Using drones to map marine litter on Portugal’s coastline

Unmanned Aerial Vehicles – known as ‘UAVs’ or more commonly ‘UAS’, Unmanned Aerial Systems – until fairly recently were specialist military technologies used to perform tasks deemed too dangerous for a person. Nowadays, UAVs are a well-established commercial market: the FFA estimates that over a million drones are used for recreational use in the USA.

The technology has proven a success in aerial filming and photography. As the technology has developed and prices fallen to meet demand for low-cost UAVs, the opportunity has arisen to use them to perform tasks of scientific importance at a relatively low cost.

Marine Litter in Beach Dunes
In Coimbra, Portugal, an interdisciplinary team of scientists have been working on using UAVs to perform a task of increasing environmental and ecological importance: tallying and identifying marine litter on local beaches.

Anything which humans cause to end up accumulating in marine habitats is considered ‘marine litter’, and this broad category of materials is becoming an increasingly common sight along the sandy beaches of Portugal’s coast. For the sake of preserving the beauty of the beach for tourists and leisure, environmental agencies, local authorities and businesses may clean up sections of the beach themselves.

However, marine litter also accumulates on coastal dunes, which receive much less attention compared to sandy shores. The wide variety of plants on dunes trap sand and build a natural ground which is an ideal feeding, breeding and roosting ground for numerous bird and insect species. These are fragile but important environments; well-worth protecting.

The first step to understand the state of these environments is documenting the level of pollution. Ecologists are generally reluctant to use traditional census techniques, where people walk on the dunes, for fears of damaging the delicate equilibrium in these ecosystems.

UAS4LITTER
Gil Gonçalves and Filipa Bessa are the coordinators of the project funded by the Portuguese Science and Technology Foundation called UAS4LITTER, which aims to bring low-cost UAVs technology – both the UAVs themselves and the software – to perform mapping and surveillance of marine litter on coastal dunes.

Literature in this area is generally focused on beaches rather than dunes, likely because dunes are less visible, less important to tourism, and are harder to study.

This UAV-based system gives a new impulse to coastal dune litter monitoring.

The study involved fitting a drone with a digital camera, and the system was completed with an application which could run on the UAV operator’s mobile phone.

Mapping Marine Litter on Quiaios Dunes
The team chose the Quiaios beach, in a location starting near car parks, houses, hotels and restaurants, and accessible via elevated walkways built to protect the dunes. A boardwalk runs along the length of the dune. The study site was a short section north of the largest car park, next to the final walkway.

Flying at a height of 40 m over the beach-dune, the UAV captured a series of overlapping images to build a georeferenced mosaic composite image measuring 200 m by 80 m.

The researchers used out-of-the-box software which could create a digital surface model. From this image the researchers built a digital model of the terrain to identify features such as sand trails, which is useful for understanding how geomorphic features like sand trails and dune breaches can affect the distribution of marine litter.

To identify marine litter, the image was divided into 2 m x 2 m tiles and screened manually by an operator. Every piece of litter above 2.5 cm was located, identified by type, numbered and identified by colour.

More than 400 items were found, plastic being the most common material (76%) with pieces of Styrofoam (23%) and plastic bottles (20%) being the most abundant. However, paper, metal, glass and processed wood were also found all along the dune.

The value of UAVs
This UAV-based system gives a new impulse to coastal dune litter monitoring. Information about types of litter and their exact location makes it possible to identify trends in how different types of material are distributed.

Almost 70% of the marine litter items were found on the backdune, mostly octopus fishing pots and Styrofoam fragments, but the smaller foredune showed the highest number of objects per m² which were trapped amongst the plants. The objects on the foredune were more related to recreational activities, most commonly objects such as plastic bottles, bags, papers and napkins.

Three major marine litter hotspots were found, which corresponded to areas known as dune blowouts. These are areas where wind coming from the sea erodes sand away from the dune, and it’s no surprise that these are also major litter pathways, highlighting the major role that wind and water play in contaminating the dunes with litter left by beach visitors.

The future of UAVs in marine ecology
This study demonstrates the value of using UAVs to perform detailed non-intrusive surveys, and gives a new impulse to coastal dune litter monitoring. Anthropogenic litter is a relatively recent phenomenon compared to the age of these ecosystems, and swift action can prevent the effects of long-term residence of marine debris which threatens the equilibrium of these natural environments.

The use of UAVs for mapping marine litter was also an advantage in terms of workload and human effort, taking about three days total from taking the images to finishing image processing, visual inspection and marine litter mapping.

Moreover, on the basis of this survey, the authors can offer advice on how to fix the problem. They recommend interventions to restore dune boundaries where blowouts have occurred, to prevent marine litter being carried further into the dunes by wind and waves, and to replace the open bins along the elevated walkways which fail to prevent litter blowing away, by covered ones.

Despite the impact and high expected residence time of marine litter on dunes, there are no national or international programmes which involve their routine clean-up. In the future, camera- and claw-arm-equipped drones could use marine litter maps generated in this study to perform minimally intrusive cleaning of marine litter from dunes.

Find out more about the UAS4Litter project: https://www.uas4litter.com