Coastal engineering structures bifurcate intertidal eco-morphodynamic trajectories in a sediment-starved delta

Damming has reduced sediment supply to most river deltas, impairing the land-building capacity and ecological services of tidal wetlands. How localised engineering structures, coupled with sediment starvation, drive shifts in intertidal ecology and morphodynamics within a deltaic system remains a critical knowledge gap. Here, high-resolution Unmanned Aerial Vehicle imagery and Terrestrial Laser Scanner topographic data are integrated to quantify the dynamics of the Nanhui wetlands, the largest marginal wetland in the Yangtze Delta, where riverine sediment input has decreased by more than 70% in the last decades. Results show that the total salt marsh area expanded from 21.80 ha in 2019 to 46.37 ha in 2022, with the area of native and invasive marsh species increasing by 58.5% and 221.1%, respectively. The salt marsh in the East Nanhui wetland rapidly increased, growing 7.2-fold to reach 31.25 ha, whereas the salt marsh extent in the South Nanhui wetland declined by 16%, shrinking to 15.12 ha. Groins promoting siltation in the East Nanhui wetland weakened the wave and tidal forcing. This facilitated the accumulation of sediment from coarser-grained sources, as well as the encroachment of native Scirpus mariqueter. On the contrary, following the reclamation of the upper intertidal and supratidal zones, the South Nanhui wetland suffered severe retreat and sustained coarsening of bed sediment, due to enhanced alongshore currents and waves. Sediment remobilised by erosion deposited in the high marsh, favouring the invasion of Spartina alterniflora. Hence, this study clarifies that anthropogenic effects coupled with changes in hydrodynamic regimes can bifurcate the intertidal eco-morphodynamics into two distinct states: prograding versus eroding. Reduced sediment availability is likely to increase erosion of native salt marshes, thereby favouring the encroachment of invasive high marsh vegetation. Our findings highlight the value of high-resolution measurements in revealing different trajectories of nature-based solutions for coastal protection in mega-deltas.