Use of RPAS to monitor coastal dune systems and beach erosion in Guardamar del Segura, Spain

Abstract

Coastal dune ecosystems had a crucial role in coastal dynamics, so it is essential to measure their movements precisely and monitor their changes over time. The natural interactions can be altered by anthropic pressure, modifying the coastal dynamics, causing alterations in beaches and dunes and affecting their stability. In recent years, the appearance of affordable low take-off weight Remotely Piloted Aircraft Systems (RPAS), together with the development of image-based computing techniques such as Structure-from-Motion (SfM), has increased the use of RPAS-based photogrammetry to produce high-resolution digital elevation models (DEMs) for the study of different surface processes, and specifically, for surveying dune ecosystems. This study aims to evaluate the applicability of RPAS to conduct periodic accurate studies at an affordable cost, to monitor the evolution of the coastal system of beaches and dunes, as well as the erosion caused by coastal regression. The study is focused on Guardamar del Segura, Alicante (Spain), an area strongly affected by anthropogenic pressure and coastal erosion. The methodology applied used a DJI Phantom 4 quadcopter, a device with a high ratio of sensor quality and performance at a very reasonable cost. The flights were planned so that the study area, with a length of 3.2 km long and 150 m wide (0.51km2), was covered in 4 passes. A series of targets distributed throughout the study area were used as ground control points (GCP) for the photogrammetric georeferencing process. Its coordinates were surveyed by a Leica Zeno FLX100 GNSS, which provides an accuracy of its measurements of 2 cm horizontal and 3 cm vertical. The SfM algorithm enables the reconstruction of a 3D scene by resolving the geometry of the images, the camera positions and their orientation simultaneously using Agisoft Methashape software. The Digital Surface Models (DSM) obtained from the RPAS with the SfM-MVS algorithm have a high density (450 pts/m2) and high accuracy, with RMSE in both the GCPS and check points < 3 cm in horizontal and < 1 cm in vertical measurements. The orthophotos and DSM generated have a spatial resolution of 2.5 cm/pixel. This very high resolution enables to accurately detect the shoreline, the dune limits and anthropogenic actions, as well as the shape of the dune and beach. Also, to monitor changes and the effects of storms precisely. Significant erosion has been detected along the entire length of the coastal area, as well as a movement towards the interior of the dune ridge was detected. In conclusion, the complex dune ecosystem and beaches of Guardamar del Segura has proven to be an excellent test site for monitoring coastal processes using a small, lightweight, easily deployable and affordable commercial RPAS. The possibility of quickly obtaining orthophotos and Digital Surface Models of very high resolution, covering large extensions at a low cost, enables us to model and monitor these rapid-changing environments with regularity and accuracy.