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- Effect of survey parameters on unmanned aerial vehicles-derived topography for coastal dune monitoringPublication . Bon de Sousa, Luísa; Costas, Susana; Ferreira, ÓscarCoastal 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.
- Identification of risk hotspots to storm events in a coastal region with high morphodynamic alongshore variabilityPublication . Celedón, Victoria; Del Río, Laura; Ferreira, Óscar; Costas, Susana; Plomaritis, Theocharis A.High-energy storm events induce hazards that promote damage and destruction of property and infrastructure. Defining high-risk areas is therefore fundamental to prioritise management actions. This work presents the application of an approach to identify hotspots of storm impact at a regional scale (tens to hundreds of kilometres). The Coastal Risk Assessment Framework Phase 1 (CRAF1) is a hotspot selection method based on a coastal index that combines the potential hazard (i.e. overwash and erosion), the exposure (based on land use) and the vulnerability (based on socio-economic data) along each kilometre of the coast to assess the risk level. The suitability of the approach was tested on the southeastern coast of the Gulf of Cadiz (South Spain). CRAF1 was applied considering a morphological worst-case scenario and events of 10/50/100-year return period. The region shows a high overwash and erosion hazard level. Nevertheless, a relatively low number of risk hotspots were identified due to the low level of occupation in the study area. Comparison against available information of previous overwash and erosion events proved the reliability of the method to identify hotspots at a regional scale, even in a coastal area with high alongshore variability (geomorphology, wave exposure and tidal range). The results support the utility of the tool for coastal managers to prioritise and support risk reduction plans. Furthermore, the method presents two aspects that enlarge its potential applicability: (1) it is relatively easy to apply at a regional scale, and (2) it can be updated with new data to test different scenarios (e.g. sea-level rise).