± these authors contributed equally to this work
Key Points:

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).