| Summary: | The mouth bar is a critical geomorphic unit in the Modaomen Estuary, playing a significant role in modulating saltwater intrusion and influencing both estuarine dynamics and regional water security. With the continuous changes in river discharge, tidal dynamics, and human interventions, the evolution of the mouth bar has become a central issue in understanding salinity processes in the Pearl River Estuary. Despite its importance, limited quantitative research has been conducted to evaluate the long-term impacts of mouth bar evolution on saltwater intrusion. Based on representative bathymetric data spanning from 1990 to 2020, this study employed the finite-volume community ocean model (FVCOM), which is well known for its unstructured grid and high-resolution simulation capabilities, to quantitatively assess how morphological changes in the mouth bar affect the intensity, distance, and duration of saltwater intrusion. The model was carefully calibrated and validated using observed hydrological and salinity data, ensuring reliability in capturing the complex interactions between hydrodynamics, sediment dynamics, and salinity transport. Results indicate that in 1990, when the mouth bar remained relatively elevated and intact, the maximum intrusion distance was restricted to 46.77 km. By contrast, in 2020, bar degradation and shrinkage led to a significant increase in intrusion distance, reaching 57.79 km. This morphological decline also extended the duration during which surface salinity exceeded the drinking water standard (0.5 psu) at the Pinggang water intake site, rising from 38 to 99 hours. Such an increase represents a substantial threat to the reliability and safety of regional water supply systems. Furthermore, the shrinkage of the mouth bar altered the spatial and temporal distribution of salinity retention. Specifically, the average duration of bottom-salinity retention in the deep channel increased by approximately 158 minutes, while that in the adjacent shoal areas decreased by about 261 minutes. These contrasting patterns highlight how morphological degradation intensifies saltwater intrusion and redistributes salinity stress across different estuarine sub-regions. The degradation of the sand body further exerted a strong influence on the salinity regime within the estuary, particularly in terms of mean salinity, which exhibited a maximum variation of up to 81%. In comparison, peak salinity levels were shown to be primarily controlled by upstream river discharge, and their variability remained relatively limited. This distinction emphasizes the dual controls of morphology and hydrology on estuarine salinity processes. Overall, this research demonstrates that the long-term evolution of the mouth bar has profound implications for saltwater intrusion dynamics in the Modaomen Estuary. The findings enrich the scientific understanding of estuarine geomorphic–hydrodynamic interactions and provide critical insights for water resources management, salinity control dispatching, and ecological protection in the Pearl River Delta. These results highlight the necessity of incorporating morphological evolution into future estuarine management strategies, particularly under the combined pressures of climate change and human development.
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