Evolution and natural recovery of a washover fan, North Adriatic Sea, Italy

Barrier islands are important landforms that protect the inland, bays, and estuaries from sea storms, and are notable for their rapid morphological changes. Transgressive processes caused by sea-level rise or storms are common modes of barrier evolution. The Marano and Grado Lagoon barrier islands (North Adriatic Sea, Italy) are a significant example of these dynamic environments in a more general human-altered and stabilised context. Between 2012 and 2013, at least two storms hit the westernmost one (Martignano), causing the island breach and the formation of a washover structure (channel and fan). The new formation occurred at the same location as a previous breach, indicating the presence of an erosive hot spot. The aims of the research are to (i) describe the washover morphology and its evolution, (ii) quantify the mode and the timing of the morphological beach recovery, and (iii) verify the existence of sedimentological signatures as diagnostic elements in figuring out the dynamics and evolution following paroxysmal events. Between 2016 and 2021, several Unmanned Aerial Vehicle surveys were performed in the area and compared to previous orthophotos from 2010 and 2014. Sedimentological sampling on significant morphologies was carried out across the entire system. The orthophotos and Digital Surface Models obtained from UAV surveys show that the washover channel was completely filled between 2016 and 2018 (the sediments are up to 0.80 m thick and the volumes accumulated approximately 10147 m3) before being vegetated in 2021, and the beach was realigned, indicating the achievement of a stable morphological phase. The relict washover fan on the back-barrier area retains its shape: it records at least two storm events, as confirmed by topographical variation in which more than one lobe can be identified, as well as different granulometric suites with reverse grading, highlighting the process dynamics orthogonal to the beach. This case represents an example of monitoring storm effects and subsequent natural recovery processes in a wave dominated environment, demonstrating the importance of episodic events in generating new landforms. Finally, it highlights the benefits of coupling the high-quality data from UAVs and the accurate sedimentological sampling, in order to recognise the effects of natural processes.