
that the progressive loss of salt marshes significantly impacted the
Venice Lagoon hydrodynamics, both directly and indirectly, by amplifying
high-tide water levels, promoting the formation of higher and more
powerful wind waves, and critically affecting tidal asymmetries across
the lagoon. We also argue that further losses of salt-marsh area would
likely have detrimental effects on the lagoon ecomorphodynamic
evolution, though with negligible impacts in terms of increased flooding
risk in lagoonal urban settlements. Compared to previous studies, our
analyses suggest that the hydrodynamic response of back-barrier systems
to salt-marsh erosion is extremely site-specific, as it depends closely
on the morphological characteristics of the embayment as well as on the
external climatic forcings.
1 Introduction
Tidal back-barrier lagoons represent critical environments at the
interface between terrestrial, freshwater, and marine habitats
(Flemming, 2012; Levin et al., 2001; Pérez-Ruzafa et al., 2019; G. M. E.
Perillo, 1995), and are especially common along the World’s coasts
(Boothroyd et al., 1985; FitzGerald & Hughes, 2019; Kennish, 2016;
Stutz & Pilkey, 2011). They consist of sheltered embayments separated
from the ocean by a system of barrier islands (Hesp, 2016) interrupted
by tidal inlets (De Swart & Zimmerman, 2009), the latter allowing for
the exchange of tides, sediments, nutrients, and biota between the
back-barrier environment and the open sea (Boothroyd et al., 1985;
Carson et al., 1988; Finkelstein & Ferland, 1987; Wei et al., 2022).
Back-barrier lagoons provide valuable ecosystem services and support
high biodiversity, densely populated urban settlements, and florid
economies (Barbier et al., 2011; Costanza et al., 1997; D’Alpaos &
D’Alpaos, 2021). However, accelerating sea-level rise, reduced sediment
supply to the coasts, enhanced storminess, and increasing anthropogenic
pressures exacerbate the threat to back-barrier lagoons and the
communities relying on them (Gilby et al., 2021; Passeri et al., 2020).
Although the current paradigm indicates that future coastal hazards will
be mostly dictated by rising sea levels (Finkelstein & Ferland, 1987;
González-Villanueva et al., 2015), previous studies demonstrated how
geomorphological changes in tidal embayments, both natural and
anthropogenically driven, can feedback into coastal hydrodynamics and
ultimately exacerbate, or mitigate, coastal hazards (Carniello et al.,
2009; Ferrarin et al., 2015; Pollard et al., 2019; Zhou et al., 2014).
Therefore, investigating the feedbacks between ecogeomorphological
changes and the hydrodynamic of back-barrier systems is of utmost
importance to provide reliable assessments of coastal hazards (Carniello
et al., 2009; Donatelli, 2020; Donatelli et al., 2018; Donatelli, Kalra,
et al., 2020; Donatelli, Zhang, et al., 2020; Ferrarin et al., 2015;
Zarzuelo et al., 2018).
Among the morphological features that characterize shallow tidal
embayments, salt marshes are especially common and provide a wide number
of precious ecosystem services, including blue-carbon sequestration
(Chmura et al., 2003), environmental remediation (Nelson & Zavaleta,
2012), shoreline protection (Möller et al., 2014; Temmerman et al.,
2013), and habitat provision (Pennings & He, 2021; G. M. E. Perillo et
al., 2019). The alarming rates of salt-marsh loss observed worldwide
(Mcowen et al., 2017; Valiela et al., 2009) have prompted extensive
studies on salt-marsh ecomorphodynamics (A. D’Alpaos et al., 2007;
Fagherazzi et al., 2012; Finotello, Alpaos, et al., 2022), as well as on
the response of these ecosystems to changing hydrodynamic forcings and
inorganic sediment supply (A. D’Alpaos et al., 2007; Finotello et al.,
2020; FitzGerald & Hughes, 2019; Gourgue et al., 2021; Hughes et al.,
2021; Mariotti, 2020; Tommasini et al., 2019). The reverse problem, in
contrast, still remains unclear, that is, how salt-marsh loss affects
hydrodynamics and the related morphodynamic evolution in shallow coastal
bays. This uncertainty is mostly due to the paucity of study cases
analyzed so far, thus calling for new insights into the mutual feedbacks
between salt-marsh loss and hydrodynamic changes in shallow back-barrier
tidal systems (Donatelli, 2020; Donatelli et al., 2018; Donatelli,
Zhang, et al., 2020; Silvestri et al., 2018).
Here we aim to fill this knowledge gap by focusing on the microtidal
Venice Lagoon (Italy), where extensive marsh losses have been documented
over the last two centuries (Carniello et al., 2009; L. D’Alpaos, 2010;
Tommasini et al., 2019). We will focus in particular on the feedback
between salt-marsh loss and changes in the lagoon’s hydrodynamics. We
will not account for the loss of biodiversity and ecosystem services
that are implicitly associated with marsh disappearance, although these
effects are of utmost relevance and should clearly be considered when
evaluating the impacts of tidal wetland loss (Barbier et al., 2011;
D’Alpaos & D’Alpaos, 2021; Mitsch et al., 2015; Mitsch & Gosselink,
2000; Peter Sheng et al., 2022). The lagoon hydrodynamics will be
investigated using a custom-built, depth-averaged numerical model
applied to several past morphological configurations of the Lagoon, each
reconstructed based on available historical topographic and bathymetric
maps. Moreover, exploratory simulations will be performed to unravel the
hydrodynamic consequences of additional future losses of salt marshes.
The remainder of the paper is organized as follows. In section 2, we
provide a brief overview of the Venice Lagoon and describe in detail the
morphological changes, both natural and manmade, observed during the
last 130 years. We then outline (Section 3) the main features of the
hydrodynamic, wind-wave numerical models employed in this study,
together with a description of the computational grids and model
forcings used. Section 4 reports the results of the numerical
simulations, which are then discussed in detail in Section 5. Concluding
remarks (Section 6) close the paper.
2 Geomorphological Setting
Located in the northern Adriatic Sea, and characterized by an area of
550 km2, the Venice Lagoon is the largest brackish
waterbody in the Mediterranean Basin. The Lagoon formed over the last
7500 years covering alluvial Late Pleistocene, silty-clayey deposits
locally known as Caranto (Zecchin et al., 2008). Its present-day
morphology is characterized by the presence of three inlets, namely,
from North to South: Lido, Malamocco, and Chioggia (Figure
1 A-D). Tides follow a semidiurnal microtidal regime, with a mean spring
tidal range of 1 m and maximum tidal oscillations of about 0.75 m around
Mean Sea Level (MSL) (e.g., D’Alpaos et al. 2013; Valle-Levinson et al.
2021). Meteorological surges often overlap astronomical tides, thus
producing significantly high (low) tides when atmospheric pressure is
low (high). In addition, wind-related processes are critical for both
the hydrodynamics and morphodynamics of the lagoon, with seasonal
wind-storm events exerting a prominent control on the medium- to
long-term morphodynamic evolution, that is from decadal to centenary
timescales (see e.g., Carniello et al. 2009, 2012).