Erosion induced coastal changes in Karasu, Türkiye: new evidence from GNSS and remote sensing data
Coastal regions are highly sensitive to both natural processes and anthropogenic pressures including settlement, tourism, industry, and transportation, in addition to the impacts of climate change. Continuous monitoring of these areas is therefore essential for sustainable coastal management and for enabling rapid response to potential erosion hazards. Such changes can be effectively monitored using geodetic and remote sensing techniques. The study area is located in the Karasu district of Sakarya Province, in the northwestern part of Türkiye, where significant coastal erosion has been observed. Erosion mitigation measures were initiated in 2010 following the identification of the problem. Since then, the shoreline has retreated approximately 100 m landward, although the construction of breakwaters has partially reduced the erosion rate. In this study, the current state of coastal change was investigated using open-access Sentinel-2 and Sentinel-1 datasets. The Normalized Difference Water Index (NDWI) was applied to optical imagery, while backscatter information derived from Synthetic Aperture Radar (SAR) data was used to delineate the land–water boundary. Additionally, shoreline data obtained from Global Navigation Satellite System (GNSS) observations were adopted as reference data for accuracy assessment. Based on GNSS surveys conducted at two-year intervals within the same period, an erosion area of 12,069 m² and an accretion area of 77,410 m² were quantified along an approximately 9 km coastal segment. The results indicate that shoreline extraction from Sentinel-2 imagery shows significantly higher consistency with GNSS-derived reference data compared to Sentinel-1 results. Overall, Sentinel-2 data provide more reliable and accurate shoreline delineation, suggesting that they are highly suitable for rapid and effective coastal erosion monitoring in support of sustainable coastal management.