Submarine geomorphology of tidal channels: case study of the Venice Lagoon, Italy

Tidal channel networks are key geomorphic and ecological components of coastal lagoons, yet their fine-scale submarine geomorphology is rarely documented. In the Venice Lagoon (Italy), an anthropogenically altered sediment-starved system, no high-resolution geomorphological map has been available to date. This study provides the first integrated, high-resolution 3D geomorphological characterization and sediment map of ten tidal channels of different depth and dimensions in the northern Venice Lagoon, from about 60 m to 480 m wide in the most internal ones to the one of the closest to the inlet. The study covers a total length of about 22 km and total mapped area of 4.05 km2, with the aim of identifying dominant landforms, sediment patterns, and their controls. Multibeam data, sediment samples, and seafloor videos were analyzed together with hydrodynamic residual currents derived from a validated 3D model. Morphometric indices were used to delineate the complex tidal channel seafloor geomorphology, describing a total of 152 erosional, depositional and anthropogenic features (24 confluence scours, 2 obstacle scours, 1 scour of unknown origin, 1 flute mark, 13 pools, 40 points bars, 71 dune fields), while a Random Forest classifier was trained to predict sediment distribution. Sediments are dominated by sandy mud to muddy sand with variable shell-fragment content. Residual current magnitude and direction were the strongest predictors of sediment classes, outperforming terrain attributes, and backscatter. The geomorphological and seabed sediment maps provide a baseline for future monitoring, modelling, assessments of sediment transport, and channel evolution. This framework supports sediment-management strategies and is transferable to other human-impacted tidal systems worldwide.