Untitled

Funder

Organizational Unit

Publications

Effect of survey parameters on unmanned aerial vehicles-derived topography for coastal dune monitoring

Publication . Bon de Sousa, Luísa; Costas, Susana; Ferreira, Óscar

Coastal dunes are fragile ecosystems emerging at the interface between marine and continental environments. They provide multiple services, among which are the protection against the impact of storms and the hosting of diverse and unique species of fauna and flora. However, changes in the topography or biological component of these systems may endanger the perpetuation of service provision. Topographic changes within dunes can significantly differ in magnitude depending on the type of process (i.e., marine or aeolian) and the temporal scale of analysis (event to annual scale), making their monitoring a challenging task. In recent years, unmanned aerial vehicles (UAVs) have been increasingly used to monitor coastal dunes, proving to be a cost-efficient methodology for the collection of topographic data. Yet, the application of UAVs in combination with the structure from motion approach to obtain digital surface models (DSMs) presents some limitations related to the level of accuracy provided for the evaluation of topographical changes in dunes with low sedimentation rates. This work explores different survey configurations using UAVs flying at low altitudes with the aim of obtaining high-quality DSMs with vertical accuracies preferably around or lower than 0.04 m. Several tests were performed to evaluate the influence of different parameters on the accuracy of the DSM, including flight altitude and orientation, density and spatial distribution of ground control points (GCPs), terrain slope, vegetation cover, and sun-related parameters. The results indicate that the intended accuracies can be obtained by combining overlapped perpendicular flights, GCPs distributed regularly following a diamond grid, with densities of at least 6 GCPs per hectare, sun altitudes between 30 and 40 deg, and a total solar radiation per hour between 1750 and 2250 KJ / m(2). In addition, better results were obtained across gentle slope areas, suggesting the eventual need to adapt to the particularities of each site to ensure the accuracy.

2022Journal article

Open access

Satellite derived shorelines at an Eexposed meso-tidal beach

Publication . Cabezas-Rabadan, Carlos; Pardo-Pascual, Josep E.; Palomar-Vazquez, Jesus; Ferreira, Oscar; Costas, Susana

Shoreline position data offer extremely valuable information for understanding coastal dynamism and beach changes. This research applies SHOREX system for defining the shoreline position from free mid-resolution Landsat-8 (L8) and Sentinel-2 (S2) satellite imagery. This system allows an automatic definition of Satellite Derived Shorelines (SDS) over large regions and periods. Accuracy and utility of the resulting SDS have been previously assessed with positive results at low energy, microtidal, Mediterranean beaches. This work assesses SDS extracted using SHOREX at a mesotidal and moderate to highly (during storms) energetic environment, namely at Faro Beach, a barrier beach located in Ria Formosa (Algarve, South Portugal). Accuracy was defined for 14 SDS derived from S2 and 10 from L8 by measuring the differences in position with respect to the shoreline inferred from profiles obtained on close dates (or simultaneously) to imagery acquisition. For non-simultaneous datasets, the water level was estimated for the time of the satellite images acquisition using oceanographic data and run-up formulations. The measured and estimated shoreline positions were then compared with the extracted SDS. The overall accuracy is good, with errors about 5 m RMSE, supporting the application of the used methodology to define shoreline dynamics and evolution at challenging environments, as mesotidal exposed and dynamic beaches.

2020Journal article

Open access

Assessment of Overwash-induced flooding at two beaches along the Southwest Algarve, Portugal

Publication . Kupfer, S.; Ferreira, Oscar; Costas, S.

At the Algarve south coast overwash-induced floods are a frequent and destructive phenomenon. In this study return periods for overwash potentials are estimated for two beaches located at the southwest coast of the Algarve (Carvoeiro and Salema) based on runup calculations for the period 1995-2017 after applying two well validated empirical formulations. Results show that overwash potentials are high even for small return periods. Additionally, the suitability of two different indicators (overwash potential and overwash depth) has been tested to represent the flood extent. Results suggest that the flood extent derived from the overwash depth presents several limitations and cannot realistically represent observations, while the flood extent given by the overwash potential (using a simple bathtub approach) fairly represents the overwash-induced flood, even though it can over/underestimate the flood extent for gentle/steep inland slopes. Flood extents derived from the overwash potential, for return periods of 10- and 100-year, show that occupied areas are potentially flooded for both return periods. It is therefore necessary to define appropriate adaptation measures for both beaches, preferably based on detailed risk assessment.

2020Journal article

Open access