Shoreline change assessment in rapidly urbanizing coastal megacities using geospatial techniques

Coastal megacities face significant threats from various climate-induced hazards, including cyclones, storm surges, rising sea levels, coastal landslides, and floods. In recent decades, rapid urbanization and a range of anthropogenic pressures have exacerbated the vulnerability of these coastal regions. To assess the current level of coastal vulnerability, we focused on two Asian coastal megacities: Shenzhen and Shanghai. Using the Digital Shoreline Analysis System (DSAS) to evaluate coastal erosion vulnerability and shoreline changes from 1990 to 2022, we employed Shoreline Change Envelope (SCE) and Net Shoreline Movement (NSM) to measure the distance of shoreline change, while the rate of shoreline change was calculated with the End Point Rate (EPR). To simplify the DSAS analysis and highlight significant findings, Shenzhen's coastline was divided into two sections: The western coast (C1) and the eastern coastline (C2). The SCE results indicated a noteworthy seaward shoreline shift of 4.5 km in section C1 from 1990 to 2010, while section C2 experienced a significant alteration, extending 2.5 km seaward during the same period. In contrast, Shanghai exhibited a remarkable shoreline change, extending 6.5 km seaward between 1990 and 2010. NSM analysis revealed that within region C2, the maximum recorded erosion from 1990 to 2022 was 920 m, with the furthest observed accretion reaching 2,483 m. Additionally, results indicated that in section C1, the lowest shoreline change rate was −11.9 m/year, illustrating erosion, while Shanghai's minimum shoreline change rate was −3.04 m/year, also indicating a trend of erosion. The findings from this study, combined with the vulnerability maps, will aid policymakers and decision-makers in developing strategies to enhance the quality of life for coastal communities.