Dynamics, vulnerabilities, and adaptive strategies for tidal flat sustainability in a mega fluvial-tidal estuary

Estuarine tidal flats are valuable natural resources, offering significant ecological, environmental, and economic benefits. However, intense human activities are increasingly altering tidal flat morphodynamics, leading to the unsustainable development of estuarine tidal flats. Moreover, there remain knowledge gaps in developing efficient resilience-enhancing strategies for vulnerable coastal communities and ecosystems. Therefore, this study integrates satellite images, bathymetric data, and hydrological records to detect the spatiotemporal dynamics and risks of four major tidal flats in the mega-Changjiang Estuary (CJE) between 2002 and 2023. Our results indicated that four tidal flats prograded overall over the past two decades, with salt marsh expansion rates in the Eastern Chongming Shoal, Hengsha Shoal, Jiuduan Shoal, and Nanhui Shoal standing at 3.13 km²/yr, 6.44 km²/yr, 3.58 km²/yr, and 5.56 km²/yr, respectively. Nevertheless, the bare flat of Jiuduan Shoal experienced significant erosion at a rate of 2.45 km²/yr since 2014. The primary risks to tidal flats in the CJE include the loss of upper intertidal zones, steepening topography, imbalanced expansion of vegetated-unvegetated areas, degeneration of tidal creeks, and Spartina alterniflora invasion. Despite declining fluvial sediment, engineered structures in the CJE have enhanced local sedimentation rates in tidal flats while concurrently increasing future instability risks. We consequently propose targeted management strategies for sustainable tidal flat restorations, prioritizing: 1) sediment replenishment pathways, 2) geomorphic rehabilitation techniques, and 3) ecological engineering solutions. These evidence-based recommendations support policy development for preserving coastal ecosystems sustainable development.