Monitoring the dune-beach system of Guardamar del Segura (Spain) using UAV, SfM and GIS techniques.

Dune ecosystems play a key role in coastal dynamics, so it is essential to measure their movements with high precision and monitor their changes over time. It is crucial to have a system that allows us to know the natural and anthropic impacts affecting these ecosystems. The aim of this study is to ascertain the historical evolution of the dune system of Guardamar del Segura (Spain) and its relationship with coastal erosion. Likewise, it is also intended to assess the state of the foredune restoration works carried out in 2011. To this end, a comparison of existing cartographic data has been undertaken by using geospatial analysis techniques through Geographic Information Systems (GIS). As a novelty, a low takeoff weight UAV (Unmanned Aerial Vehicle) has been used to produce a high-precision 3D model from two-dimensional images using photogrammetric techniques, such as Structure from Motion (SfM). This technique made it possible to obtain a digital terrain model of high density and precision (30 pt/m2 and RMSE Z of 0.173 m). The results show a constant erosion of both the beach and the foredune, with an overall loss of 143,561 m3 of material in the period analyzed (2001-2017). The anthropogenic restoration actions executed within this period have not been effective. In fact, erosion has increased in the period 2016-2017, with a significant reduction in the beach width and sea waves directly affecting the foredune. The main conclusion is that the combined use of UAV and SfM techniques is an excellent procedure to periodically supervise dune ecosystems with high precision and significant time and cost savings.