Tidal currents and wind waves controlling sediment distribution in a subtidal point bar of the Venice Lagoon (Italy)

Abstract

The present study aims to improve current understanding of the sedimentation of subtidal point bars, analyzing interaction between tidal currents and waves in shaping a submerged meander bend of the microtidal Venice Lagoon (Italy), and it is based on coupling of sedimentological studies, geophysical analyses and numerical modelling. The Venice Lagoon is characterized by an average depth of about 1Á5 m over subtidal platforms and a mean tidal range of about 1Á0 m. The morphodynamic evolution of the lagoon is strongly affected by intense seasonal windstorms, which promote the formation of wind waves triggering sediment resuspension and bottom erosion. The study channel is 70 to 100 m wide, it has a radius of curvature of about 260 m and cuts through a permanently submerged subtidal platform. Water depth ranges from 1Á0 to 5Á0 m below mean sea level on the subtidal platform and channel thalweg, respectively. Different from classical architectural models, the study point-bar beds do not show sigmoidal geometries, but consist of horizontally-bedded deposits abruptly overlying clinostratified beds. Sedimentation in the study bar is hypothesized to stem from the interaction between the in-channel secondary helical flow, as for most meander bends, and wave winnowing of the subaqueous overbank areas. Laterally accreting point-bar deposits point out that the curvature-induced helical flow redistributed sediment from the channel thalweg to the bar top and contributed to the development of the 'classical' fining-upward grain size trend. The marked truncation surface, separating clinostratified bar deposits from overlying horizontally-bedded platform sediments is interpreted here as due to bar top wave-winnowing, which also possibly promoted bank collapses. In the proposed model, sediments remobilized from bar top and subaqueous overbank areas were transported into the channel, forming peculiar 'apronlike' accumulations, where sand accumulated through avalanching processes and mud settled down from suspension.

Ontogeny of a subtidal point bar in the microtidal Venice Lagoon (Italy) revealed by three‐dimensional architectural analyses

“…The Venice Lagoon developed over the last 7500 years during Holocene transgression (McClennen & Housley, 2006; Zecchin et al ., 2008; Rizzetto et al ., 2009) and consists of tidal flats, salt marshes, and subtidal and intertidal channel deposits. Sediment ranges in grain size between mud and fine sand, although mud is largely dominant (Brivio et al ., 2016; Ghinassi et al ., 2018a; Ghinassi et al ., 2019b). The channel network comprises tidal channels with different cross‐sections, from almost 400 m to <1 m wide as the distance from the inlets rises up.…”

“…Knowledge gaps are even more relevant for subtidal channels, which have been investigated by few studies (e.g. La Croix & Dashtgard, 2015; Ghinassi et al ., 2019b) despite their widespread presence in coastal environments [for example, the Geum River estuary (Korea) and Hammocks Beach State Park, North Carolina (USA)]. Subtidal meandering channels can be found in the most seaward portion of tidal basins (Rieu et al ., 2005), where current velocities are higher because of the inlets, and they are permanently submerged.…”

“…Subtidal meandering channels can be found in the most seaward portion of tidal basins (Rieu et al ., 2005), where current velocities are higher because of the inlets, and they are permanently submerged. They usually develop on top of tidal flood deltas and their hydrodynamics is complex (Rieu et al ., 2005; Ghinassi et al ., 2019b) since the flow within the channels can be influenced by the currents flowing on the surrounding subtidal platforms and by the wind‐induced wave winnowing (Carniello & Defina, 2012; Tommasini et al ., 2019). Subtidal meanders can migrate laterally accumulating point bars that thereby do not experience any subaerial exposure.…”

Spatial distribution patterns of sandfish juveniles released in a sea ranch area in Maliwaliw Island, Philippines

“…North-east monsoon occurs from October to late March while south-west monsoon occurs from June to October. Ghinassi et al (2019) stated that wind waves trigger sediment resuspension and bottom erosion.…”

An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon (Italy)

“…The present-day configuration of the lagoon is characterized by the presence of sparse salt marshes (Finotello et al, 2020b) undergoing rapid erosion in the last two centuries (D'Alpaos, 2010;Tommasini et al, 2019), wide subtidal platforms, and tidal flats that are cut by intertidal and subtidal channels (Finotello et al, 2019;Ghinassi et al, 2019) (Fig. 1A).…”