Hybrid green-grey intertidal-flat plus wetland restoration as solution for heavily human-intervened coastline management

The world coastlines in the Anthropocene are experiencing a dramatic transition from natural dynamics to predominantly artificial modifications. The latter causes high uncertainties of future evolution, especially in densely populated mega deltas where natural mudflats are damaged by overexploitation, leading to an irreversible retreat. Based on abundant datasets and an integrated hydro-sediment-dynamics model, we i) examined decadal-centennial historical coastline change of a densely populated mega delta in the Yangtze Estuary, and ii) predicted future morphological developments under variance restoration strategies. We found that human activities, such as intertidal mudflat reclamation and diminished sediment supply from watershed, have disproportionately impacted recent intertidal area change, causing a sixfold greater impact than natural factors of tide, wave, and river runoff combined. Hindcasting using numerical model revealed a unique coastal erosion mechanism post-reclamation and Three Gorges Dam (TGD): a reduced suspended sediment deposition threshold due to sediment fining coupled with increased longshore transport due to wetland lost exacerbates coastal erosion. Various restoration strategies were modelled to identify an approach that could rehabilitate the squeezed tidal flats and degraded wetlands. Our results favor an innovative hybrid “green-grey” structure, combining traditional seawalls (“hard” engineering) with strategically designed long-armed T-groynes (“grey” solution) alongside an integrated saltmarsh (“green” solution) to promote tidal flat growth. This hybrid setup, through a biogeomorphological feedback, captures 30% more sediment than the solely “grey” infrastructure, halving the time needed for tidal flat restoration. Once the intertidal area is recovered, the restored mudflat and habitat could significantly strengthen biodiversity and landscape opportunities, supporting both flood defense and multifunctional ecosystem services.