Satellite-derived shoreline evolution in Inhambane province (Mozambique) using high-resolution imagery

Satellite imagery is crucial for studying shoreline evolution due to its ability to provide consistent, high-resolution, and large-scale data over time (Gomes da Silva et al., 2024); and it plays a crucial role in countries with limited coastal information sources. The aim of this study is to explore the use of satellite imagery to investigate shoreline evolution at three different locations in Mozambique: Morrungulo Beach, Barra Beach, and Tofo Beach in Inhambane Province. These three locations are characterized as exposed and mesotidal beaches and were selected as representative of the typical coastal archetype in the south coast of Mozambique. This study uses satellite imagery provided by Planet Labs, which is explored with the open-source code CoastSat.PlanetScope toolkit to map and analyze in detail shoreline changes in the study sites (Doherty et al., 2022). PlanetScope satellite imagery has a spatial resolution of approximately 3 meters and almost daily temporal resolution, allowing for detailed observation of shoreline features. For the automated extraction of the shoreline, CoastSat.PlanetScope takes into account the beach slope and tide to provide shoreline positions along user-defined transects, determined using a water index and pixel thresholding. The temporal scope of the satellite imagery utilized in this study extends from July 2016 to June 2024, using one image per month, offering a comprehensive dataset for examining monthly to multiannual shoreline dynamics. Shoreline positions have been evaluated using data total of 101 shorelines for Barra Beach, 94 shorelines for Morrungulo Beach, and 108 shorelines for Tofo Beach. Through this analysis, it was also possible to determine the shoreline impacts of tropical cyclones that made landfall in the region. Barra Beach revealed a strong erosion rate of 3.7 m/year as calculated using the End Point Rate (EPR) method, which measures the net shoreline change over time, and a moderate erosion rate of 0.5 m/year based on the Linear Regression (LR) method, suggesting relative stability in shoreline position when more shoreline positions are considered. Morrungulo Beach presented an accretion rate of 1.7 m/yr based on EPR, but evidenced an erosion rate of 0.4 m/yr with LR. Tofo Beach presented more consistent erosion, with a rate of 1.8 m/yr for EPR and 0.7 m/yr for LR. The analysis of shoreline changes across the three selected beaches in Mozambique highlights distinct patterns of erosion and accretion over the study period. Barra Beach demonstrated considerable differences in erosion rate according to the method, while Morrungulo Beach exhibited a mix of accretion and minor erosion, depending on the analysis method used. Conversely, Tofo Beach showed consistent erosion. These findings highlight the need to carefully consider shoreline change metrics, selecting those that better represent the coastal processes of interest to ensure site-specific management strategies along Mozambique’s coastline. This study has been funded by DOORS project (H2020 – 101000518 – DOORS), and co-funded by the FI AGAUR grant (2022 FI_B 00897).