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

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.info:eu-repo/semantics/publishedVersio

Avaliação do impacto da subida do nível do mar nas praias encastradas da costa Sul do Algarve

Dissertação de mestrado, Sistemas Marinhos e Costeiros, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2016As previsões do Painel Intergovernamental para as Alterações Climáticas (IPCC) apontam para a subida do nível médio do mar (NMM) que, num cenário mais pessimista, pode chegar a 1 m em relação ao atual. Esta previsão de subida no NMM pode ter impacto na zona costeira a nível global, quer modificando a posição da linha de costa e a morfologia costeira, quer influenciando a economia das regiões costeiras. A costa sul do Algarve, no sul de Portugal, é fortemente influenciada pelo turismo de praia, sendo que as praias encastradas são utilizadas como cartão de visita da região. A subida do NMM e a impossibilidade de migração das praias encastradas para terra pode comprometer a largura e capacidade balnear destas. Neste estudo quantificaram-se e analisaram-se as alterações morfológicas provocadas pela subida do NMM nas praias encastradas da costa sul do Algarve, de acordo com as previsões do IPCC para 2100. Para tal analisaram-se perfis 2D, representativos das morfologias de praia, onde se aplicaram dois modelos para calcular o recuo da berma e da face de praia. Dos modelos utilizados, um foi desenvolvido por Taborda e Ribeiro (2015) e outro no decorrer do presente estudo. Os modelos assumem que nos perfis com berma há um recuo e elevação da berma, e que nos perfis sem berma a face de praia torna-se mais inclinada como resposta à subida do NMM. Calculou-se também qual a capacidade balnear das praias com a diminuição da largura do areal obtida, e como poderá afetar a economia local. Conclui-se que com a subida do NMM há uma redução da largura do areal, e consequente diminuição da capacidade balnear, o que é negativo numa região dependente do turismo. Propõe-se então uma realimentação sedimentar das praias como forma de mitigar as consequências da subida do NMM

Ecogeomorphic response of a coastal dune in southern Portugal regulated by extrinsic factors

Coastal dunes are complex landforms whose morphology results from various interactions between biotic and abiotic factors. Here, we explore the longshore variability of the morphological features, plant community dis-tribution and accumulation patterns of a dune segment (1.4 km-long) located at the downdrift end of a sandy peninsula in the Ria Formosa, Portugal. To understand the main drivers of the observed variability and the implications for dune morphological response, this information was combined with recent multidecadal shore-line evolution data. The integrated results document significant differences in dune morphology, sedimentation patterns and plant zonation, with two distinct dune configurations or states identified in close proximity. One (western sector) shows a narrower dune system, vegetation cover characterised by pioneer species with low densities, and squeezed plant zonation. Conversely, the other (eastern sector) presents a wider dune system with a new foredune, a more developed plant zonation and relatively high vegetation density. Both states could be partially explained by the recent shoreline trends and inlet shifts, with stable to retreating trends in the western sector and shoreline progradation in the eastern one. Plant zonation and accumulation patterns suggest that the dune along the retreating sector is in a cycle of inland migration, encouraged by the reduced accommodation space and the low retention capacity of the vegetation across the dune stoss. Alternatively, observations along the prograding sector suggest that the greater accommodation space and the stabilising feedback between vegetation and topography promoted the seaward progradation of the system and the development of an incipient foredune. Outcomes support the importance of biogeomorphic feedbacks for the dune configuration, but they also evidence that the role of vegetation within the feedback is primarily regulated by physical factors that ultimately promote or inhibit vegetation effects on dune topographinfo:eu-repo/semantics/publishedVersio

Exploring foredune growth capacity in a coarse sandy beach

Dunes are key elements of coastal landscapes in almost every latitude. They host high levels of biodiversity and provide important benefits to society; e.g. protection against floods and erosion, or recreation. Coastal dune growth is constrained by intrinsic factors, which are critical when managing dune systems or choosing coastal dune restoration as an alternative green solution for coastal protection. Here, the evolution of a beach-dune system, characterized by a reflective coarse sandy beach and low dunes, is explored to identify the favourable and optimal conditions for dune growth in these settings. Dune growth capacity is evaluated by analysing the topographical changes observed along a coastal dune over two different temporal scales (interannual and event scale) and comparing the observations with theoretical approximations of sediment transport potentials. Observations and predictions over interannual scale document that (1) temporal variability in external conditions (wind regime) and spatial variability of estimated wind fetch length, alone, fail to explain alongshore dune growth patterns and (2) optimal conditions for dune growth occur when storms (strong winds) impact the study area, jointly with low runup levels, at zones of shoreline progradation and absence of direct human influence. Conversely, lowest values of dune accumulation are associated with areas where shoreline retreat was documented. Observations from event timescales suggest that sediment transport potential can be reached over zones with no significant signs of beach erosion, if runup levels remain low and the event duration is shorter than the time scale of sand surface depletion within the upper beach.FCT Investigator program [IF/01047/2014]FCTPortuguese Foundation for Science and TechnologyEuropean Commission [SFRH/BD/144869/2019]Portuguese Science and Technology Foundation (FCT) through National and FEDER Funds [PTDC/CTA-GFI/28949/2017](FCT) Portuguese Foundation for Science and Technology [UIDP/00350/2020]info:eu-repo/semantics/publishedVersio