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

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 1.5 m over subtidal platforms and a mean tidal range of about 1.0 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 1.0 to 5.0 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 'apron-like' accumulations, where sand accumulated through avalanching processes and mud settled down from suspension.