Shoreline evolution of Portuguese low-lying sandy coasts in the last 50 years Technical Report as support for the GRUPO DE TRABALHO DO LITORAL – GTL study Cristina Lira | Setembro de 2015 Introduction The present report aims at analyzing shoreline evolution of the low-lying Portuguese coast, mainland Portugal, in the last 50 years. Materials and Methods Shoreline evolution analysis was supported by 2 sources of information: 1. 2. Aerial photographs from USAF 1958 flight (photos taken in different days and no flight plan available to the authors) (Figure 2); Orthophotomaps of 2010 available from IGeo (URL http://mapas.igeo.pt/ortos/2010 ). Methodology for georeferencing aerial photographs was the same described in Silva et al (2013). Shoreline vectorization was made using dune toe (DT) as the shoreline indicator, which is representative of both shoreline change and the beach terrestrial limit – seaward backbeach boundary. DT was identified using the seaward vegetation limit, which restricted this study to low-lying sandy coasts whose terrestrial limit was a dune. RMS error associated to the georeferencing process was estimated in 5m and shoreline position precision estimated in 10m, value accounting for georeferecing errors, vectorization errors, etc. This 10m value is associated with a precision of 0.2 m/year, for the study period (52 years). Shoreline evolution rates were calculated using DSAS (Thieler et al., 2009) and Shoreline Change Mapper ArcGIS extensions; and transects spaced by 250m. Attention is made to Praia do Norte beach, north of Nazaré town, which was deliberately removed from this study. This area presents a very strong seasonal variation signal, which could not be accounted for because both aerial and orthophotomaps time of acquisition (summer vs winter condition) were unknown. PAGE 1 2010 Shoreline Figure 1 –Low-lying Portuguese coast 2010 shoreline – dune toe indicator (DT). PAGE 2 1958 Shoreline 1958 USAF photos Figure 3 – Low-lying Portuguese coast 1958 shoreline 1958 Shoreline– dune toe indicator (DT). Aerial Photographic cover for the year 1958 used is this study. Results Shoreline change rates between 1958 and 2010 can be observed Figure 2. Table 1 summarizes mean, minimum and maximum rate values for each littoral cell considered. Figure 3 to Figure 12 show each littoral cell in detail. PAGE 3 Shoreline Evolution Rates (m/year) Figure 2 – Shoreline evolution rates for Portuguese low-lying coast, Portugal mainland between 1958 and 2010. The Praia do Norte beach, North of Nazaré town deserves special attention. This area has a very large seasonal dynamic, and the calculated rates for the area were within seasonal values reported for this beach1. Therefore, this study excluded the calculated rates for this coastal stretch, because the inter-annual and decadal variability of this region was not represented. 1 A data das fotografias aéreas de 1958 é desconhecida, assim como a data dos ortofotomapas. PAGE 4 Table 1 – Shoreline evolution rates for each littoral cell and for all stretches. Littoral cell 1a 1b 1c 2 3 4 5 6 7 8 Mainland Portugal Minimum (m/year) -1.83 -7.35 -2.72 -1.07 -0.48 -4.79 -1.27 -0.89 -0.63 -7.82 -7.82 Maximum (m/year) 5.60 4.57 2.94 1.16 0.04 1.28 8.16 1.30 3.18 8.85 8.85 Mean (m/year) -0.18 -0.92 -0.17 -0.25 -0.16 -0.13 0.42 0.51 0.35 -0.03 -0.23 Shoreline Evolution Rates (m/year) Cell 1a Figure 3 – Shoreline evolution rates for cell 1a between 1958 and 2010. PAGE 5 Shoreline Evolution Rates (m/year) Cell 1b Figure 4 - Shoreline evolution rates for cell 1b between 1958 and 2010. Shoreline Evolution Rates (m/year) Cell 1c Figure 5 - Shoreline evolution rates for cell 1c between 1958 and 2010. PAGE 6 Shoreline Evolution Rates (m/year) Cell 2 Figure 6 - Shoreline evolution rates for cell 2 between 1958 and 2010. Shoreline Evolution Rates (m/year) Cell 3 Figure 7 - Shoreline evolution rates for cell 3 between 1958 and 2010. PAGE 7 Shoreline Evolution Rates (m/year) Cell 4 Figure 8 - Shoreline evolution rates for cell 4 between 1958 and 2010. Shoreline Evolution Rates (m/year) Cell 5 Figure 9 - Shoreline evolution rates for cell 5 between 1958 and 2010. PAGE 8 Shoreline Evolution Rates (m/year) Cell 6 Figure 10 - Shoreline evolution rates for cell 6 between 1958 and 2010. Shoreline Evolution Rates (m/year) Cell 7 Figure 11 - Shoreline evolution rates for cell 7 between 1958 and 2010. PAGE 9 Shoreline Evolution Rates (m/year) Cell 8 Figure 12 - Shoreline evolution rates for cell 8 between 1958 and 2010. Additionally to the shoreline elution rates results, Figure 13 to Figura 21 show details of this evolution with a “before and after” in more sensitive and known areas. A B Figure 13 –Ofir adjacente shoreline: A – 1958 e B – 2010. Yellow line – 1958 Dune toe position. Red line – 2010 Dune toe position. PAGE 10 A B Figure 14 – Ovar adjacent shoreline: Yellow line – 1958 Dune toe position. Red line – 2010 Dune toe position. A B Figure 15 - North Aveiro bar adjacent shoreline: Yellow line – 1958 Dune toe position. Red line – 2010 Dune toe position. PAGE 11 A B Figure 16 – South Aveiro bar adjacent shoreline: Yellow line – 1958 Dune toe position. Red line – 2010 Dune toe position. A B Figure 17 - Figueira da Foz bar adjacent shoreline: Yellow line – 1958 Dune toe position. Red line – 2010 Dune toe position. PAGE 12 A B Figure 18 - Costa da Caparica adjacent shoreline: Yellow line – 1958 Dune toe position. Red line – 2010 Dune toe position. A B Figure 19 - Tróia Peninsula adjacent shoreline: Yellow line – 1958 Dune toe position. Red line – 2010 Dune toe position. PAGE 13 A B Figure 20 – Faro bar – Ria Formosa adjacent shoreline: Yellow line – 1958 Dune toe position. Red line – 2010 Dune toe position. PAGE 14 A B Figura 21 - Evolução da linha de costa no troço litoral adjacente a Manta Rota – Ria Formosa: A – 1958 e B – 2010. Linha amarela – Base da duna 1958. Linha vermelha – Base da Duna 2010. PAGE 15 References Silva, A. N., Lira, C., Matildes, R., Andrade, C. F., Taborda, R. and Freitas, M. C., 2013 - Entregável 1.2.2.2.c Utilização de ortofotomapas e fotografias aéreas para a delimitação da linha de costa, Projeto CISML: “Consultoria para a Criação e Implementação de um Sistema de Monitorização do Litoral abrangido pela área de Jurisdição da ARH do Tejo”, Fundação da Faculdade de Ciências da Universidade de Lisboa (FFCUL) para Agência Portuguesa do Ambiente (APA). http://www.apambiente.pt/_zdata/Politicas/Agua/Ordenamento/SistemaMonitorizacaoLitoral/ E_1.2.2.2.c%20Deteao%20da%20linha%20de%20costa.pdf Thieler, E.R., Himmelstoss, E.A., Zichichi, J.L., and Ergul, Ayhan, 2009 - Digital Shoreline Analysis System (DSAS) version 4.0— An ArcGIS extension for calculating shoreline change: U.S. Geological Survey Open-File Report 2008-1278. PAGE 16