Short-term evolution of Po della Pila delta lobe from time lapse high-resolution multibeam bathymetry (2013–2016)

River deltas are highly dynamical and valuable environments and often undergo strong natural changes and human-induced pressures that need careful observation and monitoring. However, the high dynamicity of submerged deltas makes it very hard to observe how sediment are distributed across the delta area, a crucial element in understanding their morpho-dynamic evolution. To fill this observational gap, in the last few years (2013-2016) repeated high-resolution multibeam bathymetry surveys were carried out for the first time at the Po della Pila delta, the recent-most delta lobe of the Po river. The collected bathymetric and acoustic reflectivity (backscatter), seabed samples and seismic profiler datasets provided insights on the short-term morphological and sedimentological evolution of the extremely dynamic submarine portion of the delta in the framework of the hydrodynamical, seismo-stratigraphical and environmental setting. A high variety of geomorphological features and depositional bodies at different scales were observed from the mouth bar to the prodelta slope such as the alongshore and radial bars under the effect of marine currents, gravitational-instability phenomena and collapse depressions driven by fluid expulsion. Concurrently, the analysis of the seabed reflectivity and sediment samples allowed the identification of two main classes of seafloor sediment, corresponding to dominantly sandy vs. muddy substrata. The comparison among time-lapsed, high-resolution DEMs in the time frame 2013-2016 showed that the main changes occurred on the northern side of the prodelta slope in front of the active channel: mainly as a consequence of the flood of November 2014, a new lobe-shaped fine-sediment deposit up to 4.5 m thick corresponds to a newly-added sediment volume of 1.16 Mm3. Other major morphological changes occurred in the prodelta slope, where radial depositional bars showed a net S-ward migration. Moreover, a generalized seafloor deepening was documented, accompanied by changes in seabed reflectivity with a general coarsening of the delta lobe sediment distribution from the first to the third survey. We document the morphological and sedimentological complexity and the rapidity of the geomorphological changes in the shallow reaches of the Po della Pila lobe. Given the high rates of sediment accumulation and rapid erosion related to storms and currents, this area represents as well a relevant site to study the Anthropocene short-term climate-driven variability of the coastal region including the possible interaction among human-induced changes in sediment supply, coastal erosion, alongshore sediment transport, local subsidence (both natural and anthropogenic) and submarine slope instability.