Sedimentation at River Mouths bounded by Coastal Structures: A Case Study along the Emilia-Romagna Coastline, Italy

Beach retreat in the Emilia-Romagna coast, facing the north Adriatic Sea, is well-known since decades and several factors are behind this phenomenon: a scarcity of natural sediment supply by rivers, natural and anthropogenic subsidence, and a strong urbanization of the coastal zone. Several bedload measurement campaigns in one representative river of the Ravenna province (Savio River) have been carried out since 2017. At the same time, seasonal surveys of bathymetry were undertaken at the river outlet to correlate changes in sedimentation with river input. The river mouth is constrained by artificial embankments that possibly funnel out sediment offshore during river floods. However, monitoring of bedload transport correlated with bathymetric changes between July 2017 and November 2018 still found a positive budget of almost 5000 cubic meters. Local authorities in Ravenna are planning to dredge 20000 cubic metres from the river mouth to supply small nourishments outside the studied coastal cell. According to the measurements of bathy-metric changes, the Savio mouth will need almost six years to recover, assuming the occurrence of a particularly efficient rate of sediment transport, like the one observed in the monitored period. Notably, the analysed period included a very large river flood that could overshadow the modal sediment transport operated by the river. Furthermore, with deepening of the mouth, the two villages adjacent to the outlet are likely to become more exposed to sea flooding, due to the propagation of surges inside the river mouth. The paper concludes that dredging activities should be reconsidered in view of the precarious equilibrium of the system.