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 km 2 , 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 and 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.