Satellite Reveals the Accelerated Coastline Erosion of Sydney’s Sandy Beaches Since the 21st Century

Coastline change serves as a crucial indicator of environmental changes in coastal areas. By utilizing Landsat imagery time series from 2000 to 2022, we integrated the Modified Normalized Water Index, Support Vector Machine supervised classifier, and the Digital Coastline Analysis System to track coastline changes in Sydney’s sandy beaches over 2000-2022, and analyzed the potential factors. Since the beginning of the 21st century, Sydney’s sandy beaches exhibited an erosion trend of −0.17 m/a, resulting in a net coastline movement of −6.84 m, with over 80% of the coastline experiencing erosion. Before 2010, the sandy beaches, on average, accreted at a rate of 0.40 m/a. Then, from 2010 to 2019, the average beach accretion slowed down (0.07 m/a), with some beaches showing an erosion trend. However, after 2019, sandy coastline erosion in Sydney has greatly accelerated, with an average rate of −4.81 m/a. The primary factors influencing the spatial-temporal patterns of Sydney’s sandy coastline include sea level height, significant wave height, and storm events. This study provides valuable insights into the sustainable management and protection of sandy beaches, disaster response planning, and the sustainable development of coastal areas.