Quantifying five decades of shoreline change in relation to river sediment supply along the Phu Yen Coast, Central Vietnam

We examined the relationship between shoreline dynamics and significant changes in river sediment supply along the Phu Yen Coast, a typical coastal area in central Vietnam, over a 53-year period (1972–2024). Empirical Orthogonal Functions (EOFs) were applied to a demeaned dataset to identify significant modes of shoreline position changes extracted from high-quality Landsat satellite data. Overall, we identified two distinct phases of coastal evolution: (i) a relatively stable period (1972–2012) characterized by modest accretion (+1 m/yr); (ii) followed by a period of shoreline retreat (2012–2024) at a rate of −1 m/yr. Notably, areas within 3 km of the river mouth, the shoreline exhibited severe geomorphic instability with retreat rates reaching −20 m/yr. Long-term monitoring of riverine processes in the Ba River system, the region's largest fluvial system, indicates a dramatic 82 % decline in river sediment discharge, from 2.17 × 10⁶ ton/yr in 1979–2007 to 0.4 × 10⁶ ton/yr in 2008–2023. Our EOF analysis showed that the first four modes explain up to 73 % of the observed shoreline variability, with primary controlling factors including riverine sediment supply fluctuations and both longshore and cross-shore transport processes. Significantly, we observed a multi-year lag between the reduction in river sediment supply and subsequent shoreline retreat, highlighting the complex temporal dynamics of coastal sediment budgets in response to anthropogenic perturbations.