Detection of Macro Marine Litter Along a Coastal Spit Using UAV Images

Marine litter (ML) abundance has been increasing globally and is recognized as one of the main socio-economic and environmental concerns. Monitoring operations have been realized worldwide in order to collect information on the types, quantities, and distribution of marine debris or to assess the efficiency of mitigation measures. The traditional beach monitoring using 100 m stretch is generally used, but such approach is generally considered as time consuming. In addition, the results generally present a high spatio-temporal variability. In this study, we used UAV images to map (visual screening) the presence of ML on a coastal spit in the northern part of the Po Delta. We used two classes of items, which are well-identified items and non-identified items. The UAV images also provide geomorphological and habitat information. Our results show that macro-litter items are present all over the system (from the beach to the lagoon) and suggest different scenarios of plastic debris accumulation. Indeed, ML are mainly concentrated in the beach wrack, dunes, and saltmarshes highlighting the role of the vegetation in trapping ML. Moreover, ML will most probably remain trapped by the saltmarsh vegetation, while cleaning operations may remove the ML present in the beach wrack. The accumulation of floating debris in coastal dunes is mostly driven by alongshore currents and wave set up in the nearshore area, while tidal currents may be responsible for the presence of ML in the salt marsh. Finally, considering that coastal management scenarios and decisions rely on numerical models that can predict best practices for coastal sustainability, our results might help local agencies and stakeholders to manage coastal environments.