Impact of groyne lowering on tidal-flat morphodynamics in a tide-dominated estuary

Tidal-flat reclamation and coastal stabilization projects are widely implemented in the tide-dominated estuaries of eastern China, where intensive human intervention has profoundly altered sediment dynamics and morphological evolution. The Nanbei Lake reclamation area, located on the northern coast of Hangzhou Bay and the transitional reach of the Qiantang Estuary, China. This zone experiences strong semidiurnal tides, rapid current variations, and frequent typhoon impacts that shape highly dynamic geomorphic patterns. To mitigate erosion and promote siltation, a main embankment and seven rockfill groynes were constructed in 2007. However, long-term monitoring indicates that high crest elevations of the groynes have suppressed cross-shore water exchange, weakened tidal flushing, and promoted excessive sedimentation—patterns likely exacerbated by increasing storm-tide levels and evolving tidal asymmetry under climate change. To address these issues, this study evaluates a groyne-lowering scheme designed to enhance hydrodynamic connectivity while maintaining shoreline protection. A two-dimensional hydro–morphodynamic model (MIKE 21 FM) was developed using high-resolution bathymetry, tidal observations, and sediment data. The computational domain (~4000 km²) employs an unstructured mesh (minimum grid size 5 m) with 20 s time steps. Model calibration achieves strong agreement with measured tidal levels and velocities. The proposed scheme lowers the groyne crests by 0.2–2.5 m, increasing overtopping frequency during spring tides and enabling reactivation of intertidal exchange pathways. Model results reveal that groyne lowering significantly modifies the nearshore flow structure: bottom velocities increase by 0.005–0.050m/s, residual circulation strengthens between groynes, and previously stagnant zones behind the structures become reconnected. Morphodynamic responses over a spring–neap cycle indicate 0.2–0.4 m reduction in sedimentation near groyne heads, accompanied by mild accretion on the inner tidal flat, leading to a smoother, more gradually sloping intertidal profile. These changes reflect a shift toward a more dynamic and resilient morphodynamic state capable of better accommodating extreme water levels. This study highlights groyne lowering as an adaptive and nature-based intervention to counteract human-induced hydrodynamic restriction and climate-driven pressures. The findings contribute to improved understanding of eco-morphodynamic adjustment processes and offer guidance for sustainable coastal management in tide-dominated estuaries such as the Qiantang River delta.