Shoreline evolution of the Gulf of Cadiz through manually digitized and automated extraction methods

Understanding the spatial and temporal variation of the shoreline position is key to both research and engineering projects contributing to an efficient management of the coast. Accelerated climate change and its related impacts can further destabilize coastal systems, highlighting the need for studies that quantify coastal evolution,  while discussing the application of satellite remote sensing datasets and GIS methods for coastline extraction, mapping, and analysis along regional coasts.

The Gulf of Cadiz is brimming with human intervention and as such has been the target of many studies. The focus of this work covers part of this region, extending from Olhos de Água (Portugal) to the mouth of the Guadalquivir River (Spain) (~180 km). The study area is characterized by a variety of coastal morphological features, including cliffs, beaches, foredunes and inlets. Regardless of its great diversity of landforms, sandy beaches still constitute the dominant coastal environment of this region. We aim to grasp a better understanding of the Cadiz Gulf coastal dynamics through the comparing two shoreline mapping methods and indicators, covering the time span between 2014-2024 for Portugal and 2016-2022 for the Spain coast.  The applied methods include 1) the manual digitation of Wet/Dry Line (WDL) and the Instantaneous Water Line (IWL) indicators within a GIS environment, and 2) the automatized extraction of the IWL using the CoastSat toolkit (Vos et al., 2019). The WDL Marks the darkest edge of the wet area of the beach, while the IWL is the line where the water meets the sand. The manually digitized shoreline was carried out in ArcGIS Pro 3.4.0 over the orthophotomaps obtained from “Direção Geral do Território” (Portugal) and “Instituto Geografico Nacional” (Spain) websites. CoastSat python toolkit (Vos et al., 2019) was used to extract shorelines from open-source satellite imagery (Landsat and Sentinel-2).

Overall, the Gulf of Cadiz has shown average end point rates (EPR) of 1.65 m/yr and 0.6 m/yr for the manually mapped WDL and IWL, respectively. The automated approach yielded a rate of 1.84 m/yr. All the methods show net shoreline accretion, with the results heavily influenced by the significant accretion observed in the downdrift  sector, Matalascañas to Guadalquívir. When all the sectors are analyzed individually it is possible better compare the methodologies, according to all indicators. Comparisons reveal that, in most cases, the automated shorelines align more closely with the manually identified WDL rather than the expected IWL. This discrepancy raises questions about the nature of the indicator detected by the automated tool. The findings suggest that the automated extraction may primarily capture the WDL, highlighting the need for further investigation into the physical significance of indicators identified by automated methods.

This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020). The work is a contribution to the CREST project, funded by FCT through Grant 2022.05392.PTDC (doi:10.54499/2022.05392.PTDC). Authors also recognize the support of national funds through FCT, under the project LA/P/0069/2020 (doi:10.54499/LA/P/0069/2020), granted to the Associate Laboratory ARNET, and UID/00350/2020 (doi:10.54499/UIDB/00350/2020) granted to CIMA.