Abstract
Coastal wetlands are intertidal ecosystems based on a delicate balance
between hydrodynamic, morphological, and biological processes.
Increasing rates of relative sea-level rise, sediment starvation and
anthropogenic pressure challenge the existence of wetlands and the
ecosystem services they support, extending to water quality enhancement,
carbon sequestration, and shoreline protection. Therefore, to preserve
coastal wetlands and their ecosystem services, it is of utmost
importance to understand sedimentation processes that drive salt-marsh
vertical accretion and offset the effects of relative sea-level rise.
Tidal flooding propagating via the channel and creek system is
considered to be the main mechanism controlling marsh sediment supply.
However, storm-induced resuspension associated with enhanced water level
can importantly affect the marsh sediment budget, sustaining
sedimentation on the marsh surface and signing its topography, which, in
turn, affects transport processes. To better understand how tides and
storm surges affect spatial and temporal sedimentation patterns in salt
marshes, we investigated short-term sedimentation processes through
field observation in the salt marshes of the Venice Lagoon, Italy.
Sediment accumulation measurements carried out continuously from October
2018 to July 2021 in four different marshes reveal that storm-driven
sediment supply accounts on average for 70% of the total yearly
sedimentation, despite the brief duration of storm events. On marshes
bordering channels, sediment mostly accumulates close to the marsh
margin and sedimentation rapidly decreases with the distance from the
marsh edge, contributing to form a levee-shaped profile. Conversely, on
marshes facing tidal flats, where the action of wind waves is stronger,
maximum sedimentation shows an inland displacement, creating a gently
sloped, ramped transition at the marsh margin. We conclude that storm
surges importantly support marsh sediment accumulation and change the
spatial depositional patterns, which largely define the marsh
topographic profile.